Search

Sunyaev–Zel'dovich (SZ) measurements can dramatically improve our understanding of the intergalactic medium and the role of feedback processes in galaxy formation, allowing us to calibrate important astrophysical systematics in cosmological constraints from weak lensing galaxy clustering surveys. However, the signal is only measured in a two-dimensional projection, and its correct interpretation relies on understanding the connection between observable quantities and the underlying intrinsic properties of the gas, in addition to the relation between the gas and the underlying matter distribution. One way to address these challenges is through the use of hydrodynamical simulations such as the high-resolution, large-volume MillenniumTNG suite. We find that measurements of the optical depth, τ, and the Compton- y parameter, Y , receive large line-of-sight contributions that can be removed effectively by applying a compensated aperture photometry filter. In contrast with other τ probes (e.g. X-rays and fast radio bursts), the kinematic SZ-inferred τ receives most of its signal from a confined cylindrical region around the halo due to the velocity decorrelation along the line of sight. Additionally, we perform fits to the Y – M and τ– M scaling relations and report best-fitting parameters adopting the smoothly broken power law formalism. We note that subgrid physics modelling can broaden the error bar on these by 30 per cent for intermediate-mass haloes (⁠|${\sim }10^{13} \, {\rm M}_{\odot }$|⁠). The scatter of the scaling relations can be captured by an intrinsic dependence on concentration and an extrinsic dependence on tidal shear. Finally, we comment on the effect of using galaxies rather than haloes in observations, which can bias the inferred profiles by ∼20 per cent for L * galaxies. [ABSTRACT FROM AUTHOR]

The observability of Lyα emitting galaxies (LAEs) during the Epoch of Reionization can provide a sensitive probe of the evolving neutral hydrogen gas distribution, thus setting valuable constraints to distinguish different reionization models. In this study, we utilize the new thesan suite of large-volume (⁠|$L_\text{box} = 95.5\, \text{cMpc}$|⁠) cosmological radiation-hydrodynamic simulations to directly model the Lyα emission from individual galaxies and the subsequent transmission through the intergalactic medium. thesan combines the arepo-rt radiation-hydrodynamic solver with the IllustrisTNG galaxy formation model and includes high- and medium-resolution simulations designed to investigate the impacts of halo-mass-dependent escape fractions, alternative dark matter models, and numerical convergence. We find important differences in the Lyα transmission based on reionization history, bubble morphology, frequency offset from line centre, and galaxy brightness. For a given global neutral fraction, Lyα transmission reduces when low-mass haloes dominate reionization over high-mass haloes. Furthermore, the variation across sightlines for a single galaxy is greater than the variation across all galaxies. This collectively affects the visibility of LAEs, directly impacting observed Lyα luminosity functions (LFs). We employ Gaussian Process Regression using SWIFTEmulator to rapidly constrain an empirical model for dust escape fractions and emergent spectral-line profiles to match observed LFs. We find that dust strongly impacts the Lyα transmission and covering fractions of M UV ≲ −19 galaxies in |$M_\text{vir} \gtrsim 10^{11}\, \text{M}_{\bigodot }$| haloes, such that the dominant mode of removing Lyα photons in non-LAEs changes from low-IGM transmission to high dust absorption around z ∼ 7. [ABSTRACT FROM AUTHOR]

A fundamental requirement for reionizing the Universe is that a sufficient fraction of the ionizing photons emitted by galaxies successfully escapes into the intergalactic medium. However, due to the scarcity of high-redshift observational data, the sources driving reionization remain uncertain. In this work, we calculate the ionizing escape fractions (f esc) of reionization-era galaxies from the state-of-the-art thesan simulations, which combine an accurate radiation-hydrodynamic solver (arepo-rt) with the well-tested IllustrisTNG galaxy formation model to self-consistently simulate both small-scale galaxy physics and large-scale reionization throughout a large patch of the universe (⁠|$L_\text{box} = 95.5\, \text{cMpc}$|⁠). This allows the formation of numerous massive haloes (⁠|$M_\text{halo} \gtrsim 10^{10}\, {\text{M}_{\odot }}$|⁠), which are often statistically underrepresented in previous studies but are believed to be important to achieving rapid reionization. We find that low-mass galaxies (⁠|$M_\text{stars} \lesssim 10^7\, {\text{M}_{\odot }}$|⁠) are the main drivers of reionization above z ≳ 7, while high-mass galaxies (⁠|$M_\text{stars} \gtrsim 10^8\, {\text{M}_{\odot }}$|⁠) dominate the escaped ionizing photon budget at lower redshifts. We find a strong dependence of f esc on the effective star formation rate (SFR) surface density defined as the SFR per gas mass per escape area, i.e. |$\bar{\Sigma }_\text{SFR} = \text{SFR}/M_\text{gas}/R_{200}^2$|⁠. The variation in halo escape fractions decreases for higher mass haloes, which can be understood from the more settled galactic structure, SFR stability, and fraction of sightlines within each halo significantly contributing to the escaped flux. Dust is capable of reducing the escape fractions of massive galaxies, but the impact on the global f esc depends on the dust model. Finally, active galactic nuclei are unimportant for reionization in thesan and their escape fractions are lower than stellar ones due to being located near the centres of galaxy gravitational potential wells. [ABSTRACT FROM AUTHOR]

Dwarf galaxies are thought to quench primarily due to environmental processes most typically occurring in galaxy groups and clusters or around single, massive galaxies. However, at earlier epochs, (5 < z < 2), the collapse of large-scale structure (forming Zel'dovich sheets and subsequently filaments of the cosmic web) can produce volume-filling accretion shocks which elevate large swaths of the intergalactic medium (IGM) in these structures to a hot (T > 106 K) phase. We study the impact of such an event on the evolution of central dwarf galaxies (5.5 < log  M * < 8.5) in the field using a spatially large, high resolution cosmological zoom simulation which covers the cosmic web environment between two protoclusters. We find that the shock-heated sheet acts as an environmental quencher much like clusters and filaments at lower redshift, creating a population of quenched, central dwarf galaxies. Even massive dwarfs that do not quench are affected by the shock, with reductions to their sSFR and gas accretion. This process can potentially explain the presence of isolated quenched dwarf galaxies, and represents an avenue of pre-processing, via which quenched satellites of bound systems quench before infall. [ABSTRACT FROM AUTHOR]

The dividing line between galaxies that are quenched by reionization ("relics") and galaxies that survive reionization (i.e., continue forming stars) is commonly discussed in terms of a halo mass threshold. We probe this threshold in a physically more complete and accurate way than has been possible to date, using five extremely high resolution (M target = 4 M ⊙) cosmological zoom-in simulations of dwarf galaxies within the halo mass range (1–4) × 109 M ⊙. The employed LYRA simulation model features resolved interstellar medium physics and individual, resolved supernova explosions. Interestingly, two out of five of the simulated dwarf galaxies lie close to the threshold mass but are neither full reionization relics nor full reionization survivors. These galaxies initially quench at the time of reionization but merely remain quiescent for ∼500 Myr. At z ∼ 5 they recommence star formation in a synchronous way and remain star-forming until the present day. The parallel timing indicates consistent sound-crossing and cooling times between the halos. While the star formation histories we find are diverse, we show that they are directly related to the ability of a given halo to retain and cool gas. Whereas the latter is most strongly dependent on the mass (or virial temperature) of the host halo at the time of reionization, it also depends on its growth history, the UV background (and its decrease at late times), and the amount of metals retained within the halo. [ABSTRACT FROM AUTHOR]

