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We define a physically motivated measure for galactic bar length, called the dynamical length. The dynamical length of the bar corresponds to the radial extent of the trapped orbits that are the backbone supporting the bar feature. We propose a direct observational technique well suited to integral field unit spectroscopy to measure it. Identifying these orbits and using the dynamical length is a more faithful tracer of the secular evolution and influence of the bar. We demonstrate the success of our observational technique for recovering the maximal bar-parenting orbit in a range of simulations, and to show its promise we perform its measurement on a real galaxy. We also study the difference between traditionally used ellipse fit approaches to determine bar length and the dynamical length proposed here in a wide range of bar-forming N -body simulations of a stellar disc and dark matter halo. We find that ellipse fitting may severely overestimate measurements of the bar length by a factor of 1.5–2.5 relative to the extent of the orbits that are trapped and actually comprise the bar. This bias leads to overestimates of both bar mass and the ratio of corotation radius to bar length, i.e. the bar speed, affecting inferences about the evolution of bars in the real Universe. [ABSTRACT FROM AUTHOR]

Using reconstructed initial conditions in the Sloan Digital Sky Survey (SDSS) survey volume, we carry out constrained hydrodynamic simulations in three regions representing different types of the cosmic web: the Coma cluster of galaxies; the SDSS Great Wall; and a large low-density region at z ∼ 0.05. These simulations, which include star formation and stellar feedback but no active galactic nucleus formation and feedback, are used to investigate the properties and evolution of intergalactic and intracluster media. About half of the warm-hot intergalactic gas is associated with filaments in the local cosmic web. Gas in the outskirts of massive filaments and halos can be heated significantly by accretion shocks generated by mergers of filaments and halos, respectively, and there is a tight correlation between the gas temperature and the strength of the local tidal field. The simulations also predict some discontinuities associated with shock fronts and contact edges, which can be tested using observations of the thermal Sunyaevâ€"Zel’dovich effect and X-rays. A large fraction of the sky is covered by Ly α and O vi absorption systems, and most of the O vi systems and low-column-density H i systems are associated with filaments in the cosmic web. The constrained simulations, which follow the formation and heating history of the observed cosmic web, provide an important avenue to interpret observational data. With full information about the origin and location of the cosmic gas to be observed, such simulations can also be used to develop observational strategies. [ABSTRACT FROM AUTHOR]

We present the N -body simulation techniques implemented in the exp code. exp uses empirically chosen basis functions to expand the potential field of an ensemble of particles. Unlike other basis function expansions, the derived basis functions are adapted to an input mass distribution, enabling accurate expansion of highly non-spherical objects, such as Galactic discs. We measure the force accuracy in three models, one based on a spherical or aspherical halo, one based on an exponential disc, and one based on a bar-based disc model. We find that exp is as accurate as a direct-summation or tree-based calculation, and in some ways is better, while being considerably less computationally intensive. We discuss optimizing the computation of the basis function representation. We also detail numerical improvements for performing orbit integrations, including time-steps. [ABSTRACT FROM AUTHOR]

The morphological evolution of galaxy clusters occurs during multiple high-speed collisions between galaxies. High-resolution numerical simulations show that during these interactions small disk galaxies are converted into dwarf elliptical or spheroidal galaxies. The debris arcs left after the encounters provide fuel for quasars in the sub-luminous host galaxies. The simulations suggest that these galaxy interactions are different from galaxy mergers.

We develop a model to establish the interconnection between galaxies and their dark matter haloes. We use Principal Component Analysis (PCA) to reduce the dimensionality of both the mass assembly histories of haloes/subhaloes and the star formation histories of galaxies, and Gradient Boosted Decision Trees (GBDT) to transform halo/subhalo properties into galaxy properties. We use two sets of hydrodynamic simulations to motivate our model architecture and to train the transformation. We then apply the two sets of trained models to dark-matter-only (DMO) simulations to show that the transformation is reliable and statistically accurate. The model trained by a high-resolution hydrodynamic simulation, or by a set of such simulations implementing the same physics of galaxy formation, can thus be applied to large DMO simulations to make 'mock' copies of the hydrodynamic simulation. The model is both flexible and interpretable, which paves the way for future applications in which we will constrain the model using observations at different redshifts simultaneously and explore how galaxies form and evolve in dark matter haloes empirically. [ABSTRACT FROM AUTHOR]

Quasar absorption systems encode a wealth of information about the abundances, ionization structure, and physical conditions in intergalactic and circumgalactic media. Simple (often single-phase) photoionization models are frequently used to decode such data. Using five discrete absorbers from the COS Absorption Survey of Baryon Harbors (CASBaH) that exhibit a wide range of detected ions (e.g. Mg  ii , S  ii – S  vi , O  ii – O  vi , Ne  viii), we show several examples where single-phase ionization models cannot reproduce the full set of measured column densities. To explore models that can self-consistently explain the measurements and kinematic alignment of disparate ions, we develop a Bayesian multiphase ionization modelling framework that characterizes discrete phases by their unique physical conditions and also investigates variations in the shape of the UV flux field, metallicity, and relative abundances. Our models require at least two (but favour three) distinct ionization phases ranging from T ≈ 104 K photoionized gas to warm-hot phases at T ≲ 105.8 K. For some ions, an apparently single absorption 'component' includes contributions from more than one phase, and up to 30 per cent of the H  i is not from the lowest ionization phase. If we assume that all of the phases are photoionized, we cannot find solutions in thermal pressure equilibrium. By introducing hotter, collisionally ionized phases, however, we can achieve balanced pressures. The best models indicate moderate metallicities, often with subsolar N/ α , and, in two cases, ionizing flux fields that are softer and brighter than the fiducial Haardt & Madau UV background model. [ABSTRACT FROM AUTHOR]

We interpret simulations of secularly evolving disc galaxies through orbit morphology. Using a new algorithm that measures the volume of orbits in real space using a tessellation, we rapidly isolate commensurate (resonant) orbits. We identify phase-space regions occupied by different orbital families. Compared to spectral methods, the tessellation algorithm can identify resonant orbits within a few dynamical periods, crucial for understanding an evolving galaxy model. The flexible methodology accepts arbitrary potentials, enabling detailed descriptions of the orbital families. We apply the machinery to four different potential models, including two barred models, and fully characterize the orbital membership. We identify key differences in the content of orbit families, emphasizing the presence of orbit families indicative of the bar evolutionary state and the shape of the dark matter halo. We use the characterization of orbits to investigate the shortcomings of analytic and self-consistent studies, comparing our findings to the evolutionary epochs in self-consistent barred galaxy simulations. Using insight from our orbit analysis, we present a new observational metric that uses spatial and kinematic information from integral field spectrometers that may reveal signatures of commensurabilities and allow for a differentiation between models. [ABSTRACT FROM AUTHOR]

The propagation and evolution of cold galactic winds in galactic haloes is crucial to galaxy formation models. However, modelling of this process in hydrodynamic simulations of galaxy formation is oversimplified owing to a lack of numerical resolution and often neglects critical physical processes such as hydrodynamic instabilities and thermal conduction. We propose an analytic model, Physically Evolved Winds, that calculates the evolution of individual clouds moving supersonically through a uniform ambient medium. Our model reproduces predictions from very high resolution cloud-crushing simulations that include isotropic thermal conduction over a wide range of physical conditions. We discuss the implementation of this model into cosmological hydrodynamic simulations of galaxy formation as a subgrid prescription to model galactic winds more robustly both physically and numerically. [ABSTRACT FROM AUTHOR]

Many phenomenologically successful cosmological simulations employ kinetic winds to model galactic outflows. Yet systematic studies of how variations in kinetic wind scalings might alter observable galaxy properties are rare. Here we employ gadget-3 simulations to study how the baryon cycle, stellar mass function, and other galaxy and CGM predictions vary as a function of the assumed outflow speed and the scaling of the mass-loading factor with velocity dispersion. We design our fiducial model to reproduce the measured wind properties at 25 per cent of the virial radius from the Feedback In Realistic Environments simulations. We find that a strong dependence of η ∼ σ5 in low-mass haloes with |$\sigma \lt 106\mathrm{\, km\, s^{-1}}$| is required to match the faint end of the stellar mass functions at |$z$| > 1. In addition, faster winds significantly reduce wind recycling and heat more halo gas. Both effects result in less stellar mass growth in massive haloes and impact high ionization absorption in halo gas. We cannot simultaneously match the stellar content at |$z$|  = 2 and 0 within a single model, suggesting that an additional feedback source such as active galactic nucleus might be required in massive galaxies at lower redshifts, but the amount needed depends strongly on assumptions regarding the outflow properties. We run a 50 |$\mathrm{Mpc}\, h^{-1}$|⁠ , 2 × 5763 simulation with our fiducial parameters and show that it matches a range of star-forming galaxy properties at |$z$| ∼ 0–2. [ABSTRACT FROM AUTHOR]

