ALBERT

Description: Galaxy clusters contain a large population of low-mass dwarf elliptical galaxies whose exact origin is unclear: their colours, structural properties and kinematics differ substantially from those of dwarf irregulars in the field. We use the Illustris cosmological simulation to study differences in the assembly histories of dwarf galaxies (3 x 10 8 〈 M * /M 〈 10 10 ) according to their environment. We find that cluster dwarfs achieve their maximum total and stellar mass on average ~8 and ~4.5 Gyr ago (or redshifts z  = 1.0 and 0.4, respectively), around the time of infall into the clusters. In contrast, field dwarfs not subjected to environmental stripping reach their maximum mass at z  = 0. These different assembly trajectories naturally produce a colour bimodality, with blue isolated dwarfs and redder cluster dwarfs exhibiting negligible star formation today. The cessation of star formation happens over median times 3.5–5 Gyr depending on stellar mass, and shows a large scatter (~1–8 Gyr), with the lower values associated with starburst events that occur at infall through the virial radius or pericentric passages. We argue that such starbursts together with the early assembly of cluster dwarfs can provide a natural explanation for the higher specific frequency of globular clusters (GCs) in cluster dwarfs, as found observationally. We present a simple model for the formation and stripping of GCs that supports this interpretation. The origin of dwarf ellipticals in clusters is, therefore, consistent with an environmentally driven evolution of field dwarf irregulars. However, the z  = 0 field analogues of cluster dwarf progenitors have today stellar masses a factor of ~3 larger – a difference arising from the early truncation of star formation in cluster dwarfs.

Description: A halo merger tree forms the essential backbone of a semi-analytic model for galaxy formation and evolution. Recent studies have pointed out that extracting merger trees from numerical simulations of structure formation is non-trivial; different tree building algorithms can give differing merger histories. These differences should be carefully understood before merger trees are used as input for models of galaxy formation. We investigate the impact of different halo merger trees on a semi-analytic model. We find that the z  = 0 galaxy properties in our model show differences between trees when using a common parameter set. The star formation history of the universe and the properties of satellite galaxies can show marked differences between trees with different construction methods. Independently calibrating the semi-analytic model for each tree can reduce the discrepancies between the z  = 0 global galaxy properties, at the cost of increasing the differences in the evolutionary histories of galaxies. Furthermore, the underlying physics implied can vary, resulting in key quantities such as the supernova feedback efficiency differing by factors of 2. Such a change alters the regimes where star formation is primarily suppressed by supernovae. Therefore, halo merger trees extracted from a common halo catalogue using different, but reliable, algorithms can result in a difference in the semi-analytic model. Given the uncertainties in galaxy formation physics, however, these differences may not necessarily be viewed as significant.

Description: We use the Illustris simulation to study the relative contributions of in situ star formation and stellar accretion to the build-up of galaxies over an unprecedentedly wide range of masses ( M * = 10 9 -10 12 M ), galaxy types, environments, and assembly histories. We find that the ‘two-phase’ picture of galaxy formation predicted by some models is a good approximation only for the most massive galaxies in our simulation – namely, the stellar mass growth of galaxies below a few times 10 11 M is dominated by in situ star formation at all redshifts. The fraction of the total stellar mass of galaxies at z  = 0 contributed by accreted stars shows a strong dependence on galaxy stellar mass, ranging from about 10 per cent for Milky Way-sized galaxies to over 80 per cent for M * 10 12 M objects, yet with a large galaxy-to-galaxy variation. At a fixed stellar mass, elliptical galaxies and those formed at the centres of younger haloes exhibit larger fractions of ex situ stars than disc-like galaxies and those formed in older haloes. On average, ~50 per cent of the ex situ stellar mass comes from major mergers (stellar mass ratio μ 〉 1/4), ~20 per cent from minor mergers (1/10 〈 μ 〈 1/4), ~20 per cent from very minor mergers (μ 〈 1/10), and ~10 per cent from stars that were stripped from surviving galaxies (e.g. flybys or ongoing mergers). These components are spatially segregated, with in situ stars dominating the innermost regions of galaxies, and ex situ stars being deposited at larger galactocentric distances in order of decreasing merger mass ratio.