Line intensity mapping (LIM) is rapidly emerging as a powerful technique to study galaxy formation and cosmology in the high-redshift Universe. We present LIM estimates of select spectral lines originating from the interstellar medium (ISM) of galaxies and 21 cm emission from neutral hydrogen gas in the Universe using the large volume, high resolution thesan reionization simulations. A combination of subresolution photoionization modelling for H  ii regions and Monte Carlo radiative transfer calculations is employed to estimate the dust-attenuated spectral energy distributions (SEDs) of high-redshift galaxies (z ≳ 5.5). We show that the derived photometric properties such as the ultraviolet (UV) luminosity function and the UV continuum slopes match observationally inferred values, demonstrating the accuracy of the SED modelling. We provide fits to the luminosity–star formation rate relation (L –SFR) for the brightest emission lines and find that important differences exist between the derived scaling relations and the widely used low- z ones because the ISM of reionization era galaxies is generally less metal enriched than in their low-redshift counterparts. We use these relations to construct line intensity maps of nebular emission lines and cross-correlate with the 21 cm emission. Interestingly, the wavenumber at which the correlation switches sign (k transition) depends heavily on the reionization model and to a lesser extent on the targeted emission line, which is consistent with the picture that k transition probes the typical sizes of ionized regions. The derived scaling relations and intensity maps represent a timely state-of-the-art framework for forecasting and interpreting results from current and upcoming LIM experiments. [ABSTRACT FROM AUTHOR]

We present the simulation of a |$2\times 10^{9}\, \mathrm{M}_{\odot }$| halo mass cosmological dwarf galaxy run to z = 0 at 4 solar mass gas resolution with resolved supernova feedback. We compare three simple subgrid implementations for the inhomogeneous chemical enrichment from Population III stars and compare them to constraints from Local Group dwarf galaxies. The employed model, LYRA, is a novel high-resolution galaxy formation model built for the moving mesh code arepo , which is marked by a resolved multiphase interstellar medium, single stars, and individual supernova events. The resulting reionization relic is characterized by a short (<1.5 Gyr) star formation history that is repeatedly brought to a standstill by violent bursts of feedback. Star formation is reignited for a short duration due to a merger at z ≈ 4 and then again at z ≈ 0.2−0 after sustained gas accretion. Our model z = 0 galaxy matches the stellar mass, size, stellar kinematics, and metallicity relations of Local Group dwarf galaxies well. The dark matter profile does not exhibit a core in any version of the model. We show that the host halo masses of Population III stars affect the assembly history of dwarf galaxies. This manifests itself through the initial gaseous collapse in the progenitor haloes, affecting the central density of the stellar component and through the accretion of luminous substructure. [ABSTRACT FROM AUTHOR]

We post-process galaxies in the IllustrisTNG simulations with skirt radiative transfer calculations to make predictions for the rest-frame near-infrared (NIR) and far-infrared (FIR) properties of galaxies at z ≥ 4. The rest-frame K - and z -band galaxy luminosity functions from TNG are overall consistent with observations, despite |${\sim}0.5\, \mathrm{dex}$| underprediction at z = 4 for M K ≲ −25 and M z ≲ −24. Predictions for the JWST MIRI observed galaxy luminosity functions and number counts are given. Based on theoretical estimations, we show that the next-generation survey conducted by JWST can detect 500 (30) galaxies in F1000W in a survey area of |$500\, {\rm arcmin}^{2}$| at z = 6 (z = 8). As opposed to the consistency in the UV, optical, and NIR, we find that TNG, combined with our dust modelling choices, significantly underpredicts the abundance of most dust-obscured and thus most luminous FIR galaxies. As a result, the obscured cosmic star formation rate density (SFRD) and the SFRD contributed by optical/NIR dark objects are underpredicted. The discrepancies discovered here could provide new constraints on the sub-grid feedback models, or the dust contents, of simulations. Meanwhile, although the TNG predicted dust temperature and its relations with IR luminosity and redshift are qualitatively consistent with observations, the peak dust temperature of z ≥ 6 galaxies are overestimated by about |$20\, {\rm K}$|⁠. This could be related to the limited mass resolution of our simulations to fully resolve the porosity of the interstellar medium (or specifically its dust content) at these redshifts. [ABSTRACT FROM AUTHOR]

We present a novel framework to self-consistently model the effects of radiation fields, dust physics, and molecular chemistry (H2) in the interstellar medium (ISM) of galaxies. The model combines a state-of-the-art radiation hydrodynamics module with a H  and He  non-equilibrium thermochemistry module that accounts for H2 coupled to an empirical dust formation and destruction model, all integrated into the new stellar feedback framework SMUGGLE. We test this model on high-resolution isolated Milky-Way (MW) simulations. We show that the effect of radiation feedback on galactic star formation rates is quite modest in low gas surface density galaxies like the MW. The multiphase structure of the ISM, however, is highly dependent on the strength of the interstellar radiation field. We are also able to predict the distribution of H2, that allow us to match the molecular Kennicutt–Schmidt (KS) relation, without calibrating for it. We show that the dust distribution is a complex function of density, temperature, and ionization state of the gas. Our model is also able to match the observed dust temperature distribution in the ISM. Our state-of-the-art model is well-suited for performing next-generation cosmological galaxy formation simulations, which will be able to predict a wide range of resolved (∼10 pc) properties of galaxies. [ABSTRACT FROM AUTHOR]

Hot gaseous atmospheres that permeate galaxies and extend far beyond their stellar distribution, where they are commonly referred to as the circumgalactic medium, imprint important information about feedback processes powered by the stellar populations of galaxies and their central supermassive black holes (SMBHs). In this work, we study the properties of this hot X-ray emitting medium using the IllustrisTNG cosmological simulations. We analyse their mock X-ray spectra, obtained from the diffuse and metal-enriched gas in TNG100 and TNG50, and compare the results with X-ray observations of nearby early-type galaxies. The simulations reproduce the observed X-ray luminosities (L X) and temperature (T X) at small (< R e) and intermediate (<5 R e) radii reasonably well. We find that the X-ray properties of lower mass galaxies depend on their star formation rates. In particular, in the magnitude range where the star-forming and quenched populations overlap, |$M_{\rm K}\sim -24\ (M_*\sim 10^{10.7}\, \mathrm{M}_\odot)$|⁠ , we find that the X-ray luminosities of star-forming galaxies are on average about an order of magnitude higher than those of their quenched counterparts. We show that this diversity in L X is a direct manifestation of the quenching mechanism in the simulations, where the galaxies are quenched due to gas expulsion driven by SMBH kinetic feedback. The observed dichotomy in L X is thus an important observable prediction for the SMBH feedback-based quenching mechanisms implemented in state-of-the-art cosmological simulations. While the current X-ray observations of star-forming galaxies are broadly consistent with the predictions of the simulations, the observed samples are small and more decisive tests are expected from the sensitive all-sky X-ray survey with eROSITA. [ABSTRACT FROM AUTHOR]

