We use a complete and uniform sample of almost half a million galaxies from the Sloan Digital Sky Survey to characterize the distribution of stellar mass in the low-redshift Universe. Galaxy ...abundances are well determined over almost four orders of magnitude in stellar mass and are reasonably but not perfectly fit by a Schechter function with characteristic stellar mass m*= 6.7 × 1010 M⊙ and with faint-end slope α=−1.155. For a standard cosmology and a standard stellar initial mass function, only 3.5 per cent of the baryons in the low-redshift Universe are locked up in stars. The projected autocorrelation function of stellar mass is robustly and precisely determined for rp < 30 h−1 Mpc. Over the range 10 h−1 kpc < rp < 10 h−1 Mpc, it is extremely well represented by a power law. The corresponding three-dimensional autocorrelation function is ξ*(r) = (r/6.1 h−1 Mpc)−1.84. Relative to the dark matter, the bias of the stellar mass distribution is approximately constant on large scales, but varies by a factor of 5 for rp < 1 h−1 Mpc. This behaviour is approximately but not perfectly reproduced by current models for galaxy formation in the concordance Λcold dark matter cosmology. Detailed comparison suggests that a fluctuation amplitude σ8∼ 0.8 is preferred to the somewhat larger value adopted in the Millennium Simulation models with which we compare our data. This comparison also suggests that observations of stellar mass autocorrelations as a function of redshift might provide a powerful test for the nature of Dark Energy.
We present a new statistical method to determine the relationship between the stellar masses of galaxies and the masses of their host dark matter haloes over the entire cosmic history from z ∼ 4 to ...the present. This multi-epoch abundance matching (MEAM) model self-consistently takes into account that satellite galaxies first become satellites at times earlier than they are observed. We employ a redshift-dependent parametrization of the stellar-to-halo-mass relation to populate haloes and subhaloes in the Millennium simulations with galaxies, requiring that the observed stellar mass functions at several redshifts are reproduced simultaneously. We show that physically meaningful growth of massive galaxies is consistent with these data only if observational mass errors are taken into account. Using merger trees extracted from the dark matter simulations in combination with MEAM, we predict the average assembly histories of galaxies, separating into star formation within the galaxies (in situ) and accretion of stars (ex situ). Our main results are the peak star formation efficiency decreases with redshift from 23 per cent at z = 0 to 9 per cent at z =4 while the corresponding halo mass increases from 1011.8 to 1012.5 M. The star formation rate of central galaxies peaks at a redshift which depends on halo mass; for massive haloes this peak is at early cosmic times while for low-mass galaxies the peak has not been reached yet. In haloes similar to that of the Milky Way about half of the central stellar mass is assembled after z = 0.7. In low-mass haloes, the accretion of satellites contributes little to the assembly of their central galaxies, while in massive haloes more than half of the central stellar mass is formed ex situ with significant accretion of satellites at z < 2. We find that our method implies a cosmic star formation history and an evolution of specific star formation rates which are consistent with those inferred directly. We present convenient fitting functions for stellar masses, star formation rates and accretion rates as functions of halo mass and redshift.
We study the mass, velocity dispersion and anisotropy profiles of Λ cold dark matter (ΛCDM) haloes using a suite of N-body simulations of unprecedented numerical resolution. The Aquarius Project ...follows the formation of six different galaxy-sized haloes simulated several times at varying numerical resolution, allowing numerical convergence to be assessed directly. The highest resolution simulation represents a single dark matter halo using 4.4 billion particles, of which 1.1 billion end up within the virial radius. Our analysis confirms a number of results claimed by earlier work, and clarifies a few issues where conflicting claims may be found in the recent literature. The mass profile of ΛCDM haloes deviates slightly but systematically from the form proposed by Navarro, Frenk & White. The spherically averaged density profile becomes progressively shallower inwards and, at the innermost resolved radius, the logarithmic slope is γ≡− d ln ρ/d ln r≲ 1. Asymptotic inner slopes as steep as the recently claimed ρ∝r−1.2 are clearly ruled out. The radial dependence of γ is well approximated by a power law, γ∝rα (the Einasto profile). The shape parameter, α, varies slightly but significantly from halo to halo, implying that the mass profiles of ΛCDM haloes are not strictly universal: different haloes cannot, in general, be rescaled to look identical. Departures from similarity are also seen in velocity dispersion profiles and correlate with those in density profiles so as to preserve a power-law form for the spherically averaged pseudo-phase-space density, ρ/σ3∝r−1.875. The index here is identical to that of Bertschinger's similarity solution for self-similar infall on to a point mass from an otherwise uniform Einstein–de Sitter universe. The origin of this striking behaviour is unclear, but its robustness suggests that it reflects a fundamental structural property of ΛCDM haloes. Our conclusions are reliable down to radii below 0.4 per cent of the virial radius, providing well-defined predictions for halo structure when baryonic effects are neglected, and thus an instructive theoretical template against which the modifications induced by the baryonic components of real galaxies can be judged.