We track the angular momentum transfer in N -body simulations of barred galaxies by measuring torques to understand the dynamical mechanisms responsible for the evolution of the bar–disc–dark matter halo system. We find evidence for three distinct phases of barred galaxy evolution: assembly, secular growth, and steady-state equilibrium. Using a decomposition of the disc into orbital families, we track bar mass and angular momentum through time and correlate the quantities with the phases of evolution. We follow the angular momentum transfer between particles and identify the dominant torque channels. We find that the halo model mediates the assembly and growth of the bar for a high central density halo, and the outer disc mediates the assembly and growth of the bar in a low central density halo model. Both galaxies exhibit a steady-state equilibrium phase where the bar is neither lengthening nor slowing. The steady-state equilibrium results from the balance of torque between particles that are gaining and losing angular momentum. We propose observational metrics for barred galaxies that can be used to help determine the evolutionary phase of a barred galaxy, and discuss the implications of the phases for galaxy evolution as a whole. [ABSTRACT FROM AUTHOR]

We test and improve the numerical schemes in our smoothed particle hydrodynamics (SPH) code for cosmological simulations, including the pressure–entropy formulation (PESPH), a time-dependent artificial viscosity, a refined time-step criterion, and metal-line cooling that accounts for photoionization in the presence of a recently refined Haardt & Madau model of the ionizing background. The PESPH algorithm effectively removes the artificial surface tension present in the traditional SPH formulation, and in our test simulations, it produces better qualitative agreement with mesh-code results for Kelvin–Helmholtz instability and cold cloud disruption. Using a set of cosmological simulations, we examine many of the quantities we have studied in previous work. Results for galaxy stellar and H  i mass functions, star formation histories, galaxy scaling relations, and statistics of the Lyα forest are robust to the changes in numerics and microphysics. As in our previous simulations, cold gas accretion dominates the growth of high-redshift galaxies and of low-mass galaxies at low redshift, and recycling of winds dominates the growth of massive galaxies at low redshift. However, the PESPH simulation removes spurious cold clumps seen in our earlier simulations, and the accretion rate of hot gas increases by up to an order of magnitude at some redshifts. The new numerical model also influences the distribution of metals among gas phases, leading to considerable differences in the statistics of some metal absorption lines, most notably Ne  viii. [ABSTRACT FROM AUTHOR]

We examine the properties of the galaxies and dark matter haloes residing in the cluster infall region surrounding the simulated Λ cold dark matter galaxy cluster studied by Elahi et al. at z = 0. The 1.1 × 1015 h-1⊙ galaxy cluster has been simulated with eight different hydrodynamical codes containing a variety of hydrodynamic solvers and sub-grid schemes. All models completed a dark-matter-only, non-radiative and full-physics run from the same initial conditions. The simulations contain dark matter and gas with mass resolution mDM = 9.01 × 108 h-1⊙ and mgas = 1.9 × 108 h-1⊙, respectively. We find that the synthetic cluster is surrounded by clear filamentary structures that contain ~60 per cent of haloes in the infall region with mass ~1012.5-1014 h-1⊙, including 2-3 group-sized haloes (>1013 h-1⊙). However, we find that only ~10 per cent of objects in the infall region are sub-haloes residing in haloes, which may suggest that there is not much ongoing pre-processing occurring in the infall region at z = 0. By examining the baryonic content contained within the haloes, we also show that the code-to-code scatter in stellar fraction across all halo masses is typically ~2 orders of magnitude between the two most extreme cases, and this is predominantly due to the differences in sub-grid schemes and calibration procedures that each model uses. Models that do not include active galactic nucleus feedback typically produce too high stellar fractions compared to observations by at least ~1 order of magnitude. [ABSTRACT FROM AUTHOR]

We investigate the complex interactions between the stellar disc and the dark-matter halo during bar formation and evolution using N-body simulations with fine temporal resolution and optimally chosen spatial resolution. We find that the forming stellar bar traps dark matter in the vicinity of the stellar bar into bar-supporting orbits. We call this feature the shadow bar. The shadow bar modifies both the location and magnitude of the angular momentum transfer between the disc and dark matter halo and adds 10 per cent to the mass of the stellar bar over 4 Gyr. The shadow bar is potentially observable by its density and velocity signature in spheroid stars and by direct dark matter detection experiments. Numerical tests demonstrate that the shadow bar can diminish the rate of angular momentum transport from the bar to the dark matter halo by more than a factor of 3 over the rate predicted by dynamical friction with an untrapped dark halo, and thus provides a possible physical explanation for the observed prevalence of fast bars in nature. [ABSTRACT FROM AUTHOR]

We have simulated the formation of a massive galaxy cluster (Mcrit 200 = 1.1 × 1015 h-1M☉) in a ⋀ cold dark matter universe using 10 different codes (RAMSES, 2 incarnations of AREPO and 7 of GADGET), modelling hydrodynamics with full radiative subgrid physics. These codes include smoothed-particle hydrodynamics (SPH), spanning traditional and advanced SPH schemes, adaptive mesh and moving mesh codes. Our goal is to study the consistency between simulated clusters modelled with different radiative physical implementations - such as cooling, star formation and thermal active galactic nucleus (AGN) feedback. We compare images of the cluster at z = 0, global properties such as mass, and radial profiles of various dynamical and thermodynamical quantities. We find that, with respect to non-radiative simulations, dark matter is more centrally concentrated, the extent not simply depending on the presence/absence of AGN feedback. The scatter in global quantities is substantially higher than for non-radiative runs. Intriguingly, adding radiative physics seems to have washed away the marked code-based differences present in the entropy profile seen for non-radiative simulations in Sembolini et al.: radiative physics+classic SPH can produce entropy cores, at least in the case of non cool-core clusters. Furthermore, the inclusion/absence of AGN feedback is not the dividing line -as in the case of describing the stellar content - for whether a code produces an unrealistic temperature inversion and a falling central entropy profile. However, AGN feedback does strongly affect the overall stellar distribution, limiting the effect of overcooling and reducing sensibly the stellar fraction. [ABSTRACT FROM AUTHOR]

We analyse the low-redshift (z≈0.2) circumgalactic medium (CGM) by comparing absorption-line data from the COS-Halos survey to absorption around a matched galaxy sample from two cosmological hydrodynamic simulations. The models include different prescriptions for galactic outflows, namely hybrid energy/momentum driven wind (ezw), and constant winds (cw). We compare equivalent widths, covering factors, ion ratios, and kinematics. Both wind models show generally ≲1σ agreement with these observations for H I and certain low-ionization metal lines, but poorer agreement with higher ionization metal lines including Si III and O VI that are well observed by COS-Halos. This suggests that both models predict too much cool, metal-enriched gas and not enough hot gas, and / or that metals are not sufficiently mixed. This may reflect our model assumption of ejecting outflows as cool and unmixing gas. Our ezw simulation includes a heuristic prescription to quench massive galaxies by superheating interstellar medium gas. This produces low-ionization absorption broadly consistent with observations, but substantial O VI absorption inconsistent with data, suggesting that gas around quenched galaxies in the real Universe does not cool. At impact parameters of ≲50 kpc, recycling winds dominate the absorption of low ions and even H I, while O VI generally arises from metals ejected ≳1 Gyr ago. The similarity between the wind models is surprising, since they differ substantially in the amount and phase distribution of halo gas.We show that this similarity owes mainly to our comparison at fixed stellar (not halo) mass, suggesting that CGM properties are more closely tied to galaxy's stellar (versus halo) mass. [ABSTRACT FROM AUTHOR]

Building on the initial results of the nIFTy simulated galaxy cluster comparison, we compare and contrast the impact of baryonic physics with a single massive galaxy cluster, run with 11 state-of-the-art codes, spanning adaptive mesh, moving mesh, classic and modern smoothed particle hydrodynamics (SPH) approaches. For each code represented we have a dark-matter-only (DM) and non-radiative (NR) version of the cluster, as well as a full physics (FP) version for a subset of the codes. We compare both radial mass and kinematic profiles, as well as global measures of the cluster (e.g. concentration, spin, shape), in the NR and FP runs with that in the DM runs. Our analysis reveals good consistency ⪅20 per cent) between global properties of the cluster predicted by different codes when integrated quantities are measured within the virial radius R200. However, we see larger differences for quantities within R2500, especially in the FP runs. The radial profiles reveal a diversity, especially in the cluster centre, between the NR runs, which can be understood straightforwardly from the division of codes into classic SPH and non-classic SPH (including the modern SPH, adaptive and moving mesh codes); and between the FP runs, which can also be understood broadly from the division of codes into those that include active galactic nucleus feedback and those that do not. The variation with respect to the median is much larger in the FP runs with different baryonic physics prescriptions than in the NR runs with different hydrodynamics solvers. [ABSTRACT FROM AUTHOR]