We introduce the Stars and MUltiphase Gas in GaLaxiEs – SMUGGLE model, an explicit and comprehensive stellar feedback model for the moving-mesh code arepo. This novel sub-resolution model resolves the multiphase gas structure of the interstellar medium and self-consistently generates gaseous outflows. The model implements crucial aspects of stellar feedback including photoionization, radiation pressure, energy, and momentum injection from stellar winds and from supernovae. We explore this model in high-resolution isolated simulations of Milky Way like disc galaxies. Stellar feedback regulates star formation to the observed level and naturally captures the establishment of a Kennicutt–Schmidt relation. This result is achieved independent of the numerical mass and spatial resolution of the simulations. Gaseous outflows are generated with average mass loading factors of the order of unity. Strong outflow activity is correlated with peaks in the star formation history of the galaxy with evidence that most of the ejected gas eventually rains down on to the disc in a galactic fountain flow that sustains late-time star formation. Finally, the interstellar gas in the galaxy shows a distinct multiphase distribution with a coexistence of cold, warm, and hot phases. [ABSTRACT FROM AUTHOR]

We analyse the IllustrisTNG simulations to study the mass, volume fraction, and phase distribution of gaseous baryons embedded in the knots, filaments, sheets, and voids of the Cosmic Web from redshift z  = 8 to redshift z  = 0. We find that filaments host more star-forming gas than knots, and that filaments also have a higher relative mass fraction of gas in this phase than knots. We also show that the cool, diffuse intergalactic medium [IGM; |$T\lt 10^5 \, {\rm K}$|⁠ , |$n_{\rm H}\lt 10^{-4}(1+z) \, {\rm cm^{-3}}$| ] and the warm-hot intergalactic medium [WHIM; |$10^5 \lt T\lt 10^7 \, {\rm K}$|⁠ , |$n_{\rm H} \lt 10^{-4}(1+z)\, {\rm cm^{-3}}$| ] constitute |${\sim } 39$| and |${\sim } 46{{\ \rm per\ cent}}$| of the baryons at redshift z  = 0, respectively. Our results indicate that the WHIM may constitute the largest reservoir of missing baryons at redshift z  = 0. Using our Cosmic Web classification, we predict the WHIM to be the dominant baryon mass contribution in filaments and knots at redshift z  = 0, but not in sheets and voids where the cool, diffuse IGM dominates. We also characterize the evolution of WHIM and IGM from redshift z  = 4 to redshift z  = 0, and find that the mass fraction of WHIM in filaments and knots evolves only by a factor of ∼2 from redshift z  = 0 to 1, but declines faster at higher redshift. The WHIM only occupies |$4\!-\!11{{\ \rm per\ cent}}$| of the volume at redshift 0 ≤ z ≤ 1. We predict the existence of a significant number of currently undetected O  vii and Ne  ix absorption systems in cosmic filaments, which could be detected by future X-ray telescopes like Athena. [ABSTRACT FROM AUTHOR]

The coevolution of galaxies and their metal content serves as an important test for galaxy feedback models. We analyse the distribution and evolution of metals within the IllustrisTNG simulation suite with a focus on the gas-phase mass−metallicity relation (MZR). We find that the IllustrisTNG model broadly reproduces the slope and normalization evolution of the MZR across the redshift range 0 < z < 2 and mass range 109 < M */M⊙ < 1010.5. We make predictions for the high-redshift (2 < z < 10) metal content of galaxies which is described by a gradual decline in the normalization of the metallicity with an average high-redshift (z > 2) evolution fit by d log(Z)/d z ≈ −0.064. Our simulations indicate that the metal retention efficiency of the interstellar medium (ISM) is low: a majority of gas-phase metals (∼85 per cent at z  = 0) live outside of the ISM, either in an extended gas disc, the circumgalactic medium, or outside the halo. Nevertheless, the redshift evolution in the simulated MZR normalization is driven by the higher gas fractions of high-redshift galaxies, not by changes to the metal retention efficiency. The scatter in the simulated MZR contains a clear correlation with the gas-mass or star formation rate of the system, in agreement with the observed fundamental metallicity relation. The scatter in the MZR is driven by a competition between periods of enrichment- and accretion-dominated metallicity evolution. We expect that while the normalization of the MZR declines with redshift, the slope of the correlation between metallicity and gas-mass at fixed stellar mass is not a strong function of redshift. Our results indicate that the gas fraction dependence of 'regulator' style models allows them to simultaneously explaining the shape, redshift evolution, and existence of correlated scatter with gas fraction about the MZR. [ABSTRACT FROM AUTHOR]

We introduce a set of four very high resolution cosmological simulations exploring f (R) gravity, with 20483 particles in 768 and |$1536 \, h^{-1} \, {\rm Mpc}$| simulation boxes, for |$|\bar{f}_{R0}| = 10^{-5}$| and Λ cold dark matter (ΛCDM), making the set the largest simulations of f (R) gravity to date. To mimic real observations, the simulations include a continuous 2D- and 3D-light-cone output dedicated to study lensing and clustering statistics. We present a detailed analysis and resolution study for the matter power spectrum in f (R) gravity over a wide range of scales. We also analyse the angular matter power spectrum and lensing convergence on the light-cone. In addition, we investigate the impact of modified gravity on the halo mass function, matter, and halo autocorrelation functions, linear halo bias, and the concentration–mass relation. The impact of f (R) gravity is generally larger on smaller scales and smaller redshift. Comparing our simulations to state-of-the-art hydrodynamical simulations, we confirm a degeneracy between f (R) gravity and baryonic feedback in the matter power spectrum on small scales, but also find that scales around |$k = 1 \, h\, {\rm Mpc}^{-1}$| are promising to distinguish both effects. The lensing convergence power spectrum is increased in f (R) gravity. Numerical fits are in good agreement with our simulations for both standard and modified gravity, but tend to overestimate their relative difference on non-linear scales. The halo bias is lower in f (R) gravity, whereas halo concentrations are increased for unscreened haloes. [ABSTRACT FROM AUTHOR]

We use the IllustrisTNG (TNG) cosmological simulations to provide theoretical expectations for the dark matter mass fractions (DMFs) and circular velocity profiles of galaxies. TNG predicts flat circular velocity curves for |$z$| = 0 Milky Way (MW)-like galaxies beyond a few kpc from the galaxy centre, in better agreement with observational constraints than its predecessor, Illustris. TNG also predicts an enhancement of the dark matter mass within the 3D stellar half-mass radius (⁠|$r_{\rm half}$|⁠ ; |$M_{\rm 200c} = 10^{10}-10^{13}\, \mathrm{M_{\odot }}$|⁠, |$z$| ≤ 2) compared to its dark matter only and Illustris counterparts. This enhancement leads TNG present-day galaxies to be dominated by dark matter within their inner regions, with |$f_{\rm DM}(\lt r_{\rm half})\gtrsim 0.5$| at all masses and with a minimum for MW-mass galaxies. The 1σ scatter is ≲10 per cent at all apertures, which is smaller than that inferred by some observational data sets, e.g. 40 per cent from the sluggs survey. TNG agrees with the majority of the observationally inferred values for elliptical galaxies once a consistent initial mass function is adopted (Chabrier) and the DMFs are measured within the same apertures. The DMFs measured within |$r_{\rm half}$| increase towards lower redshifts: this evolution is dominated by the increase in galaxy size with time. At |$z$| ∼ 2, the DMF in disc-like TNG galaxies decreases with increasing galaxy mass, with |$f_{\rm DM}(\lt r_{\rm half}) \sim 0.10\hbox{--}0.65$| for 1010≲ M stars/M |$\odot$| ≲ 1012, and are two times higher than if TNG galaxies resided in Navarro–Frenk–White dark matter haloes unaffected by baryonic physics. It remains to be properly assessed whether recent observational estimates of the DMFs at |$z$| ∼ 2 rule out the contraction of the dark matter haloes predicted by the TNG model. [ABSTRACT FROM AUTHOR]