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 (100 h−1 Mpc) with five times better spatial resolution (a Plummer equivalent softening of 1.0 h−1 kpc) and with 125 times better mass resolution (a particle mass of 6.9 × 106 h−1 M⊙). 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.
We present an update to the multiphase smoothed particle hydrodynamics galaxy formation code by Scannapieco et al. We include a more elaborate treatment of the production of metals, cooling rates ...based on individual element abundances and a scheme for the turbulent diffusion of metals. Our supernova feedback model now transfers energy to the interstellar medium (ISM) in kinetic and thermal form, and we include a prescription for the effects of radiation pressure from massive young stars on the ISM. We calibrate our new code on the well-studied Aquarius haloes and then use it to simulate a sample of 16 galaxies with halo masses between 1 × 1011 and 3 × 1012 M. In general, the stellar masses of the sample agree well with the stellar mass to halo mass relation inferred from abundance matching techniques for redshifts z = 0-4. There is however a tendency to overproduce stars at z > 4 and to underproduce them at z < 0.5 in the least massive haloes. Overly high star formation rates (SFRs) at z < 1 for the most massive haloes are likely connected to the lack of active galactic nuclei feedback in our model. The simulated sample also shows reasonable agreement with observed SFRs, sizes, gas fractions and gas-phase metallicities at z = 0-3. Remaining discrepancies can be connected to deviations from predictions for star formation histories from abundance matching. At z = 0, the model galaxies show realistic morphologies, stellar surface density profiles, circular velocity curves and stellar metallicities, but overly flat metallicity gradients. 15 out of 16 of our galaxies contain disc components with kinematic disc fraction ranging between 15 and 65 per cent. The disc fraction depends on the time of the last destructive merger or misaligned infall event. Considering the remaining shortcomings of our simulations we conclude that even higher kinematic disc fractions may be possible for Λ cold dark matter haloes with quiet merger histories, such as the Aquarius haloes.
Abstract
We present emerge, an Empirical ModEl for the foRmation of GalaxiEs, describing the evolution of individual galaxies in large volumes from z ∼ 10 to the present day. We assign a star ...formation rate to each dark matter halo based on its growth rate, which specifies how much baryonic material becomes available, and the instantaneous baryon conversion efficiency, which determines how efficiently this material is converted to stars, thereby capturing the baryonic physics. Satellites are quenched following the delayed-then-rapid model, and they are tidally disrupted once their subhalo has lost a significant fraction of its mass. The model is constrained with observed data extending out to high redshift. The empirical relations are very flexible, and the model complexity is increased only if required by the data, assessed by several model selection statistics. We find that for the same final halo mass galaxies can have very different star formation histories. Galaxies that are quenched at z = 0 typically have a higher peak star formation rate compared to their star-forming counterparts. emerge predicts stellar-to-halo mass ratios for individual galaxies and introduces scatter self-consistently. We find that at fixed halo mass, passive galaxies have a higher stellar mass on average. The intracluster mass in massive haloes can be up to eight times larger than the mass of the central galaxy. Clustering for star-forming and quenched galaxies is in good agreement with observational constraints, indicating a realistic assignment of galaxies to haloes.
We investigate the recent and current star formation activity of galaxies as function of distance from the cluster centre in a sample of 521 Sloan Digital Sky Survey clusters at z < 0.1. We ...characterize the recent star formation history (SFH) by the strength of the 4000 Å break and the strength of the Balmer absorption lines, and thus probe the SFH over the last ∼2 Gyr. We show that when the brightest cluster galaxies are excluded from the galaxy sample, there is no evidence for mass segregation in the clusters, so that differences in cluster and field populations cannot simply be attributed to different mass functions. We find a marked star formation–radius relation in that almost all galaxies in the cluster core are quiescent, i.e. have terminated star formation a few Gyr ago. This star formation–radius relation is most pronounced for low-mass galaxies and is very weak or absent beyond the virial radius. The typical star formation rate (SFR) of non-quiescent galaxies declines by approximately a factor of 2 towards the cluster centre. However, the fraction of galaxies with young stellar populations indicating a recently completed starburst or a truncation of star formation does not vary significantly with radius. These results favour a scenario in which star formation is quenched slowly, on time-scales similar to the cluster crossing time, i.e. a few Gyr. The fraction of star-forming galaxies which host a powerful optical active galactic nucleus (AGN) is also independent of clustercentric radius, indicating that the link between star formation and AGN in these galaxies operates independent of environment. The fraction of red galaxies which host a weak optical AGN decreases, however, towards the cluster centre, with a similar time-scale as the decline of star-forming galaxies. Our results can be fully explained by a gradual decline of SFR upon infall into the cluster, and rule out significant contributions from more violent processes, at least beyond cluster radii ≳0.1R200.