We examine subhaloes and galaxies residing in a simulated Λ cold dark matter galaxy cluster (M crit 200 =1.1 1015 h-1 M⊙) produced by hydrodynamical codes ranging from classic smooth particle hydrodynamics (SPH), newer SPH codes, adaptive and moving mesh codes. These codes use subgrid models to capture galaxy formation physics. We compare how well these codes reproduce the same subhaloes/galaxies in gravity-only, non-radiative hydrodynamics and full feedback physics runs by looking at the overall subhalo/galaxy distribution and on an individual object basis. We find that the subhalo population is reproduced to within ≲10 percent for both dark matter only and non-radiative runs, with individual objects showing code-to-code scatter of ≲0.1 dex, although the gas in non-radiative simulations shows significant scatter. Including feedback physics significantly increases the diversity. Subhalo mass and Vmax distributions vary by ≈20 percent. The galaxy populations also show striking code-to-code variations. Although the Tully-Fisher relation is similar in almost all codes, the number of galaxies with 109 h-1M⊙ ≲ M* ≲ 1012 h-1M⊙ can differ by a factor of 4. Individual galaxies show code-to-code scatter of ~0.5 dex in stellar mass. Moreover, systematic differences exist, with some codes producing galaxies 70 percent smaller than others. The diversity partially arises from the inclusion/absence of active galactic nucleus feedback. Our results combined with our companion papers demonstrate that subgrid physics is not just subject to fine-tuning, but the complexity of building galaxies in all environments remains a challenge. We argue that even basic galaxy properties, such as stellar mass to halo mass, should be treated with errors bars of ~0.2-0.4 dex. [ABSTRACT FROM AUTHOR]

We have simulated the formation of a galaxy cluster in a Λ cold dark matter universe using 13 different codes modelling only gravity and non-radiative hydrodynamics (RAMSES, ART, AREPO, HYDRA and nine incarnations of GADGET). This range of codes includes particle-based, moving and fixed mesh codes as well as both Eulerian and Lagrangian fluid schemes. The various GADGET implementations span classic and modern smoothed particle hydrodynamics (SPH) schemes. The goal of this comparison is to assess the reliability of cosmological hydrodynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be non-radiative. We compare images of the cluster at z = 0, global properties such as mass and radial profiles of various dynamical and thermodynamical quantities. The underlying gravitational framework can be aligned very accurately for all the codes allowing a detailed investigation of the differences that develop due to the various gas physics implementations employed. As expected, the mesh-based codes RAMSES, ART and AREPO form extended entropy cores in the gas with rising central gas temperatures. Those codes employing classic SPH schemes show falling entropy profiles all the way into the very centre with correspondingly rising density profiles and central temperature inversions. We show that methods with modern SPH schemes that allow entropy mixing span the range between these two extremes and the latest SPH variants produce gas entropy profiles that are essentially indistinguishable from those obtained with grid-based methods. [ABSTRACT FROM AUTHOR]

The thermal and chemical properties of the hot diffuse intragroup medium (IGrM) provide important constraints on the feedback processes associated with massive galaxy formation and evolution. Here we explore these constraints via a detailed analysis of the global properties of simulated z < 3 galaxy groups drawn from a cosmological simulation that includes a wellconstrained prescription for momentum-driven, stellar/supernova-powered galactic outflows but no active galactic nucleus (AGN) feedback. Our simulation successfully reproduces the key observed IGrM properties, including X-ray trends, for all but the most massive groups. The z < 1 redshift evolution of these is also consistent with the observations. Contrary to expectations, the simulated groups' IGrM does not suffer catastrophic cooling. Yet, the z = 0 group stellar mass is ∼2? too large. We show that this is due to the build-up of cold gas in the massive galaxies before they are incorporated inside groups. This indicates that other feedback mechanisms must activate in real galaxies once they grow toM* a few 1010M⊙ and that these must be powerful enough to expel a significant fraction of the gas from the galactic haloes. Gentle 'maintenance-mode' AGN feedback would be insufficient to bring the stellar and baryonic fractions into agreement with the observations. Just as importantly, we find that the stellar/supernova-powered winds are essential for understanding the IGrM metal abundances. Our simulation is able to reproduce the observed relationship between the global IGrM iron and silicon abundances and the group X-ray temperature, and these results should be relatively insensitive to the addition of AGN feedback. [ABSTRACT FROM AUTHOR]

The instantaneous HI content of galaxies is thought to be governed by recent accretion and environment. We examine these effects within a cosmological hydrodynamic simulation that includes a heuristic galactic outflow model that reproduces basic observed trends of HI in galaxies. We show that this model reproduces the observed HI mass function in bins of stellar mass, as well as the HI richness (MHI/M⊙) versus local galaxy density. For satellite galaxies in massive (≳1012M) haloes, the HI richness distribution is bimodal and the median drops towards the largest halo masses. The depletion time-scale of HI entering a massive halo is more rapid, in contrast to the specific star formation rate which shows little variation in the attenuation rate versus halo mass. This suggests that, up to the halo mass scales probed here (≲ 1014M⊙), star formation is mainly attenuated by starvation, but HI is additionally removed by stripping once a hot gaseous halo is present. In low-mass haloes, the HI richness of satellites is independent of radius, while in very massive haloes they become gas-poor towards the centre, confirming the increasing strength of the stripping with halo mass. Mergers somewhat increase the HI richness and its scatter about the mean relation, tracking the metallicity in a way consistent with it arising from inflow fluctuations, while star formation is significantly boosted relative to HI. [ABSTRACT FROM AUTHOR]

The empirical model of Lu et al. is updated with recent data of galaxy stellar mass functions (SMFs). The model predicts that the slope of galaxy SMFs at z > 2 should be quite steep at the low-mass end, beyond the current detection limit, and it is a strong prediction that can be tested against future observations. The model is used to investigate the galaxy star formation and assembly or merger histories in detail. Most of the stars in cluster centrals, corresponding to brightest cluster galaxies in observations, formed earlier than z ≈ 2 but have been assembled much later. Typically, they have experienced ≈5 major mergers since their star formation was quenched. Milky Way mass galaxies have had on-going star formation without significant mergers since z ≈ 2, and are thus free of significant (classic) bulges produced by major mergers. Dwarf galaxies in haloes with Mh < 1011 h-1 M⊙ or M⋆ < 109 M⊙ have experienced a star formation burst at z > 2, followed by a nearly constant star formation rate after z = 1, and the stellar age decreases with stellar mass, contrary to the ‘downsizing’ trend for more massive galaxies. Major mergers are not uncommon during the early burst phase and may result in the formation of old spheroids in dwarf galaxies. We also characterize the stellar population of halo stars in different haloes. [ABSTRACT FROM AUTHOR]

We examine how H i and metal absorption lines within low-redshift galaxy haloes trace the dynamical state of circumgalactic gas, using cosmological hydrodynamic simulations that include a well-vetted heuristic model for galactic outflows. We categorize inflowing, outflowing, and ambient gas based on its history and fate as tracked in our simulation. Following our earlier work, showing that the ionization level of absorbers was a primary factor in determining the physical conditions of absorbing gas, we show here that it is also a governing factor for its dynamical state. Low-ionization metal absorbers (e.g. Mg ii) tend to arise in gas that will fall on to galaxies within several Gyr, while high-ionization metal absorbers (e.g. O vi) generally trace material that was deposited by outflows many Gyr ago. Inflowing gas is dominated by enriched material that was previously ejected in an outflow; hence, accretion at low redshifts is typically substantially enriched. Recycling wind material is preferentially found closer to galaxies, and is more dominant in lower mass haloes since high-mass haloes have more hot gas that is able to support itself against infall. Low-mass haloes also tend to re-eject more of their accreted material, owing to our outflow prescription that employs higher mass loading factors for lower mass galaxies. Typical H i absorbers trace unenriched ambient material that is not participating in the baryon cycle, but stronger H i absorbers arise in cool, enriched inflowing gas. Instantaneous radial velocity measures of absorbers are generally poor at distinguishing between inflowing and outflowing gas, except in the case of very recent outflows. These results suggest that probing halo gas using a range of absorbers can provide detailed information about the amount and physical conditions of material that is participating in the baryon cycle. [ABSTRACT FROM PUBLISHER]