We explore the abundance, spatial distribution, and physical properties of the OVI, OVII, and OVIII ions of oxygen in circumgalactic and intergalactic media (the CGM, IGM, and WHIM). We use the TNG100 and TNG300 large volume cosmological magnetohydrodynamical simulations. Modelling the ionization states of simulated oxygen, we find good agreement with observations of the low-redshiftOVI column density distribution function (CDDF), and present its evolution for all three ions from z = 0 to z = 4. Producing mock quasar absorption line spectral surveys, we show that the IllustrisTNG simulations are fully consistent with constraints on the OVI content of the CGM from COS-haloes and other low-redshift observations, producing columns as high as observed. We measure the total amount of mass and average column densities of each ion using hundreds of thousands of simulated galaxies spanning 1011 < Mhalo/M⊙<1015 corresponding to 109 < M⊙/M⊙<1012 in stellar mass. Stacked radial profiles of OVI are computed in 3D number density and 2D projected column density, decomposing into 1-halo and 2-halo terms. Relating halo OVI to properties of the central galaxy, we find a correlation between the (g - r) colour of a galaxy and the total amount of OVI in its CGM. In comparison to the COS-Haloes finding, this leads to a dichotomy of columns around star-forming versus passive galaxies at fixed stellar (or halo) mass. We demonstrate that this correlation is a direct result of black hole feedback associated with quenching and represents a causal consequence of galactic-scale baryonic feedback impacting the physical state of the circumgalactic medium. [ABSTRACT FROM AUTHOR]

In the interstellar medium of galaxies and the intracluster gas of galaxy clusters, the charged particles making up cosmic rays are moving almost exclusively along (but not across) magnetic field lines. The resulting anisotropic transport of cosmic rays in the form of diffusion or streaming not only affects the gas dynamics but also rearranges the magnetic fields themselves. The coupled dynamics of magnetic fields and cosmic rays can thus impact the formation and evolution of galaxies and the thermal evolution of galaxy clusters in critical ways. Numerically studying these effects requires solvers for anisotropic diffusion that are accurate, efficient, and robust, requirements that have proved difficult to be satisfied in practice. Here, we present an anisotropic diffusion solver on an unstructured moving mesh that is conservative, does not violate the entropy condition, allows for semi-implicit time integration with individual timesteps, and only requires solving a single linear system of equations per timestep. We apply our new scheme to a large number of test problems and show that it works as well or better than previous implementations. Finally, we demonstrate for a numerically demanding simulation of the formation of an isolated disc galaxy that our local time-stepping scheme reproduces the results obtained with global time-stepping at a fraction of the computational cost. [ABSTRACT FROM AUTHOR]

We performed a hydrodynamical cosmological simulation of the formation of a Milky Waylike galaxy in a warm dark matter (WDM) cosmology. Smooth and dense filaments, several comoving mega parsec long, form generically above z ~ 2 in this model. Atomic line cooling allows gas in the centres of these filaments to cool to the base of the cooling function, resulting in a very striking pattern of extended Lyman-limit systems (LLSs). Observations of the correlation function of LLSs might hence provide useful limits on the nature of the dark matter. We argue that the self-shielding of filaments may lead to a thermal instability resulting in star formation. We implement a sub-grid model for this, and find that filaments rather than haloes dominate star formation until z ~ 6, although this depends on how stars form in WDM. Reionization decreases the gas density in filaments, and the more usual star formation in haloes dominates below z ~ 6, although star formation in filaments continues until z = 2. 15 per cent of the stars of the z = 0 galaxy formed in filaments. At higher redshift, these stars give galaxies a stringy appearance, which, if observed, might be a strong indication that the dark matter is warm. [ABSTRACT FROM AUTHOR]

We examine the abundance, clustering and metallicity of Damped Lyman α Absorbers (DLAs) in a suite of hydrodynamic cosmological simulations using the moving mesh code arepo. We incorporate models of supernova and AGN feedback, as well as molecular hydrogen formation. We compare our simulations to the column density distribution function at z = 3, the total DLA abundance at z = 2–4, the measured DLA bias at z = 2.3 and the DLA metallicity distribution at z = 2–4. Our preferred models produce populations of DLAs in good agreement with most of these observations. The exception is the DLA abundance at z < 3, which we show requires stronger feedback in 1011–12 h−1 M⊙ mass haloes. While the DLA population probes a wide range of halo masses, we find the cross-section is dominated by haloes of mass 1010–1011 h−1 M⊙ and virial velocities 50–100 km s−1. The simulated DLA population has a linear theory bias of 1.7, whereas the observations require 2.17 ± 0.2. We show, however, that non-linear growth increases the bias in our simulations to 2.3 at k = 1 h Mpc−1, the smallest scale observed. The scale-dependence of the bias is, however, very different in the simulations compared against the observations. We show that, of the observations we consider, the DLA abundance and column density function provide the strongest constraints on the feedback model. [ABSTRACT FROM AUTHOR]

We analyse the properties of the circumgalactic medium and the metal content of the stars comprising the central galaxy in eight hydrodynamical ‘zoom-in’ simulations of disc galaxy formation. We use these properties as a benchmark for our model of galaxy formation physics implemented in the moving-mesh code arepo, which succeeds in forming quite realistic late-type spirals in the set of ‘Aquarius’ initial conditions of Milky-Way-sized haloes. Galactic winds significantly influence the morphology of the circumgalactic medium and induce bipolar features in the distribution of heavy elements. They also affect the thermodynamic properties of the circumgalactic gas by supplying an energy input that sustains its radiative losses. Although a significant fraction of the heavy elements are transferred from the central galaxy to the halo, and even beyond the virial radius, enough metals are retained by stars to yield a peak in their metallicity distributions at about Z⊙. All our default runs overestimate the stellar [O/Fe] ratio, an effect that we demonstrate can be rectified by an increase of the adopted Type Ia supernova rate. Nevertheless, the models have difficulty in producing stellar metallicity gradients of the same strength as observed in the Milky Way. [ABSTRACT FROM PUBLISHER]