Description: We present an analysis of the evolving comoving cumulative number density of galaxy populations found in the Illustris simulation. Cumulative number density is commonly used to link galaxy populations across different epochs by assuming that galaxies preserve their number density in time. Our analysis allows us to examine the extent to which this assumption holds in the presence of galaxy mergers or when rank ordering is broken owing to variable stellar growth rates. Our primary results are as follows: (1) the inferred average stellar mass evolution obtained via a constant comoving number density assumption is systematically biased compared to the merger tree results at the factor of ~2(4) level when tracking galaxies from redshift z  = 0 to 2(3); (2) the median number density evolution for galaxy populations tracked forward in time is shallower than for galaxy populations tracked backward; (3) a similar evolution in the median number density of tracked galaxy populations is found regardless of whether number density is assigned via stellar mass, stellar velocity dispersion, or halo mass; (4) explicit tracking reveals a large diversity in the stellar and dark matter assembly histories that cannot be captured by constant number density analyses; (5) the significant scatter in galaxy linking methods is only marginally reduced (~20 per cent) by considering additional physical galaxy properties. We provide fits for the median evolution in number density for use with observational data and discuss the implications of our analysis for interpreting multi-epoch galaxy property observations.

Description: Massive quiescent galaxies have much smaller physical sizes at high redshift than today. The strong evolution of galaxy size may be caused by progenitor bias, major and minor mergers, adiabatic expansion, and/or renewed star formation, but it is difficult to test these theories observationally. Herein, we select a sample of 35 massive, compact galaxies ( M * = 1–3 x 10 11 M , M * / R 1.5 〉 10 10.5 M /kpc 1.5 ) at z  = 2 in the cosmological hydrodynamical simulation Illustris and trace them forwards to z  = 0 to uncover their evolution and identify their descendants. By z  = 0, the original factor of 3 difference in stellar mass spreads to a factor of 20. The dark matter halo masses similarly spread from a factor of 5 to 40. The galaxies’ evolutionary paths are diverse: about half acquire an ex situ envelope and are the core of a more massive descendant, a third survive undisturbed and gain very little mass, 15 per cent are consumed in a merger with a more massive galaxy, and a small remainder are thoroughly mixed by major mergers. The galaxies grow in size as well as mass, and only ~10 per cent remain compact by z  = 0. The majority of the size growth is driven by the acquisition of ex situ mass. The most massive galaxies at z  = 0 are the most likely to have compact progenitors, but this trend possesses significant dispersion which precludes a direct linkage to compact galaxies at z  = 2. The compact galaxies’ merger rates are influenced by their z  = 2 environments, so that isolated or satellite compact galaxies (which are protected from mergers) are the most likely to survive to the present day.

Description: We present an overview of galaxy evolution across cosmic time in the Illustris simulation. Illustris is an N -body/hydrodynamical simulation that evolves 2 1820 3 resolution elements in a (106.5 Mpc) 3 box from cosmological initial conditions down to z  = 0 using the arepo moving-mesh code. The simulation uses a state-of-the-art set of physical models for galaxy formation that was tuned to reproduce the z  = 0 stellar mass function and the history of the cosmic star formation rate density. We find that Illustris successfully reproduces a plethora of observations of galaxy populations at various redshifts, for which no tuning was performed, and provide predictions for future observations. In particular, we discuss (a) the buildup of galactic mass, showing stellar mass functions and the relations between stellar mass and halo mass from z  = 7 to 0, (b) galaxy number density profiles around massive central galaxies out to z  = 4, (c) the gas and total baryon content of both galaxies and their haloes for different redshifts, and as a function of mass and radius, and (d) the evolution of galaxy specific star formation rates up to z  = 8. In addition, we (i) present a qualitative analysis of galaxy morphologies from z  = 5 to 0, for the stellar as well as the gaseous components, and their appearance in Hubble Space Telescope mock observations, (ii) follow galaxies selected at z  = 2 to their z  = 0 descendants, and quantify their growth and merger histories, and (iii) track massive z  = 0 galaxies to high redshift and study their joint evolution in star formation activity and compactness. We conclude with a discussion of several disagreements with observations, and lay out possible directions for future research.

