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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]

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 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 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]

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]

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 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]

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 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 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 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]

We analyse the flux power spectrum and its covariance using simulated Lyα forests. We find that pseudo-hydro techniques are good approximations of hydrodynamical simulations at high redshift. However, the pseudo-hydro techniques fail at low redshift because they are insufficient for characterizing some components of the low-redshift intergalactic medium, notably the warm–hot intergalactic medium. Hence, to use the low-redshift Lyα flux power spectrum to constrain cosmology, one would need realistic hydrodynamical simulations. By comparing (one-dimensional) mass statistics with flux statistics, we show that the non-linear transform between density and flux quenches the fluctuations so that the flux power spectrum is much less sensitive to cosmological parameters than the one-dimensional mass power spectrum. The covariance of the flux power spectrum is nearly Gaussian. As such, the uncertainties of the underlying mass power spectrum could still be large, even though the flux power spectrum can be precisely determined from a small number of lines of sight. [ABSTRACT FROM AUTHOR]

We examine the temperature history of gas accreted by forming galaxies in smoothed particle hydrodynamics simulations. About half of the gas follows the track expected in the conventional picture of galaxy formation, shock heating to roughly the virial temperature of the galaxy potential well ( for a Milky Way type galaxy) before cooling, condensing and forming stars. However, the other half radiates its acquired gravitational energy at much lower temperatures, typically , and the histogram of maximum gas temperatures is clearly bimodal. The ‘cold mode’ of gas accretion dominates for low-mass galaxies (baryonic mass or halo mass ), while the conventional ‘hot mode’ dominates the growth of high-mass systems. Cold accretion is often directed along filaments, allowing galaxies to efficiently draw gas from large distances, while hot accretion is quasi-spherical. The galaxy and halo mass dependence leads to redshift and environment dependence of cold and hot accretion rates, with the cold mode dominating at high redshift and in low-density regions today, and the hot mode dominating in group and cluster environments at low redshift. The simulations reproduce an important feature of the observed relation between the galaxy star formation rate (SFR) and the environment, namely a break in star formation rates at surface densities , outside the virial radii of large groups and clusters. The cosmic SFR tracks the overall history of gas accretion, and its decline at low redshift follows the combined decline of cold and hot accretion rates. The drop in cold accretion is driven by the decreasing infall rate on to haloes, while for hot accretion this slower mass growth is further modified by the longer cooling times within haloes. If we allowed hot accretion to be suppressed by conduction or active galactic nuclei feedback, then the simulation predictions would change in interesting ways, perhaps resolving conflicts with the colours of ellipticals and the cut-off of the galaxy luminosity function. The transition at between cold-mode domination and hot-mode domination is similar to that found by Birnboim & Dekel using one-dimensional simulations and analytic arguments. The corresponding baryonic mass is tantalizingly close to the scale at which Kauffmann et al. find a marked shift in galaxy properties, and we speculate on possible connections between these theoretical and observational transitions. [ABSTRACT FROM AUTHOR]