We use N-body simulations to investigate the radial dependence of the density, ρ, and velocity dispersion, σ, in cold dark matter (CDM) haloes. In particular, we explore how closely , a surrogate measure of the phase-space density, follows a power law in radius. Our study extends earlier work by considering, in addition to spherically averaged profiles, local Q estimates for individual particles, ; profiles based on the ellipsoidal radius dictated by the triaxial structure of the halo, ; and by carefully removing substructures in order to focus on the profile of the smooth halo, . The resulting profiles follow closely a power law near the centre, but show a clear upturn from this trend near the virial radius, r200. The location and magnitude of the deviations are in excellent agreement with the predictions from Bertschinger's spherical secondary-infall similarity solution. In this model, in the inner, virialized regions, but departures from a power-law occur near r200 because of the proximity of this radius to the location of the first shell crossing – the shock radius in the case of a collisional fluid. Particles there have not yet fully virialized, and so Q departs from the inner power-law profile. Our results imply that the power-law nature of Q profiles only applies to the inner regions and cannot be used to predict accurately the structure of CDM haloes beyond their characteristic scale radius. [ABSTRACT FROM AUTHOR]

If dark matter is composed of neutralinos, one of the most exciting prospects for its detection lies in observations of the gamma-ray radiation created in pair annihilations between neutralinos, a process that may contribute significantly to the extragalactic gamma-ray background (EGB) radiation. We here use the high-resolution Millennium-II simulation of cosmic structure formation to produce the first full sky maps of the expected radiation coming from extragalactic dark matter structures. Our map-making procedure takes into account the total gamma-ray luminosity from all haloes and their subhaloes, and includes corrections for unresolved components of the emission as well as an extrapolation to the damping scale limit of neutralinos. Our analysis also includes a proper normalization of the signal according to a specific supersymmetric model based on minimal supergravity. The new simulated maps allow a study of the angular power spectrum of the gamma-ray background from dark matter annihilation, which has distinctive features associated with the nature of the annihilation process and may be detectable in forthcoming observations by the recently launched Fermi satellite. Our results are in broad agreement with analytic models for the gamma-ray background, but they also include higher order correlations not readily accessible in analytic calculations and, in addition, provide detailed spectral information for each pixel. In particular, we find that difference maps at different energies can reveal cosmic large-scale structure at low and intermediate redshifts. If the intrinsic emission spectrum is characterized by an emission peak, cosmological tomography with gamma-ray annihilation radiation is in principle possible. [ABSTRACT FROM AUTHOR]

If the accelerated expansion of the Universe at the present epoch is driven by a dark energy scalar field, there may well be a non-trivial coupling between the dark energy and the cold dark matter (CDM) fluid. Such interactions give rise to new features in cosmological structure growth, like an additional long-range attractive force between CDM particles, or variations of the dark matter particle mass with time. We have implemented these effects in the N-body codegadget-2 and present results of a series of high-resolution N-body simulations where the dark energy component is directly interacting with the CDM. As a consequence of the new physics, CDM and baryon distributions evolve differently both in the linear and in the non-linear regime of structure formation. Already on large scales, a linear bias develops between these two components, which is further enhanced by the non-linear evolution. We also find, in contrast with previous work, that the density profiles of CDM haloes are less concentrated in coupled dark energy cosmologies compared with ΛCDM, and that this feature does not depend on the initial conditions setup, but is a specific consequence of the extra physics induced by the coupling. Also, the baryon fraction in haloes in the coupled models is significantly reduced below the universal baryon fraction. These features alleviate tensions between observations and the ΛCDM model on small scales. Our methodology is ideally suited to explore the predictions of coupled dark energy models in the fully non-linear regime, which can provide powerful constraints for the viable parameter space of such scenarios. [ABSTRACT FROM AUTHOR]

Hydrodynamic cosmological simulations at present usually employ either the Lagrangian smoothed particle hydrodynamics (SPH) technique or Eulerian hydrodynamics on a Cartesian mesh with (optional) adaptive mesh refinement (AMR). Both of these methods have disadvantages that negatively impact their accuracy in certain situations, for example the suppression of fluid instabilities in the case of SPH, and the lack of Galilean invariance and the presence of overmixing in the case of AMR. We here propose a novel scheme which largely eliminates these weaknesses. It is based on a moving unstructured mesh defined by the Voronoi tessellation of a set of discrete points. The mesh is used to solve the hyperbolic conservation laws of ideal hydrodynamics with a finite-volume approach, based on a second-order unsplit Godunov scheme with an exact Riemann solver. The mesh-generating points can in principle be moved arbitrarily. If they are chosen to be stationary, the scheme is equivalent to an ordinary Eulerian method with second-order accuracy. If they instead move with the velocity of the local flow, one obtains a Lagrangian formulation of continuum hydrodynamics that does not suffer from the mesh distortion limitations inherent in other mesh-based Lagrangian schemes. In this mode, our new method is fully Galilean invariant, unlike ordinary Eulerian codes, a property that is of significant importance for cosmological simulations where highly supersonic bulk flows are common. In addition, the new scheme can adjust its spatial resolution automatically and continuously, and hence inherits the principal advantage of SPH for simulations of cosmological structure growth. The high accuracy of Eulerian methods in the treatment of shocks is also retained, while the treatment of contact discontinuities improves. We discuss how this approach is implemented in our new codearepo, both in 2D and in 3D, and is parallelized for distributed memory computers. We also discuss techniques for adaptive refinement or de-refinement of the unstructured mesh. We introduce an individual time-step approach for finite-volume hydrodynamics, and present a high-accuracy treatment of self-gravity for the gas that allows the new method to be seamlessly combined with a high-resolution treatment of collisionless dark matter. We use a suite of test problems to examine the performance of the new code and argue that the hydrodynamic moving-mesh scheme proposed here provides an attractive and competitive alternative to current SPH and Eulerian techniques. [ABSTRACT FROM AUTHOR]

We simulate the growth of isolated dark matter haloes from self-similar and spherically symmetric initial conditions. Our N-body code integrates the geodesic deviation equation in order to track the streams and caustics associated with individual simulation particles. The radial orbit instability causes our haloes to develop major-to-minor axis ratios approaching 10 to 1 in their inner regions. They grow similarly in time and have similar density profiles to the spherical similarity solution, but their detailed structure is very different. The higher dimensionality of the orbits causes their stream and caustic densities to drop much more rapidly than in the similarity solution. This results in a corresponding increase in the number of streams at each point. At 1 per cent of the turnaround radius (corresponding roughly to the Sun's position in the Milky Way), we find of the order of 106 streams in our simulations, as compared to 102 in the similarity solution. The number of caustics in the inner halo increases by a factor of several, because a typical orbit has six turning points rather than one, but caustic densities drop by a much larger factor. This reduces the caustic contribution to the annihilation radiation. For the region between 1 and 50 per cent of the turnaround radius, this is 4 per cent of the total in our simulated haloes, as compared to 6.5 per cent in the similarity solution. Caustics contribute much less at smaller radii. These numbers assume a neutralino with present-day velocity dispersion , but reducing the dispersion by 10 orders of magnitude only doubles the caustic luminosity. We conclude that caustics will be unobservable in the inner parts of haloes. Only the outermost caustic might potentially be detectable. [ABSTRACT FROM AUTHOR]