Description: Observationally, the fraction of blue satellite galaxies decreases steeply with host halo mass, and their radial distribution around central galaxies is significantly shallower in massive ( M * ≥ 10 11 M ) than in Milky Way-like systems. Theoretical models, based primarily on semi-analytical techniques, have had a long-standing problem with reproducing these trends, instead predicting too few blue satellites in general but also estimating a radial distribution that is too shallow, regardless of primary mass. In this Letter, we use the Illustris cosmological simulation to study the properties of satellite galaxies around isolated primaries. For the first time, we find good agreement between theory and observations. We identify the main source of this success relative to earlier work to be a consequence of the large gas contents of satellites at infall, a factor ~5–10 times larger than in semi-analytical models. Because of their relatively large gas reservoirs, satellites can continue to form stars long after infall, with a typical time-scale for star-formation to be quenched ~2 Gyr in groups but more than ~5 Gyr for satellites around Milky Way-like primaries. The gas contents we infer are consistent with z  = 0 observations of H i gas in galaxies, although we find large discrepancies among reported values in the literature. A testable prediction of our model is that the gas-to-stellar mass ratio of satellite progenitors should vary only weakly with cosmic time.

Description: Massive, quiescent galaxies at high redshift have been found to be considerably more compact than galaxies of similar mass in the local universe. How these compact galaxies formed has yet to be determined, though several progenitor populations have been proposed. Here we investigate the formation processes and quantify the assembly histories of such galaxies in Illustris, a suite of hydrodynamical cosmological simulations encompassing a sufficiently large volume to include rare objects, while simultaneously resolving the internal structure of galaxies. We select massive (~10 11 M ) and compact (stellar half-mass radius 〈2 kpc) galaxies from the simulation at z  = 2. Within the Illustris suite, we find that these quantities are not perfectly converged, but are reasonably reliable for our purposes. The resulting population is composed primarily of quiescent galaxies, but we also find several star-forming compact galaxies. The simulated compact galaxies are similar to observed galaxies in star formation activity and appearance. We follow their evolution at high redshift in the simulation and find that there are multiple pathways to form these compact galaxies, dominated by two mechanisms: (i) intense, centrally concentrated starbursts generally triggered by gas-rich major mergers between z  ~ 2–4, reducing the galaxies’ half-mass radii by a factor of a few to below 2 kpc, and (ii) assembly at very early times when the universe was much denser; the galaxies formed compact and remained so until z  ~ 2.

Description: We use the Illustris simulations to gain insight into the build-up of the outer, low-surface brightness regions which surround galaxies. We characterize the stellar haloes by means of the logarithmic slope of the spherically averaged stellar density profiles, α STARS at z  = 0, and we relate these slopes to the properties of the underlying dark matter (DM) haloes, their central galaxies, and their assembly histories. We analyse a sample of ~5000 galaxies resolved with more than 5 10 4 particles each, and spanning a variety of morphologies and halo masses (3 10 11 ≤ M vir 10 14 M ). We find a strong trend between stellar halo slope and total halo mass, where more massive objects have shallower stellar haloes than the less massive ones (–5.5 ± 0.5 〈 α STARS  〈 –3.5 ± 0.2 in the studied mass range). At fixed halo mass, we show that disc-like, blue, young, and more massive galaxies are surrounded by significantly steeper stellar haloes than elliptical, red, older, and less massive galaxies. Overall, the stellar density profiles fall off much more steeply than the underlying DM, and no clear trend holds between stellar slope and DM halo concentration. However, DM haloes which formed more recently, or which accreted larger fractions of stellar mass from infalling satellites, exhibit shallower stellar haloes than their older analogues with similar masses, by up to α STARS  ~ 0.5–0.7. Our findings, combined with the most recent measurements of the strikingly different stellar power-law indices for M31 and the Milky Way, appear to favour a massive M31, and a Milky Way characterized by a much quieter accretion history over the past 10 Gyr than its companion.

Description: Merger trees are routinely used to follow the growth and merging history of dark matter haloes and subhaloes in simulations of cosmic structure formation. Srisawat et al. compared a wide range of merger-tree-building codes. Here we test the influence of output strategies and mass resolution on tree-building. We find that, somewhat surprisingly, building the tree from more snapshots does not generally produce more complete trees; instead, it tends to shorten them. Significant improvements are seen for patching schemes that attempt to bridge over occasional dropouts in the underlying halo catalogues or schemes that combine the halo-finding and tree-building steps seamlessly. The adopted output strategy does not affect the average number of branches (bushiness) of the resultant merger trees. However, mass resolution has an influence on both main branch length and the bushiness. As the resolution increases, a halo with the same mass can be traced back further in time and will encounter more small progenitors during its evolutionary history. Given these results, we recommend that, for simulations intended as precursors for galaxy formation models where of the order of 100 or more snapshots are analysed, the tree-building routine should be integrated with the halo finder, or at the very least be able to patch over multiple adjacent snapshots.