We infer mechanisms of galaxy formation for a broad family of semi-analytic models (SAMs) constrained by the K-band luminosity function and H i mass function of local galaxies using tools of Bayesian analysis. Even with a broad search in parameter space the whole model family fails to match to constraining data. In the best-fitting models, the star formation and feedback parameters in low-mass haloes are tightly constrained by the two data sets, and the analysis reveals several generic failures of models that similarly apply to other existing SAMs. First, based on the assumption that baryon accretion follows the dark matter accretion, large mass-loading factors are required for haloes with circular velocities lower than 200 km s−1, and most of the wind mass must be expelled from the haloes. Second, assuming that the feedback is powered by Type II supernovae with a Chabrier initial mass function, the outflow requires more than 25 per cent of the available supernova kinetic energy. Finally, the posterior predictive distributions for the star formation history are dramatically inconsistent with observations for masses similar to or smaller than the Milky Way mass. The inferences suggest that the current model family is still missing some key physical processes that regulate the gas accretion and star formation in galaxies with masses below that of the Milky Way. [ABSTRACT FROM PUBLISHER]

We present a budget and accounting of metals in and around star-forming galaxies at z ∼ 0. We combine empirically derived star formation histories with updated supernova and asymptotic giant branch yields and rates to estimate the total mass of metals produced by galaxies with present-day stellar mass of 109.3-1011.6 M☼. On the accounting side of the ledger, we show that a surprisingly constant 20%-25% mass fraction of produced metals remain in galaxies' stars, interstellar gas and interstellar dust, with little dependence of this fraction on the galaxy stellar mass (omitting those metals immediately locked up in remnants). Thus, the bulk of metals are outside of galaxies, produced in the progenitors of today's L* galaxies. The COS-Halos survey is uniquely able to measure the mass of metals in the circumgalactic medium (CGM, to impact parameters of <150 kpc) of low-redshift ∼L* galaxies. Using these data, we map the distribution of CGM metals as traced by both the highly ionized O VI ion and a suite of low-ionization species; combined with constraints on circumgalactic dust and hotter X-ray emitting gas out to similar impact parameters, we show that ∼40% of metals produced by M⋆ ∼ 1010 M☼ galaxies can be easily accounted for out to 150 kpc. With the current data, we cannot rule out a constant mass of metals within this fixed physical radius. This census provides a crucial boundary condition for the eventual fate of metals in galaxy evolution models. [ABSTRACT FROM AUTHOR]

We study the high-ion content and kinematics of the circumgalactic medium around low-redshift galaxies using a sample of 23 Lyman limit systems (LLSs) at 0.08 < z < 0.93 observed with the Cosmic Origins Spectrograph on board the Hubble Space Telescope. In Lehner et al., we recently showed that low-z LLSs have a bimodal metallicity distribution. Here we extend that analysis to search for differences between the high-ion and kinematic properties of the metal-poor and metal-rich branches. We find that metal-rich LLSs tend to show higher O VI columns and broader O VI profiles than metal-poor LLSs. The total H I line width (Δv90 statistic) in LLSs is not correlated with metallicity, indicating that the H I kinematics alone cannot be used to distinguish inflow from outflow and gas recycling. Among the 17 LLSs with O VI detections, all but two show evidence of kinematic sub-structure, in the form of O VI-H I centroid offsets, multiple components, or both. Using various scenarios for how the metallicities in the high-ion and low-ion phases of each LLS compare, we constrain the ionized hydrogen column in the O VI phase to lie in the range log N(H II) ∼ 17.6-20. The O VI phase of LLSs is a substantial baryon reservoir, with M(high-ion) ∼ 108.5-10.9 (r/150 kpc)2M☼, similar to the mass in the low-ion phase. Accounting for the O VI phase approximately doubles the contribution of low-z LLSs to the cosmic baryon budget. [ABSTRACT FROM AUTHOR]

We present the design and methods of the COS-Halos survey, a systematic investigation of the gaseous halos of 44 z = 0.15-0.35 galaxies using background QSOs observed with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. This survey has yielded 39 spectra of zem ≃ 0.5 QSOs with S/N ∼10-15 per resolution element. The QSO sightlines pass within 150 physical kpc of the galaxies, which span early and late types over stellar mass log M*/M☼ = 9.5-11.5. We find that the circumgalactic medium exhibits strong H I, averaging ≃ 1 Å in Lyα equivalent width out to 150 kpc, with 100% covering fraction for star-forming galaxies and 75% covering for passive galaxies. We find good agreement in column densities between this survey and previous studies over similar range of impact parameter. There is weak evidence for a difference between early- and late-type galaxies in the strength and distribution of H I. Kinematics indicate that the detected material is bound to the host galaxy, such that ≳ 90% of the detected column density is confined within ±200 km s–1 of the galaxies. This material generally exists well below the halo virial temperatures at T ≲ 105 K. We evaluate a number of possible origin scenarios for the detected material, and in the end favor a simple model in which the bulk of the detected H I arises in a bound, cool, low-density photoionized diffuse medium that is generic to all L* galaxies and may harbor a total gaseous mass comparable to galactic stellar masses. [ABSTRACT FROM AUTHOR]

We study the physical conditions of the circumgalactic medium (CGM) around z = 0.25 galaxies as traced by H i and metal line absorption, using cosmological hydrodynamic simulations that include galactic outflows. Using lines of sight targeted at impact parameters from 10 kpc to 1 Mpc around galaxies with halo masses from 1011–1013 M⊙, we study the physical conditions and their variation with impact parameter b and line-of-sight velocity Δv in the CGM as traced by H i, Mg ii, Si iv, C iv, O vi and Ne viii absorbers. All ions show a strong excess of absorption near galaxies compared to random lines of sight. The excess continues beyond 1 Mpc, reflecting the correlation of metal absorption with large-scale structure. Absorption is particularly enhanced within about Δv < 300 km s−1 and roughly 300 kpc of galaxies (with distances somewhat larger for the highest ion), approximately delineating the CGM; this range contains the majority of global metal absorption. Low ions like Mg ii and Si iv predominantly arise in denser gas closer to galaxies and drop more rapidly with b, while high ions O vi and Ne viii trace more diffusely distributed gas with a comparatively flat radial profile; C iv is intermediate. All ions predominantly trace T ∼ 104-4.5 K photoionized gas at all b, but when hot CGM gas is present (mostly in larger haloes), we see strong collisionally ionized O vi and Ne viii at b ≤ 100 kpc. Larger halo masses generally produce more absorption, though overall the trends are not as strong as that with impact parameter. These findings arise using our favoured outflow scalings as expected for momentum-driven winds; with no winds, the CGM gas remains mostly unenriched, while our outflow model with a constant velocity and mass loading factor produce hotter, more widely dispersed metals. [ABSTRACT FROM AUTHOR]

ABSTRACT Subhalo abundance matching (also known as SHAM) is a technique for populating simulated dark matter distributions with galaxies, assuming a monotonic relation between a galaxy's stellar mass or luminosity and the mass of its parent dark matter halo or subhalo. We examine the accuracy of SHAM in two cosmological smoothed particle hydrodynamics (SPH) simulations, one of which includes momentum-driven galactic winds. The SPH simulations indeed show a nearly monotonic relation between stellar mass and halo mass provided that, for satellite galaxies, we use the mass of the subhalo at the epoch zsat when it became a satellite. In each simulation, the median relation for central and satellite galaxies is nearly identical, though a somewhat larger fraction of satellites is outliers because of stellar mass loss. SHAM-assigned masses (at z= 0-2), luminosities ( R-band at z= 0) or star formation rates (at z= 2) have a 68 per cent scatter of 0.09-0.15 dex relative to the true simulation values. When we apply SHAM to the subhalo population of a collisionless N-body simulation with the same initial conditions as the SPH runs, we find generally good agreement for the halo occupation distributions and halo radial profiles of galaxy samples defined by thresholds in stellar mass. However, because a small fraction of SPH galaxies suffer severe stellar mass loss after becoming satellites, SHAM slightly overpopulates high-mass haloes; this effect is more significant for the wind simulation, which produces galaxies that are less massive and more fragile. SHAM recovers the two-point correlation function of the SPH galaxies in the no-wind simulation to better than 10 per cent at scales 0.1 < r < 10 h−1 Mpc. For the wind simulation, agreement is better than 15 per cent at r > 2 h−1 Mpc, but overpopulation of massive haloes increases the correlation function by a factor of ∼2.5 on small scales. The discrepancy in the wind simulation is greatly reduced if we raise the stellar mass threshold from 6 × 109 to 3 × 1010 M⊙; in this case SHAM overpredicts the SPH galaxy correlation function by ∼20 per cent at r < 1 h−1 Mpc but agrees well with SPH clustering at larger scales. [ABSTRACT FROM AUTHOR]