We employ cosmological hydrodynamical simulations to study the growth of massive black holes (BHs) at high redshifts subject to BH merger recoils from gravitational wave emission. As a promising host system of a powerful high-redshift quasar, we select the most massive dark matter halo at from the Millennium simulation, and resimulate its formation at much higher resolution including gas physics and a model for BH seeding, growth and feedback. Assuming that the initial BH seeds are relatively massive, of the order of , and that seeding occurs around in dark matter haloes of mass , we find that it is possible to build up supermassive BHs (SMBHs) by that assemble most of their mass during extended Eddington-limited accretion periods. The properties of the simulated SMBHs are consistent with observations of quasars in terms of the estimated BH masses and bolometric luminosities, the amount of star formation occurring within the host halo, and the presence of highly enriched gas in the innermost regions of the host galaxy. After a peak in the BH accretion rate at , the most massive BH has become sufficiently massive for the growth to enter into a much slower phase of feedback-regulated accretion. We extend our basic BH model by incorporating prescriptions for the BH recoils caused by gravitational wave emission during BH merger events, taking into account the newest numerical relativity simulations of merging BH binaries. In order to explore the full range of expected recoils and radiative efficiencies, we also consider models with spinning BHs. In the most ‘pessimistic’ case where BH spins are initially high, we find that the growth of the SMBHs can be potentially hampered if they grow mostly in isolation and experience only a small number of mergers. On the other hand, whereas BH kicks can expel a substantial fraction of low-mass BHs, they do not significantly affect the build-up of the SMBHs. On the contrary, a large number of BH mergers has beneficial consequences for the growth of the SMBHs by considerably reducing their spin. We also track the fate of our SMBH by performing cosmological simulations all the way to . This allows us to study the history of BH mass assembly over a large time-span and to establish a clear signal of ‘downsizing’ of the BH accretion rate for the population of BHs as a whole. We further find that the descendents of the most luminous quasar correspond most likely to the most massive BHs today, characterized by a low activity level and masses of the order of . [ABSTRACT FROM AUTHOR]

We explore galaxy properties and their link with environment and clustering using a population of ∼1000 galaxies formed in a high-resolution hydrodynamic simulation of the Λ cold dark matter cosmology. At the redshift we concentrate on, , the spatial resolution is 1.4 proper and the mass resolution is such that Milky Way sized disc galaxies contain particles within their virial radii. The simulations include supermassive black hole accretion and feedback as well as a multiphase model for star formation. Overall, we find that a number of familiar qualitative relationships hold approximately between galaxy properties, for example we observe galaxies as lying between two broad extremes of type, where ‘late’ types tend to be smaller in size, have lower circular velocities, younger stars, higher star formation rates, larger disc to bulge ratios and lower Sersic indices than ‘early types’. We find that as in previous studies of small numbers of resimulated galaxies the stellar component of disc galaxies is not as rotationally supported as in observations. Bulges contain too much of the stellar mass, although exponential discs do have scalelengths compatible with observations, and the stellar mass Tully–Fisher relation at is reproduced. The addition of black hole physics to the simulations does not appear to have made a large impact on the angular momentum results compared to these other studies, nor do we find that running an identical simulation with significantly lower mass resolution affects this aspect (although we are unable to test higher mass resolution, which other authors have shown can alleviate the problem). Despite this, we can profitably use the rank order of either disc-to-total ratio, Sersic index, or galaxy age to separate galaxies into morphological classes and examine the density–morphology relation and morphology dependence of clustering. We find that while at redshift the well-known preponderance of early types in dense environments is seen, at the density–morphology relation becomes flatter and late-type galaxies are even seen to have a higher clustering amplitude than early types. [ABSTRACT FROM AUTHOR]

We present the Millennium-II Simulation (MS-II), a very large N-body simulation of dark matter evolution in the concordance Λ cold dark matter (ΛCDM) cosmology. The MS-II assumes the same cosmological parameters and uses the same particle number and output data structure as the original Millennium Simulation (MS), but was carried out in a periodic cube one-fifth the size with five times better spatial resolution (a Plummer equivalent softening of ) and with 125 times better mass resolution (a particle mass of ). By comparing results at MS and MS-II resolution, we demonstrate excellent convergence in dark matter statistics such as the halo mass function, the subhalo abundance distribution, the mass dependence of halo formation times, the linear and non-linear autocorrelations and power spectra, and halo assembly bias. Together, the two simulations provide precise results for such statistics over an unprecedented range of scales, from haloes similar to those hosting Local Group dwarf spheroidal galaxies to haloes corresponding to the richest galaxy clusters. The ‘Milky Way’ haloes of the Aquarius Project were selected from a lower resolution version of the MS-II and were then resimulated at much higher resolution. As a result, they are present in the MS-II along with thousands of other similar mass haloes. A comparison of their assembly histories in the MS-II and in resimulations of 1000 times better resolution shows detailed agreement over a factor of 100 in mass growth. We publicly release halo catalogues and assembly trees for the MS-II in the same format within the same archive as those already released for the MS. [ABSTRACT FROM AUTHOR]

We study the spatial distribution of X-ray selected active galactic nuclei (AGN) in the framework of hierarchical coevolution of supermassive black holes and their host galaxies and dark matter haloes. To this end, we have applied the theoretical model developed by Croton et al., De Lucia & Blaizot and Marulli et al. to the output of the Millennium Run and obtained hundreds of realizations of past light cones from which we have extracted realistic mock AGN catalogues that mimic the Chandra deep fields. We find that the model AGN number counts are in fair agreement with observations both in the soft and in the hard X-ray bands, except at fluxes , where the model systematically overestimates the observations. However, a large fraction of these faint objects are typically excluded from the spectroscopic AGN samples of the Chandra fields. We find that the spatial two-point correlation function predicted by the model is well described by a power-law relation out to , in close agreement with observations. Our model matches the correlation length r0 of AGN in the Chandra Deep Field-North but underestimates it in the Chandra Deep Field-South. When fixing the slope to , as in Gilli et al., the statistical significance of the mismatch is 2σ–2.5σ, suggesting that the predicted cosmic variance, which dominates the error budget, may not account for the different correlation length of the AGN in the two fields. However, the overall mismatch between the model and the observed correlation function decreases when both r0 and γ are allowed to vary, suggesting that more realistic AGN models and a full account of all observational errors may significantly reduce the tension between AGN clustering in the two fields. While our results are robust to changes in the model prescriptions for the AGN light curves, the luminosity dependence of the clustering is sensitive to the different light-curve models adopted. However, irrespective of the model considered, the luminosity dependence of the AGN clustering in our mock fields seems to be weaker than in the real Chandra fields. The significance of this mismatch needs to be confirmed using larger data sets. [ABSTRACT FROM AUTHOR]

We have constructed the first all-sky cosmic microwave background (CMB) temperature and polarization lensed maps based on a high-resolution cosmological N-body simulation, the Millennium Simulation (MS). We have exploited the lensing potential map obtained using a previously developed map-making procedure which integrates along the line-of-sight the MS dark matter distribution by stacking and randomizing the simulation boxes up to , and which semi-analytically supplies the large-scale power in the angular lensing potential that is not correctly sampled by the N-body simulation. The lensed sky has been obtained by properly modifying the latest version of the LensPix code to account for the MS structures. We have also produced all-sky lensed maps of the so-called and potentials, which are directly related to the electric and magnetic types of polarization. The angular power spectra of the simulated lensed temperature and polarization maps agree well with semi-analytic estimates up to , while on smaller scales we find a slight excess of power which we interpret as being due to non-linear clustering in the MS. We also observe how non-linear lensing power in the polarized CMB is transferred to large angular scales by suitably misaligned modes in the CMB and the lensing potential. This work is relevant in view of the future CMB probes, as a way to analyse the lensed sky and disentangle the contribution from primordial gravitational waves. [ABSTRACT FROM AUTHOR]