Abstract
We use publicly available data for the Millennium Simulation to explore the implications of the recent detection of assembly bias and splashback signatures in a large sample of galaxy ...clusters. These were identified in the Sloan Digital Sky Survey/Data Release 8 (SDSS/DR8) photometric data by the redMaPPer algorithm and split into high- and low-concentration subsamples based on the projected positions of cluster members. We use simplified versions of these procedures to build cluster samples of similar size from the simulation data. These match the observed samples quite well and show similar assembly bias and splashback signals. Previous theoretical work has found the logarithmic slope of halo density profiles to have a well-defined minimum whose depth decreases and whose radius increases with halo concentration. Projected profiles for the observed and simulated cluster samples show trends with concentration which are opposite to these predictions. In addition, for high-concentration clusters the minimum slope occurs at significantly smaller radius than predicted. We show that these discrepancies all reflect confusion between splashback features and features imposed on the profiles by the cluster identification and concentration estimation procedures. The strong apparent assembly bias is not reflected in the three-dimensional distribution of matter around clusters. Rather it is a consequence of the preferential contamination of low-concentration clusters by foreground or background groups.
ABSTRACT
We present constraints on the emergence and evolution of passive galaxies with the empirical model emerge, which reproduces the evolution of stellar mass functions (SMFs), specific and ...cosmic star formation rates since $z$ ≈ 10, ‘quenched’ galaxy fractions, and correlation functions. At fixed halo mass, present-day passive galaxies are more massive than active galaxies, whereas at fixed stellar mass passive galaxies populate more massive haloes in agreement with observations. This effect naturally results from the shape and scatter of the stellar-to-halo mass relation. The stellar mass assembly of present-day passive galaxies is dominated by ‘in situ’ star formation below ∼3 × 1011 M⊙ and by merging and accretion of ‘ex situ’ formed stars at higher mass. The mass dependence is in tension with current cosmological simulations. Lower mass passive galaxies show extended star formation towards low redshift in agreement with IFU surveys. All passive galaxies have main progenitors on the ‘main sequence of star formation’ with the ‘red sequence’ appearing at $z$ ≈ 2. Above this redshift, over 95 per cent of the progenitors of passive galaxies are active. More than 90 per cent of $z$ ≈ 2 ‘main sequence’ galaxies with m* > 1010 M⊙ evolve into present-day passive galaxies. Above redshift 6, more than 80 per cent of the observed SMFs above 109 M⊙ can be accounted for by progenitors of passive galaxies with m* > 1010 M⊙. This implies that high-redshift observations mainly probe the birth of present-day passive galaxies. emerge is available at github.com/bmoster/emerge.
Abstract
We study satellite galaxy abundances by counting photometric galaxies from the Eighth Data Release of the Sloan Digital Sky Survey (SDSS/DR8) around isolated bright primary galaxies from ...SDSS/DR7. We present results as a function of the luminosity, stellar mass and colour of the satellites, and of the stellar mass and colour of the primaries. For massive primaries (log M
★/M⊙ > 11.1) the luminosity and stellar mass functions of satellites with log M
★/M⊙ > 8 are similar in shape to those of field galaxies, but for lower mass primaries they are significantly steeper, even accounting for exclusion effects due to the isolation criteria. The steepening is particularly marked for the stellar mass function. Satellite abundance increases strongly with primary stellar mass, approximately in proportion to expected dark halo mass. For log M
★/M⊙ > 10.8, red primaries have more satellites than blue ones of the same stellar mass. The effect exceeds a factor of 2 at log M
★/M⊙ ∼ 11.2. Satellite galaxies are systematically redder than field galaxies of the same stellar mass, except around primaries with log M
★/M⊙ < 10.8, where their colours are similar or even bluer. Satellites are also systematically redder around more massive primaries. At fixed primary mass, they are redder around red primaries. We select similarly isolated galaxies from mock catalogues based on the galaxy formation simulations of Guo et al. and analyse them in parallel with the SDSS data. The simulation reproduces all the above trends qualitatively, except for the steepening of the satellite luminosity and stellar mass functions with decreasing primary mass. Model satellites, however, are systematically redder than in the SDSS, particularly at low mass and around low-mass primaries. Simulated haloes of a given mass have satellite abundances that are independent of central galaxy colour, but red centrals tend to have lower stellar masses, reflecting earlier quenching of star formation by feedback. This explains the correlation between satellite abundance and primary colour in the simulation. The correlation between satellite colour and primary colour arises because red centrals live in haloes which are more massive, older and more gas rich, so that satellite quenching is more efficient.
PDF