ABSTRACT We conduct Bayesian model inferences from the observed K-band luminosity function of galaxies in the local Universe, using the semi-analytic model (SAM) of galaxy formation introduced in Lu et al. The prior distributions for the 14 free parameters include a large range of possible models. We find that some of the free parameters, e.g. the characteristic scales for quenching star formation in both high-mass and low-mass haloes, are already tightly constrained by the single data set. The posterior distribution includes the model parameters adopted in other SAMs. By marginalizing over the posterior distribution, we make predictions that include the full inferential uncertainties for the colour-magnitude relation, the Tully-Fisher relation, the conditional stellar mass function of galaxies in haloes of different masses, the H i mass function, the redshift evolution of the stellar mass function of galaxies and the global star formation history. Using posterior predictive checking with the available observational results, we find that the model family (i) predicts a Tully-Fisher relation that is curved; (ii) significantly overpredicts the satellite fraction; (iii) vastly overpredicts the H i mass function; (iv) predicts high- z stellar mass functions that have too many low-mass galaxies and too few high-mass ones and (v) predicts a redshift evolution of the stellar mass density and the star formation history that are in moderate disagreement. These results suggest that some important processes are still missing in the current model family, and we discuss a number of possible solutions to solve the discrepancies, such as interactions between galaxies and dark matter haloes, tidal stripping, the bimodal accretion of gas, preheating and a redshift-dependent initial mass function. [ABSTRACT FROM AUTHOR]

ABSTRACT We investigate the metallicity evolution and metal content of the intergalactic medium (IGM) and galactic halo gas from z= 2 to 0 using 110-million-particle cosmological hydrodynamic simulations. We focus on the detectability and physical properties of ultraviolet resonance metal-line absorbers observable with Hubble's Cosmic Origins Spectrograph (COS). We confirm that galactic superwind outflows are required to enrich the IGM to observed levels down to z= 0 using three wind prescriptions contrasted to a no-wind simulation. Our favoured momentum-conserved wind prescription deposits metals closer to galaxies owing to its moderate energy input, while the more energetic constant wind model enriches the warm-hot IGM 6.4 times more. Despite these significant differences, all wind models produce metal-line statistics within a factor of 2 of existing observations. This is because , , and absorbers primarily arise from T < 105 K, photoionized gas that is enriched to similar levels in the three feedback schemes. absorbers trace the diffuse phase with , which is enriched to ∼1/50 Z⊙ at z= 0, although the absorbers themselves usually exceed 0.3 Z⊙ and arise from inhomogeneously distributed, unmixed winds. Turbulent broadening is required to match the observed equivalent width and column density statistics for . and absorbers trace primarily T∼ 104 K gas inside haloes (), although there appear to be too many absorbers relative to observations. We predict the COS will observe a population of photoionized absorbers tracing T < 105 K, gas with equivalent widths of 10-20 mÅ. and are rarely detected in COS signal-to-noise ratio 30 simulated sight-lines (d n/d z≪ 1), although simulated detections trace halo gas at T= 106-107 K. In general, the IGM is enriched in an outside-in manner, where wind-blown metals released at higher redshift reach lower overdensities, resulting in higher ionization species tracing lower density, older metals. At z= 0, 90 per cent of baryons outside galaxies are enriched to , but 65 per cent of unbound baryons in the IGM have and contain only 4 per cent of all metals, a large decline from 20 per cent at z= 2, because metals from early winds often re-accrete on to galaxies while later winds are less likely to escape their haloes. We emphasize that our results are sensitive to how metal mixing is treated in the simulations, and argue that the lack of mixing in our scheme may be the largest difference from other similar publications. [ABSTRACT FROM AUTHOR]

ABSTRACT We believe that a wide range of physical processes conspire to shape the observed galaxy population, but we remain unsure of their detailed interactions. The semi-analytic model (SAM) of galaxy formation uses multidimensional parametrizations of the physical processes of galaxy formation and provides a tool to constrain these underlying physical interactions. Because of the high dimensionality, the parametric problem of galaxy formation may be profitably tackled with a Bayesian-inference-based approach, which allows one to constrain theory with data in a statistically rigorous way. In this paper, we develop a SAM in the framework of Bayesian inference. We show that, with a parallel implementation of an advanced Markov chain Monte Carlo algorithm, it is now possible to rigorously sample the posterior distribution of the high-dimensional parameter space of typical SAMs. As an example, we characterize galaxy formation in the current Λ cold dark matter cosmology using the stellar mass function of galaxies as an observational constraint. We find that the posterior probability distribution is both topologically complex and degenerate in some important model parameters, suggesting that thorough explorations of the parameter space are needed to understand the models. We also demonstrate that because of the model degeneracy, adopting a narrow prior strongly restricts the model. Therefore, the inferences based on SAMs are conditional to the model adopted. Using synthetic data to mimic systematic errors in the stellar mass function, we demonstrate that an accurate observational error model is essential to meaningful inference. [ABSTRACT FROM AUTHOR]

ABSTRACT We study the behaviour of multiple radiative-cooling algorithms implemented in seven semi-analytic models (SAMs) of galaxy formation, including a new model we propose in this paper. We use versions of the models without feedback and apply them to dark matter haloes growing in a cosmological context, which have final virial masses that range from 1011 to 1014 M⊙. First, using simplified smoothly growing halo models, we demonstrate that the different algorithms predict cooling rates and final cold gas masses that differ by a factor of ∼5 for massive haloes (≥1012 M⊙). The algorithms are in better agreement for less massive haloes because they cool efficiently and, therefore, their cooling rates are largely limited by the halo accretion rate. However, for less massive haloes, all the SAMs predict less cooling than corresponding 1D hydrodynamic models. Secondly, we study the gas-accretion history of the central galaxies of dark matter haloes using merger trees. The inclusion of mergers alters the cooling history of haloes by locking up gas in galaxies within small haloes at early times. For realistic halo models, the dispersion in the cold gas mass predicted by the algorithms is 0.5 dex for high-mass haloes and 0.1 dex for low-mass haloes, while the dispersion in the accretion rate is about two times larger. Comparing to cosmological smoothed particle hydrodynamic simulations, we find that most SAMs systematically underpredict the gas-accretion rates for low-mass haloes but overpredict the gas-accretion rates for massive haloes. Although the models all include both 'rapid'- and 'slow'-mode accretion, the transition between the two accretion modes varies between models and also differs from the simulations. Finally, we construct a new phenomenological model that explicitly incorporates cold halo gas and a gradual transition between the cold and hot modes of gas accretion to illustrate that such a class of models can better match the results from cosmological hydrodynamic simulations. The large dispersion in cooling rates between different SAMs influences parameter choices for other galaxy physics, including star formation and feedback. Therefore, careful parametrizations of the multimode gas cooling and accretion mechanisms in simulations are necessary to ensure that the predictions from SAMs are reliable. [ABSTRACT FROM AUTHOR]

We introduce a new galaxy image decomposition tool, (GALaxy PHotometric ATtributes), which is a front-end application of the Bayesian Inference Engine (), a parallel Markov chain Monte Carlo package, to provide full posterior probability distributions and reliable confidence intervals for all model parameters. The relies on to compute the likelihood function. generates scale-free cumulative image tables for the desired model family with precise error control. Interpolation of this table yields accurate pixellated images with any centre, scale and inclination angle. then rotates the image by position angle using a Fourier shift theorem, yielding high-speed, accurate likelihood computation. We benchmark this approach using an ensemble of simulated Sérsic model galaxies over a wide range of observational conditions: the signal-to-noise ratio S/N, the ratio of galaxy size to the point spread function (PSF) and the image size, and errors in the assumed PSF; and a range of structural parameters: the half-light radius and the Sérsic index . We characterize the strength of parameter covariance in the Sérsic model, which increases with S/N and , and the results strongly motivate the need for the full posterior probability distribution in galaxy morphology analyses and later inferences. The test results for simulated galaxies successfully demonstrate that, with a careful choice of Markov chain Monte Carlo algorithms and fast model image generation, is a powerful analysis tool for reliably inferring morphological parameters from a large ensemble of galaxies over a wide range of different observational conditions. [ABSTRACT FROM AUTHOR]