We study the formation of galaxies in a Λ cold dark matter (ΛCDM) universe using high-resolution hydrodynamical simulations with a multiphase treatment of gas, cooling and feedback, focusing on the formation of discs. Our simulations follow eight isolated haloes similar in mass to the Milky Way and extracted from a large cosmological simulation without restriction on spin parameter or merger history. This allows us to investigate how the final properties of the simulated galaxies correlate with the formation histories of their haloes. We find that, at , none of our galaxies contains a disc with more than 20 per cent of its total stellar mass. Four of the eight galaxies nevertheless have well-formed disc components, three have dominant spheroids and very small discs, and one is a spheroidal galaxy with no disc at all. The spheroids are made of old stars, while discs are younger and formed from the inside-out. Neither the existence of a disc at nor the final disc-to-total mass ratio seems to depend on the spin parameter of the halo. Discs are formed in haloes with spin parameters as low as 0.01 and as high as 0.05; galaxies with little or no disc component span the same range in spin parameter. Except for one of the simulated galaxies, all have significant discs at , regardless of their morphologies. Major mergers and instabilities which arise when accreting cold gas is misaligned with the stellar disc trigger a transfer of mass from the discs to the spheroids. In some cases, discs are destroyed, while in others, they survive or reform. This suggests that the survival probability of discs depends on the particular formation history of each galaxy. A realistic ΛCDM model will clearly require weaker star formation at high redshift and later disc assembly than occurs in our models. [ABSTRACT FROM AUTHOR]

The formation of dark matter haloes tends to occur anisotropically along the filaments of the cosmic web, which induces ellipticity–ellipticity (EE) correlations between the shapes of haloes, as well as ellipticity–direction (ED) cross-correlations between halo shapes and the directions to neighbouring haloes. We analyse the halo catalogue and the semi-analytic galaxy catalogue of the recent Millennium Run Simulation to measure the EE and ED correlations numerically at four different redshifts ( and 2). For the EE correlations, we find that (i) the major-axis correlation is strongest while the intermediate-axis correlation is weakest; (ii) the signal is significant at distances out to ; (iii) the signal decreases as z decreases and (iv) its behaviour depends strongly on the halo mass scale, with larger masses showing stronger correlations at large distances. For the ED correlations, we find that (i) the correlations are much stronger than the EE correlations, and are significant even out to distances of ; (ii) the signal also decreases as z decreases and (iii) it increases with halo mass at all distances. We also provide empirical fitting functions for the EE and ED correlations. The EE correlations are found to scale linearly with the linear density correlation function, ξ( r), while the ED cross-correlation is found to scale as ξ1/2( r) at large distances beyond . The best-fitting values of the fitting parameters for the EE and the ED correlations are all determined through -statistics. Our results may be useful for quantifying the filamentary distribution of dark matter haloes over a wide range of scales. [ABSTRACT FROM AUTHOR]

We present a new and completely general technique for calculating the fine-grained phase-space structure of dark matter (DM) throughout the Galactic halo. Our goal is to understand this structure on the scales relevant for direct and indirect detection experiments. Our method is based on evaluating the geodesic deviation equation along the trajectories of individual DM particles. It requires no assumptions about the symmetry or stationarity of the halo formation process. In this paper we study general static potentials which exhibit more complex behaviour than the separable potentials studied previously. For ellipsoidal logarithmic potentials with a core, phase mixing is sensitive to the resonance structure, as indicated by the number of independent orbital frequencies. Regions of chaotic mixing can be identified by the very rapid decrease in the real-space density of the associated DM streams. We also study the evolution of stream-density in ellipsoidal NFW haloes with radially varying isopotential shape, showing that if such a model is applied to the Galactic halo, at least 105 streams are expected near the Sun. The most novel aspect of our approach is that general non-static systems can be studied through implementation in a cosmological N-body code. Such an implementation allows a robust and accurate evaluation of the enhancements in annihilation radiation due to fine-scale structure such as caustics. We embed the scheme in the current state-of-the-art codegadget-3 and present tests which demonstrate that N-body discreteness effects can be kept under control in realistic configurations. [ABSTRACT FROM AUTHOR]

We use the Millennium Simulation (MS) to study the statistics of Λ cold dark matter (ΛCDM) halo concentrations at . Our results confirm that the average halo concentration declines monotonically with mass; the concentration–mass relation is well fitted by a power law over three decades in mass, up to the most massive objects that form in a ΛCDM universe . This is in clear disagreement with the predictions of the model proposed by Bullock et al. for these rare objects, and agrees better with the original predictions of Navarro, Frenk & White. The large volume surveyed, together with the unprecedented numerical resolution of the MS, allows us to estimate with confidence the distribution of concentrations and, consequently, the abundance of systems with unusual properties. About one in a hundred cluster haloes have concentrations exceeding , a result that may be useful in interpreting the likelihood of unusually strong massive gravitational lenses, such as Abell 1689, in the ΛCDM cosmogony. A similar fraction of about 1 per cent of galaxy-sized haloes have and this could be relevant to models that attempt to reconcile the ΛCDM cosmology with rotation curves of low surface brightness galaxies by appealing to haloes of unexpectedly low concentration. We find that halo concentrations are independent of spin once haloes manifestly out of equilibrium have been removed from the sample. Compared to their relaxed brethren, the concentrations of out-of-equilibrium haloes tend to be lower and have more scatter, while their spins tend to be higher. A number of previously noted trends within the halo population are induced primarily by these properties of unrelaxed systems. Finally, we compare the result of predicting halo concentrations using the mass assembly history of the main progenitor with predictions based on simple arguments regarding the assembly time of all progenitors. The latter are typically as good or better than the former, suggesting that halo concentration depends not only on the evolutionary path of a halo's main progenitor, but on how and when all of its constituents collapsed to form non-linear objects. [ABSTRACT FROM AUTHOR]

Deep Chandra exposures of the hot galaxy cluster 1E0657−56 have revealed a highly dynamical state of the system due to an ongoing merger with a massive subcluster. The system is observed shortly after the first core-passage of the infalling subcluster, which moves approximately in the plane of the sky and is preceded by a prominent bow shock with Mach number . The inferred shock velocity of has been commonly interpreted as the velocity of the ‘bullet’ subcluster itself in the rest frame of the parent cluster. This velocity is unexpectedly high in the Lambda cold dark matter (ΛCDM) cosmology, which may require non-trivial modifications in the dark sector such as additional long-range scalar forces if taken at face value. Here we present explicit hydrodynamical toy models of galaxy cluster mergers which very well reproduce the observed dynamical state of 1E0657−56 and the mass models inferred from gravitational lensing observations. However, despite a shock speed of , the subcluster's mass centroid is moving only with in the rest frame of the parent cluster. The difference arises in part due to a gravitationally induced inflow velocity of the gas ahead of the shock towards the bullet, which amounts to for our assumed 10:1 mass ratio of the merger. A second effect is that the shock front moves faster than the subcluster itself, enlarging the distance between the subcluster and the bow shock with time. We also discuss the expected location of the lensing mass peak relative to the hydrodynamical features of the flow, and show that their spatial separation depends sensitively on the relative concentrations and gas fractions of the merging clusters, in addition to being highly time dependent. A generic ΛCDM collision model, where a bullet subcluster with concentration merges with a parent cluster with concentration on a zero-energy orbit, reproduces all the main observational features seen in 1E0657−56 with good accuracy, suggesting that 1E0657−56 is well in line with expectations from standard cosmological models. In theories with an additional ‘fifth’ force in the dark sector, the bullet subcluster can be accelerated beyond the velocity reached in ΛCDM, and the spatial offset between the X-ray peak and the mass centroid of the subcluster can be significantly enlarged. Our results stress the need for explicit hydrodynamical models for the proper interpretation of actively merging systems such as 1E0657−56. [ABSTRACT FROM AUTHOR]

We use a set of cosmological N-body simulations to investigate the structural shape of galaxy-sized cold dark matter (CDM) haloes. Unlike most previous work on the subject – which dealt with shapes as measured by the inertia tensor – we focus here on the shape of the gravitational potential, a quantity more directly relevant to comparison with observational probes. A further advantage is that the potential is less sensitive to the effects of substructure and, as a consequence, the isopotential surfaces are typically smooth and well approximated by concentric ellipsoids. Our main result is that the asphericity of the potential increases rapidly towards the centre of the halo. The radial trend is more pronounced than expected from constant flattening in the mass distribution, and reflects a strong tendency for dark matter haloes to become increasingly aspherical inwards. Near the centre the halo potential is approximately prolate on average [ ], but it becomes less axisymmetric and more spherical in the outer regions. The principal axes of the isopotential surfaces remain well aligned, and in most haloes the angular momentum tends to be parallel to the minor axis and perpendicular to the major axis. This suggests that galactic discs may form in a plane where the potential is elliptical and where its ellipticity varies rapidly with radius. This can result in significant deviations from circular motion in systems such as low surface brightness (LSB) galaxies, even for relatively minor deviations from circular symmetry. Simulated long-slit rotation curves often appear similar to those of LSBs cited as evidence for constant density ‘cores’. This suggests that taking into account the 3D shape of the dark mass distribution might help to reconcile such evidence with the cuspy mass profile of CDM haloes. [ABSTRACT FROM AUTHOR]

We present high-resolution mass reconstructions for five massive cluster-lenses spanning a redshift range from to 0.57 utilizing archival Hubble Space Telescope data and applying galaxy–galaxy lensing techniques. These detailed mass models were obtained from the observations by combining constraints from the strong and weak lensing regimes. We ascribe local weak distortions in the shear maps to perturbations induced by the presence of galaxy haloes around individual bright early-type cluster member galaxies. This technique constrains the mass enclosed within an aperture for these subhaloes. We are sensitive to a specific mass range for these subhaloes, , which we associate with galaxy-scale subhaloes. Adopting a parametric model for the subhaloes, we also derive their velocity dispersion function and the aperture radius function. The mass spectrum of substructure in the inner regions of the observed clusters is directly compared with that in simulated clusters extracted from the Millennium Simulation. The mass function, aperture radii and velocity dispersion function are compared in detail. Overall, we find good agreement between the distribution of substructure properties retrieved using the lensing analysis and those obtained from the simulation. We find that the fraction of total cluster mass associated with individual subhaloes within the inner 0.5–0.8 h−1 Mpc of our clusters ranges from 10–20 per cent, in broad agreement with simulations. Our work provides a powerful test of the Lambda cold dark matter model, which appears consistent with the amount of observed substructure in massive, lensing clusters based on the present data. [ABSTRACT FROM AUTHOR]

We discuss the cosmological simulation code gadget-2, a new massively parallel TreeSPH code, capable of following a collisionless fluid with the N-body method, and an ideal gas by means of smoothed particle hydrodynamics (SPH). Our implementation of SPH manifestly conserves energy and entropy in regions free of dissipation, while allowing for fully adaptive smoothing lengths. Gravitational forces are computed with a hierarchical multipole expansion, which can optionally be applied in the form of a TreePM algorithm, where only short-range forces are computed with the ‘tree’ method while long-range forces are determined with Fourier techniques. Time integration is based on a quasi-symplectic scheme where long-range and short-range forces can be integrated with different time-steps. Individual and adaptive short-range time-steps may also be employed. The domain decomposition used in the parallelization algorithm is based on a space-filling curve, resulting in high flexibility and tree force errors that do not depend on the way the domains are cut. The code is efficient in terms of memory consumption and required communication bandwidth. It has been used to compute the first cosmological N-body simulation with more than 1010 dark matter particles, reaching a homogeneous spatial dynamic range of 105 per dimension in a three-dimensional box. It has also been used to carry out very large cosmological SPH simulations that account for radiative cooling and star formation, reaching total particle numbers of more than 250 million. We present the algorithms used by the code and discuss their accuracy and performance using a number of test problems. gadget-2 is publicly released to the research community. [ABSTRACT FROM AUTHOR]

We describe techniques for incorporating feedback from star formation and black hole (BH) accretion into simulations of isolated and merging galaxies. At present, the details of these processes cannot be resolved in simulations on galactic scales. Our basic approach therefore involves forming coarse-grained representations of the properties of the interstellar medium (ISM) and BH accretion starting from basic physical assumptions, so that the impact of these effects can be included on resolved scales. We illustrate our method using a multiphase description of star-forming gas. Feedback from star formation pressurizes highly overdense gas, altering its effective equation of state (EOS). We show that this allows the construction of stable galaxy models with much larger gas fractions than possible in earlier numerical work. We extend the model by including a treatment of gas accretion onto central supermassive BHs in galaxies. Assuming thermal coupling of a small fraction of the bolometric luminosity of accreting BHs to the surrounding gas, we show how this feedback regulates the growth of BHs. In gas-rich mergers of galaxies, we observe a complex interplay between starbursts and central active galactic nuclei (AGN) activity when the tidal interaction triggers intense nuclear inflows of gas. Once an accreting supermassive BH has grown to a critical size, feedback terminates its further growth and expels gas from the central region in a powerful quasar-driven wind. Our simulation methodology is therefore able to address the coupled processes of gas dynamics, star formation and BH accretion during the formation of galaxies. [ABSTRACT FROM AUTHOR]

The cold dark matter model has become the leading theoretical picture for the formation of structure in the Universe. This model, together with the theory of cosmic inflation, makes a clear prediction for the initial conditions for structure formation and predicts that structures grow hierarchically through gravitational instability. Testing this model requires that the precise measurements delivered by galaxy surveys can be compared to robust and equally precise theoretical calculations. Here we present a simulation of the growth of dark matter structure using 2,1603 particles, following them from redshift z= 127 to the present in a cube-shaped region 2.230 billion lightyears on a side. In postprocessing, we also follow the formation and evolution of the galaxies and quasars. We show that baryon-induced features in the initial conditions of the Universe are reflected in distorted form in the low-redshift galaxy distribution, an effect that can be used to constrain the nature of dark energy with future generations of observational surveys of galaxies. [ABSTRACT FROM AUTHOR]