Recently Ménard et al. (hereafter MSFR) detected a subtle but systematic change in the mean colour of quasars as a function of their projected separation from foreground galaxies, extending to comoving separations of , which they interpret as a signature of reddening by intergalactic dust. We present theoretical models of this remarkable observation, using smoothed particle hydrodynamic cosmological simulations of a volume. Our primary model uses a simulation with galactic winds and assumes that dust traces the intergalactic metals. The predicted galaxy-dust correlation function is similar in form to the galaxy-mass correlation function, and reproducing the MSFR data requires a dust-to-metal mass ratio of 0.24, about half the value in the Galactic interstellar medium (ISM). Roughly half of the reddening arises in dust that is more than from the nearest massive galaxy. We also examine a simulation with no galactic winds, which predicts a much smaller fraction of intergalactic metals (3 per cent versus 35 per cent) and therefore requires an unphysical dust-to-metal ratio of 2.18 to reproduce the MSFR data. In both models, the signal is dominated by sightlines with . The no-wind simulation can be reconciled with the data if we also allow reddening to arise in galaxies up to several . The wind model predicts a mean visual extinction of mag out to , with a sightline-to-sightline dispersion similar to the mean, which could be significant for future supernova cosmology studies. Reproducing the MSFR results in these simulations requires that a large fraction of ISM dust survive its expulsion from galaxies and its residence in the intergalactic medium. Future observational studies that provide higher precision and measure the dependence on galaxy type and environment will allow detailed tests for models of enriched galactic outflows and the survival of intergalactic dust. [ABSTRACT FROM AUTHOR]

The intergalactic medium (IGM) is the dominant reservoir of baryons at all cosmic epochs. In this paper, we investigate the evolution of the IGM from in , 110-million particle cosmological hydrodynamic simulations using three prescriptions for galactic outflows. We focus on the evolution of IGM physical properties, and how such properties are traced by Lyα absorption as detectable using Hubble's Cosmic Origins Spectrograph (COS). Our results broadly confirm the canonical picture that most Lyα absorbers arise from highly ionized gas tracing filamentary large-scale structure. Growth of structure causes gas to move from the diffuse photoionized IGM into other cosmic phases, namely stars, cold and hot gas within galaxy haloes, and the unbound and shock-heated warm-hot intergalactic medium (WHIM). By today, baryons are comparably divided between bound phases (35 per cent in our favoured outflow model), the diffuse IGM (41 per cent) and the WHIM (24 per cent). Here we (re)define the WHIM as gas with overdensities lower than that in haloes ( today) and temperatures , to more closely align it with the 'missing baryons' that are not easily detectable in emission or Lyα absorption. Strong galactic outflows can have a noticeable impact on the temperature of the IGM, though with our favoured momentum-driven wind scalings they do not. When we (mildly) tune our assumed photoionizing background to match the observed evolution of the Lyα mean flux decrement, we obtain line count evolution statistics that broadly agree with available (pre-COS) observations. We predict a column density distribution slope of for our favoured wind model, in agreement with recent observational estimates, and it becomes shallower with redshift. Winds have a mostly minimal impact, but they do result in a shallower column density slope and more strong lines. With improved statistics, the frequency of strong lines can be a valuable diagnostic of outflows, and the momentum-driven wind model matches existing data significantly better than the two alternatives we consider. The relationship between column density and physical density broadens mildly from , and evolves as for diffuse absorbers, consistent with previous studies. Linewidth distributions are quite sensitive to spectral resolution; COS should yield significantly broader lines than higher resolution data. Thermal contributions to linewidths are typically subdominant, so linewidths only loosely reflect the temperature of the absorbing gas. This will hamper attempts to quantify the WHIM using broad Lyα absorbers, though it may still be possible to do so statistically. Together, COS data and simulations such as these will provide key insights into the physical conditions of the dominant reservoir of baryons over the majority of cosmic time. [ABSTRACT FROM AUTHOR]

We analyse cosmological hydrodynamic simulations that include theoretically and observationally motivated prescriptions for galactic outflows. If these simulated winds accurately represent winds in the real Universe, then material previously ejected in winds provides the dominant source of gas infall for new star formation at redshifts . This recycled wind accretion, or wind mode, provides a third physically distinct accretion channel in addition to the ‘hot’ and ‘cold’ modes emphasized in recent theoretical studies. The recycling time of wind material ( trec) is shorter in higher mass systems owing to the interaction between outflows and the increasingly higher gas densities in and around higher mass haloes. This differential recycling plays a central role in shaping the present-day galaxy stellar mass function (GSMF), because declining leads to increasing wind mode galaxy growth in more massive haloes. For the three feedback models explored, the wind mode dominates above a threshold mass that primarily depends on wind velocity; the shape of the GSMF therefore can be directly traced back to the feedback prescription used. If we remove all particles that were ever ejected in a wind, then the predicted GSMFs are much steeper than observed. In this case, galaxy masses are suppressed both by the ejection of gas from galaxies and by the hydrodynamic heating of their surroundings, which reduces subsequent infall. With wind recycling included, the simulation that incorporates our favoured momentum-driven wind scalings reproduces the observed GSMF for stellar masses . At higher masses, wind recycling leads to excessive galaxy masses and star formation rates relative to observations. In these massive systems, some quenching mechanism must suppress not only the direct accretion from the primordial intergalactic medium but the re-accretion of gas ejected from star-forming galaxies. In short, as has long been anticipated, the form of the GSMF is governed by outflows; the unexpected twist here for our simulated winds is that it is not primarily the ejection of material but how the ejected material is re-accreted that governs the GSMF. [ABSTRACT FROM AUTHOR]

In the low-density intergalactic medium that gives rise to the Lyman α (Lyα) forest, gas temperature and density are tightly correlated. The velocity scale of thermal broadening and the Hubble flow across the gas Jeans scale are of similar magnitude . To separate the effects of gas pressure support and thermal broadening on the Lyα forest, we compare spectra extracted from two smoothed particle hydrodynamics simulations evolved with different photoionization heating rates (and thus different Jeans scales) and from the pressureless dark matter distribution, imposing different temperature–density relations on the evolved particle distributions. The dark matter spectra are similar but not identical to those created from the full gas distributions, showing that thermal broadening sets the longitudinal (line-of-sight) scale of the Lyα forest. The turnover scales in the flux power spectrum and flux autocorrelation function are determined mainly by thermal broadening rather than pressure. However, the insensitivity to pressure arises partly from a cancellation effect with a sloped temperature–density relation ( in our simulations): the high-density peaks in the colder, lower pressure simulation are less smoothed by pressure support than in the hotter simulation, and it is this higher density gas that experiences the strongest thermal broadening. Changes in thermal broadening and pressure support have comparably important effects on the flux probability distribution, which responds directly to the gas overdensity distribution rather than the scale on which it is smooth. Tests on a lower resolution simulation ( versus particles in a comoving box) show that our statistical results are converged even at this lower resolution. While thermal broadening generally dominates the longitudinal structure in the Lyα forest, we show in Peeples et al. that pressure support determines the transverse coherence of the forest observed towards close quasar pairs. [ABSTRACT FROM AUTHOR]

We examine the impact of gas pressure on the transverse coherence of high-redshift Lyman α (Lyα) forest absorption along neighbouring lines of sight that probe the gas Jeans scale (projected separation comoving, angular separation ). We compare predictions from two smoothed particle hydrodynamics simulations that have different photoionization heating rates and thus different temperature–density relations in the intergalactic medium (IGM). We also compare spectra computed from the gas distributions to those computed from the pressureless dark matter. The coherence along neighbouring sightlines is markedly higher for the hotter, higher pressure simulation and lower for the dark matter spectra. We quantify this coherence using the flux cross-correlation function and the conditional distribution of flux decrements as a function of transverse and line-of-sight (velocity) separation. Sightlines separated by are ideal for probing this transverse coherence. Higher pressure decreases the redshift-space anisotropy of the flux correlation function, while higher thermal broadening increases the anisotropy. In contrast to the longitudinal (line-of-sight) structure of the Lyα forest, the transverse structure on these scales is dominated by pressure effects rather than thermal broadening. With the rapid recent growth in the number of known close quasar pairs, paired line-of-sight observations offer a promising new route to probe the IGM temperature–density relation and test the unexpectedly high temperatures that have been inferred from single sightline analyses. [ABSTRACT FROM AUTHOR]

We study the nature of rapidly star-forming galaxies at in cosmological hydrodynamic simulations, and compare their properties to observations of submillimetre galaxies (SMGs). We identify simulated SMGs as the most rapidly star-forming systems that match the observed number density of SMGs. In our models, SMGs are massive galaxies sitting at the centres of large potential wells, being fed by smooth infall and gas-rich satellites at rates comparable to their star formation rates (SFRs). They are not typically undergoing major mergers that significantly boost their quiescent SFR, but they still often show complex gas morphologies and kinematics. Our simulated SMGs have stellar masses of , SFRs of , a clustering length of and solar metallicities. The SFRs are lower than those inferred from far-infrared (far-IR) data by , which we suggest may owe to one or more systematic effects in the SFR calibrations. SMGs at live in haloes, and by they mostly end up as brightest group galaxies in haloes. We predict that higher M* SMGs should have on average lower specific SFRs, less disturbed morphologies and higher clustering. We also predict that deeper far-IR surveys will smoothly join SMGs on to the massive end of the SFR– M* relationship defined by lower mass galaxies. Overall, our simulated rapid star-formers provide as good a match to available SMG data as merger-based scenarios, offering an alternative scenario that emerges naturally from cosmological simulations. [ABSTRACT FROM AUTHOR]

We investigate the physical mechanisms of tidal heating and satellite disruption in cold dark matter host haloes using N-body simulations based on cosmological initial conditions. We show the importance of resonant shocks and resonant torques with the host halo to satellite heating. A resonant shock (torque) couples the radial (tangential) motion of a satellite in its orbit to its phase space. For a satellite on a circular orbit, an inner Lindblad resonance (ILR)-like resonance dominates the heating and this heating results in continuous satellite mass loss. We estimate the requirements for simulations to achieve these dynamics using perturbation theory. Both resonant shocks and resonant torques affect satellites on eccentric orbits. We demonstrate that satellite mass loss is an outside-in process in energy space; a satellite's stars and gas are thus protected by their own halo against tidal stripping. We simulate the evolution of a halo similar to the Large Magellanic Cloud (LMC) in our Galactic dark matter halo and conclude that the LMC stars have not yet been stripped. Finally, we present a simple algorithm for estimating the evolution of satellite mass that includes both shock heating and resonant torques. [ABSTRACT FROM AUTHOR]

We examine the accretion and merger histories of central and satellite galaxies in a smoothed particle hydrodynamics (SPH) cosmological simulation that resolves galaxies down to . Most friends-of-friends haloes in the simulation have a distinct central galaxy, typically 2–5 times more massive than the most massive satellite. As expected, satellites have systematically higher assembly redshifts than central galaxies of the same baryonic mass, and satellites in more massive haloes form earlier. However, contrary to the simplest expectations, satellite galaxies continue to accrete gas and convert it to stars; the gas accretion declines steadily over a period of 0.5–1 Gyr after the satellite halo merges with a larger parent halo. Satellites in a cluster mass halo eventually begin to lose baryonic mass. Typically, satellites in our simulation are 0.1–0.2 mag bluer than in models that assume no gas accretion on to satellites after a halo merger. Since , 27 per cent of central galaxies (above ) and 22 per cent of present-day satellite galaxies have merged with a smaller system above a 1:4 mass ratio; about half of the satellite mergers occurred after the galaxy became a satellite and half before. In effect, satellite galaxies can remain ‘central’ objects of halo substructures, with continuing accretion and mergers, making the transition in assembly histories and physical properties a gradual one. Implementing such a gradual transformation in semi-analytic models would improve their agreement with observed colour distributions of satellite galaxies in groups and with the observed colour dependence of galaxy clustering. [ABSTRACT FROM AUTHOR]

Using a statistically representative sample of 911 central galaxies (CENs) from the Sloan Digital Sky Survey (SDSS) Data Release 4 group catalogue, we study how the structure (shape and size) of the first rank (by stellar mass) group and cluster members depends on (1) galaxy stellar mass , (2) the global environment defined by the dark matter halo mass of the host group and (3) the local environment defined by their special halocentric position. We quantify the structure of SDSS galaxies with agalfit-based pipeline that fits two-dimensional Sérsic models to the r-band image data. Through tests with simulated and real galaxy images, we demonstrate that our pipeline can recover Sérsic parameters without significant bias. We find that the fitting results are most sensitive to the background sky level determination, and we strongly recommend using the SDSS global value. We also find that uncertainties in the background sky level translate into a strong covariance between the total magnitude, the half-light radius ( r50) and the Sérsic index ( n), especially for bright/massive galaxies. Applying our pipeline to the CEN sample, we find that n of CENs depends strongly on , but only weakly or not at all on . The relation holds for CENs over the full range of halo masses that we consider. Less massive CENs tend to be disc like and high-mass systems are typically spheroids, with a considerable scatter in n at all galaxy masses. Similarly, CEN sizes depend on galaxy stellar mass and luminosity, with early- and late-type galaxies exhibiting different slopes for the size–luminosity and the size–stellar mass scaling relations. Moreover, to test the impact of local environment on CENs, we compare the structure of CENs with that satellite galaxies (SATs) of comparable . We find that low-mass SATs have somewhat larger median Sérsic indices than CENs of a similar . Furthermore, low-mass, late-type SATs are moderately smaller in size than late-type CENs of the same stellar mass. However, we find no size differences between early-type CENs and SATs and no structural differences between CENs and SATs when they are matched in both optical colour and stellar mass. The similarity in the structure of massive SATs and CENs demonstrates that the halocentric distinction has no significant impact on the structure of spheroids. We conclude that is the most fundamental property determining the basic structural shape and size of a galaxy. In contrast, the lack of a significant relation rules out a clear distinct group mass for producing spheroids. The existence of spheroid CENs in low- and high-mass groups suggests that the morphological transformation processes that produce spheroids must occur at the centres of groups spanning a wide range of masses. [ABSTRACT FROM AUTHOR]

We compare the properties of galaxies that form in a cosmological simulation without strong feedback to observations of the galaxy population. We confirm previous findings that models without strong feedback overproduce the observed galaxy baryonic mass function, especially at the low- and high-mass extremes. Through post-processing we investigate what kinds of feedback would be required to reproduce the statistics of observed galaxy masses and star formation rates. To mimic an extreme form of ‘preventive’ feedback, such as a highly efficient active galactic nucleus ‘radio mode’, we remove all baryonic mass that was originally accreted from shock-heated gas (‘hot-mode’ accretion). This removal does not bring the high-mass end of the galaxy mass function into agreement with observations because much of the stellar mass in these systems formed at high redshift from baryons that originally accreted via ‘cold mode’ on to lower mass progenitors. An efficient ‘ejective’ feedback mechanism, such as supernova-driven galactic winds, must reduce the masses of these progenitors before they merge to form today's massive galaxies. Feedback must also reduce the masses of lower mass galaxies, which assemble at lower redshifts and have much lower star formation rates. If we monotonically remap galaxy masses to reproduce the observed mass function, but retain the simulation-predicted star formation rates, we obtain fairly good agreement with the observed sequence of star-forming galaxies. However, we fail to recover the observed population of passive, low star formation rate galaxies, especially at the high-mass end. Suppressing all hot-mode accretion improves the agreement for high-mass galaxies, but it worsens the agreement at intermediate masses. Reproducing these observations requires a feedback mechanism that dramatically suppresses star formation in a fraction of galaxies, increasing with mass, while leaving star formation rates of other galaxies essentially unchanged. [ABSTRACT FROM AUTHOR]

We study the formation of galaxies in a large volume ( particles) cosmological simulation, evolved using the entropy and energy-conserving smoothed particle hydrodynamics (SPH) codegadget-2. Most of the baryonic mass in galaxies of all masses is originally acquired through filamentary ‘cold mode’ accretion of gas that was never shock heated to its halo virial temperature, confirming the key feature of our earlier results obtained with a different SPH code. Atmospheres of hot, virialized gas develop in haloes above , a transition mass that is nearly constant from to 0. Cold accretion persists in haloes above the transition mass, especially at . It dominates the growth of galaxies in low-mass haloes at all times, and it is the main driver of the cosmic star formation history. Our results suggest that the cooling of shock-heated virialized gas, which has been the focus of many analytic models of galaxy growth spanning more than three decades, might be a relatively minor element of galaxy formation. At high redshifts, satellite galaxies have gas accretion rates similar to central galaxies of the same baryonic mass, but at the accretion rates of low-mass satellites are well below those of comparable central galaxies. Relative to our earlier simulations, thegadget-2 simulations predict much lower rates of ‘hot mode’ accretion from the virialized gas component. Hot accretion rates compete with cold accretion rates near the transition mass, but only at . Hot accretion is inefficient in haloes above , with typical rates lower than at , even though our simulation does not include active galactic nuclei (AGN) heating or other forms of ‘preventive’ feedback. Instead, the accretion rates are low because the inner density profiles of hot gas in these haloes are shallow, with long associated cooling times. The cooling recipes typically used in semi-analytic models can overestimate the accretion rates in these haloes by orders of magnitude, so these models may overemphasize the role of preventive feedback in producing observed galaxy masses and colours. A fraction of the massive haloes develop cuspy profiles and significant cooling rates between and 0, a redshift trend similar to the observed trend in the frequency of cooling flow clusters. [ABSTRACT FROM AUTHOR]

We investigate the incidence of major mergers creating massive galaxies in present-day groups and clusters. Using a volume-limited sample of 845 groups with dark matter halo masses above , we isolate 221 galaxy pairs with -band magnitude differences, ≤30 kpc projected separations and combined masses above . We fit the r-band images of each pair as the line-of-sight projection of symmetric models and identify 38 mergers by the presence of residual asymmetric structure associated with both progenitors, such as non-concentric isophotes, broad and diffuse tidal tails and dynamical friction wakes. In other words, at the resolution and sensitivity of the Sloan Digital Sky Survey (SDSS), 16 per cent of massive major pairs in dense environments have mutual tidal interaction signatures; relying on automated searches of major pairs from the SDSS spectroscopic galaxy sample will result in missing 70 per cent of these mergers owing to spectroscopic incompleteness in high-density regions. We find that 90 per cent of these mergers are between two nearly equal-mass progenitors with red-sequence colours and centrally concentrated morphologies, in agreement with numerical simulations that predict that an important mechanism for the formation of massive elliptical galaxies is the dissipationless (gas-poor or so-called dry) major merging of spheroid-dominated galaxies. We identify seven additional massive mergers with disturbed morphologies and semiresolved double nuclei; thus, per cent of galaxies in large groups are involved in the major merger assembly of massive galaxies. Mergers at the centres of these groups are more common than between two satellites, but both types are morphologically indistinguishable and we tentatively conclude that the latter are likely located at the dynamical centres of large subhaloes that have recently been accreted by their host halo. Based on reasonable assumptions, the centres of group and cluster-sized haloes are gaining stellar mass at a rate of 2–9 per cent per Gyr on average. Our results indicate that the massive end of the galaxy population continues to evolve hierarchically at a measurable level, and that massive mergers are more likely to occur in large galaxy groups than in massive clusters. [ABSTRACT FROM AUTHOR]

We investigate the dynamical mechanisms responsible for producing tidal tails from dwarf satellites using N-body simulations. We describe the essential dynamical mechanisms and morphological consequences of tail production in satellites with masses greater than 0.0001 of the host halo virial mass. We identify two important dynamical coconspirators: (1) the points where the attractive force of the host halo and satellite are balanced (X-points) do not occur at equal distances from the satellite centre or at the same equipotential value for massive satellites, breaking the morphological symmetry of the leading and trailing tails and (2) the escaped ejecta in the leading (trailing) tail continues to be decelerated (accelerated) by the satellite's gravity leading to large offsets of the ejecta orbits from the satellite orbit. The effect of the satellite's self-gravity decreases only weakly with a decreasing ratio of satellite mass to host halo mass, proportional to , demonstrating the importance of these effects over a wide range of subhalo masses. Not only will the morphology of the leading and trailing tails for massive satellites be different, but the observed radial velocities of the tails will be displaced from that of the satellite orbit; both the displacement and the maximum radial velocity is proportional to satellite mass. If the tails are assumed to follow the progenitor satellite orbits, the tails from satellites with masses greater than 0.0001 of the host halo virial mass in a spherical halo will appear to indicate a flattened halo. Therefore, a constraint on the Milky Way halo shape using tidal streams requires mass-dependent modelling. Similarly, we compute the distribution of tail orbits both in space and in space, advocated for identifying satellite stream relics. The acceleration of ejecta by a massive satellite during escape spreads the velocity distribution and obscures the signature of a well-defined ‘moving group’ in phase space. Although these findings complicate the interpretation of stellar streams and moving groups, the intrinsic mass dependence provides additional leverage on both halo and progenitor satellite properties. [ABSTRACT FROM AUTHOR]

The total extragalactic background radiation can be an important test of the global star formation history (SFH). Using direct observational estimates of the SFH, along with standard assumptions about the initial mass function (IMF), we calculate the total extragalactic background radiation and the observed stellar density today. We show that plausible SFHs allow a significant range in each quantity, but that their ratio is very tightly constrained. Current estimates of the stellar mass and extragalactic background are difficult to reconcile, as long as the IMF is fixed to the Salpeter slope above . The joint confidence interval of these two quantities only agrees with that determined from the allowed range of SFH fits at the 3σ level, and for our best-fitting values the discrepancy is about a factor of 2. Alternative energy sources that contribute to the background, such as active galactic nuclei (AGN), Population III stars or decaying particles, appear unlikely to resolve the discrepancy. However, changes to the IMF allow plausible solutions to the background problem. The simplest is an average IMF with an increased contribution from stars around . A ‘paunchy’ IMF of this sort could emerge as a global average if low-mass star formation is suppressed in galaxies experiencing rapid starbursts. Such an IMF is consistent with observations of star-forming regions, and would help to reconcile the fossil record of star formation with the directly observed SFH. [ABSTRACT FROM AUTHOR]

We compare the galaxy population of a smoothed particle hydrodynamics (SPH) simulation to those predicted by the GalICS (Galaxies In Cosmological Simulations) N-body + semi-analytic model and a stripped down version of GalICS that omits the effects of supernova and active galactic nucleus (AGN) feedback. The SPH simulation and the no-feedback GalICS model make similar predictions for the baryonic mass functions of galaxies and for the dependence of these mass functions on environment and redshift. The two methods also make similar predictions for the galaxy content of dark matter haloes as a function of halo mass and for the gas accretion history of galaxies. There is a fairly good correspondence between the ‘cold’ and ‘hot’ accretion modes of the SPH simulation and the rapid and slow cooling regimes of the GalICS calculation. Both the SPH and no-feedback GalICS models predict a bimodal galaxy population at . The ‘red’ sequence of gas poor, old galaxies is populated mainly by satellite systems, which are starved of fresh gas after they begin orbiting in larger haloes, while, contrary to observations, the central galaxies of massive haloes lie on the ‘blue’ star-forming sequence as a result of continuing hot gas accretion at late times. Furthermore, both models overpredict the observed baryonic mass function, especially at the high-mass end. In the full GalICS model, supernova-driven outflows reduce the masses of low and intermediate mass galaxies by about a factor of 2. AGN feedback suppresses gas cooling in large haloes, producing a sharp cut-off in the baryonic mass function and moving the central galaxies of these massive haloes to the red sequence. Our results imply that the observational failings of the SPH simulation and the no-feedback GalICS model are a consequence of missing input physics rather than computational inaccuracies. Truncating the accretion of gas in satellite galaxies automatically produces a bimodal distribution with a quenched population, but explaining the star formation shutdown in the most massive galaxies requires a mechanism like AGN feedback that suppresses the accretion of gas on to central galaxies in large haloes. [ABSTRACT FROM AUTHOR]

A galaxy remains near equilibrium for most of its history. Only through resonances can non-axisymmetric features, such as spiral arms and bars, exert torques over large scales and change the overall structure of the galaxy. In this paper, we describe the resonant interaction mechanism in detail, derive explicit criteria for the particle number required to simulate these dynamical processes accurately using N-body simulations, and illustrate them with numerical experiments. To do this, we perform a direct numerical solution of perturbation theory, in short, by solving for each orbit in an ensemble and make detailed comparisons with N-body simulations. The criteria include: sufficient particle coverage in phase space near the resonance and enough particles to minimize gravitational potential fluctuations that will change the dynamics of the resonant encounter. These criteria are general in concept and can be applied to any dynamical interaction. We use the bar–halo interaction as our primary example owing to its technical simplicity and astronomical ubiquity. Some of our more surprising findings are as follows. First, the inner Lindblad like resonance, responsible for coupling the bar to the central halo cusp, requires more than equal-mass particles within the virial radius or inside the bar radius for a Milky Way like bar in a Navarro, Frenk & White profile. Secondly, orbits that linger near the resonance receive more angular momentum than orbits that move through the resonance quickly. Small-scale fluctuations present in state-of-the-art particle–particle simulations can knock orbits out of resonance, preventing them from lingering and, thereby, decrease the torque per orbit. This can be offset by the larger number of orbits affected by the resonance due to the diffusion. However, noise from orbiting substructure remains at least an order of magnitude too small to be of consequence. Applied to N-body simulations, the required particle numbers are sufficiently high for scenarios of interest that apparent convergence in particle number is misleading: the convergence with N may still be in the noise-dominated regime. State-of-the-art simulations are not adequate to follow all aspects of secular evolution driven by the bar–halo interaction. It is not possible to derive particle number requirements that apply to all situations, for example, more subtle interactions may be even more difficult to simulate. Therefore, we present a procedure to test the requirements for individual N-body codes to the actual problem of interest. [ABSTRACT FROM AUTHOR]