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: We use cosmological hydrodynamical simulations of the APOSTLE project along with high-quality rotation curve observations to examine the fraction of baryons in CDM haloes that collect into galaxies. This ‘galaxy formation efficiency’ correlates strongly and with little scatter with halo mass, dropping steadily towards dwarf galaxies. The baryonic mass of a galaxy may thus be used to place a lower limit on total halo mass and, consequently, on its asymptotic maximum circular velocity. A number of observed dwarfs seem to violate this constraint, having baryonic masses up to 10 times higher than expected from their rotation speeds, or, alternatively, rotating at only half the speed expected for their mass. Taking the data at face value, either these systems have formed galaxies with extraordinary efficiency – highly unlikely given their shallow potential wells – or their dark matter content is much lower than expected from CDM haloes. This ‘missing dark matter’ is reminiscent of the inner mass deficit of galaxies with slowly rising rotation curves, but cannot be explained away by star formation-induced ‘cores’ in the dark mass profile, since the anomalous deficit applies to regions larger than the luminous galaxies themselves. We argue that explaining the structure of these galaxies would require either substantial modification of the standard CDM paradigm or else significant revision to the uncertainties in their inferred mass profiles, which should be much larger than reported. Systematic errors in inclination may provide a simple resolution to what would otherwise be a rather intractable problem for the current paradigm.

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 study how optical galaxy morphology depends on mass and star formation rate (SFR) in the Illustris Simulation. To do so, we measure automated galaxy structures in 10 808 simulated galaxies at z  = 0 with stellar masses 10 9.7  〈  M * /M  〈 10 12.3 . We add observational realism to idealized synthetic images and measure non-parametric statistics in rest-frame optical and near-IR images from four directions. We find that Illustris creates a morphologically diverse galaxy population, occupying the observed bulge strength locus and reproducing median morphology trends versus stellar mass, SFR, and compactness. Morphology correlates realistically with rotation, following classification schemes put forth by kinematic surveys. Type fractions as a function of environment agree roughly with data. These results imply that connections among mass, star formation, and galaxy structure arise naturally from models matching global star formation and halo occupation functions when simulated with accurate methods. This raises a question of how to construct experiments on galaxy surveys to better distinguish between models. We predict that at fixed halo mass near 10 12 M , disc-dominated galaxies have higher stellar mass than bulge-dominated ones, a possible consequence of the Illustris feedback model. While Illustris galaxies at M *  ~ 10 11 M have a reasonable size distribution, those at M *  ~ 10 10 M have half-light radii larger than observed by a factor of 2. Furthermore, at M *  ~ 10 10.5 –10 11 M , a relevant fraction of Illustris galaxies have distinct ‘ring-like’ features, such that the bright pixels have an unusually wide spatial extent.

Description: We analyse the Aquarius simulations to characterize the shape of dark matter haloes with peak circular velocity in the range 8 〈  V max  〈 200 km s –1 , and perform a convergence study using the various Aquarius resolution levels. For the converged objects, we determine the principal axis ( a ≥ b ≥ c ) of the normalized inertia tensor as a function of radius. We find that the triaxiality of field haloes is an increasing function of halo mass, so that the smallest haloes in our sample are ~40–50 per cent rounder than Milky Way-like objects at the radius where the circular velocity peaks, r max . We find that the distribution of subhalo axis ratios is consistent with that of field haloes of comparable V max . Inner and outer contours within each object are well aligned, with the major axis preferentially pointing in the radial direction for subhaloes closest to the centre of their host halo. We also analyse the dynamical structure of subhaloes likely to host luminous satellites comparable to the classical dwarf spheroidals in the Local Group. These haloes have axis ratios that increase with radius, and which are mildly triaxial with 〈 b / a 〉 ~ 0.75 and 〈 c / a 〉 ~ 0.60 at r  ~ 1 kpc. Their velocity ellipsoid become strongly tangentially biased in the outskirts as a consequence of tidal stripping.

Description: The relative impact of radiation pressure and photoionization feedback from young stars on surrounding gas is studied with hydrodynamic radiative transfer (RT) simulations. The calculations focus on the single-scattering ( direct radiation pressure) and optically thick regime, and adopt a moment-based RT-method implemented in the moving-mesh code arepo . The source luminosity, gas density profile and initial temperature are varied. At typical temperatures and densities of molecular clouds, radiation pressure drives velocities of the order of ~20 km s –1 over 1–5 Myr; enough to unbind the smaller clouds. However, these estimates ignore the effects of photoionization that naturally occur concurrently. When radiation pressure and photoionization act together, the latter is substantially more efficient, inducing velocities comparable to the sound speed of the hot ionized medium (10–15 km s –1 ) on time-scales far shorter than required for accumulating similar momentum with radiation pressure. This mismatch allows photoionization to dominate the feedback as the heating and expansion of gas lowers the central densities, further diminishing the impact of radiation pressure. Our results indicate that a proper treatment of the impact of young stars on the interstellar medium needs to primarily account for their ionization power whereas direct radiation pressure appears to be a secondary effect. This conclusion may change if extreme boosts of the radiation pressure by photon trapping are assumed.

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: We present our methods for generating a catalogue of 7000 synthetic images and 40 000 integrated spectra of redshift z  = 0 galaxies from the Illustris simulation. The mock data products are produced by using stellar population synthesis models to assign spectral energy distributions (SED) to each star particle in the galaxies. The resulting synthetic images and integrated SEDs therefore properly reflect the spatial distribution, stellar metallicity distribution, and star formation history of the galaxies. From the synthetic data products, it is possible to produce monochromatic or colour-composite images, perform SED fitting, classify morphology, determine galaxy structural properties, and evaluate the impacts of galaxy viewing angle. The main contribution of this paper is to describe the production, format, and composition of the image catalogue that makes up the Illustris Simulation Observatory. As a demonstration of this resource, we derive galactic stellar mass estimates by applying the SED fitting code fast to the synthetic galaxy products, and compare the derived stellar masses against the true stellar masses from the simulation. We find from this idealized experiment that systematic biases exist in the photometrically derived stellar mass values that can be reduced by using a fixed metallicity in conjunction with a minimum galaxy age restriction.

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: The scaling of disc galaxy rotation velocity with baryonic mass (the ‘baryonic Tully–Fisher’ relation, BTF) has long confounded galaxy formation models. It is steeper than the M   V 3 scaling relating halo virial masses and circular velocities and its zero-point implies that galaxies comprise a very small fraction of available baryons. Such low galaxy formation efficiencies may, in principle, be explained by winds driven by evolving stars, but the tightness of the BTF relation argues against the substantial scatter expected from such a vigorous feedback mechanism. We use the apostle / eagle simulations to show that the BTF relation is well reproduced in cold dark matter (CDM) simulations that match the size and number of galaxies as a function of stellar mass. In such models, galaxy rotation velocities are proportional to halo virial velocity and the steep velocity-mass dependence results from the decline in galaxy formation efficiency with decreasing halo mass needed to reconcile the CDM halo mass function with the galaxy luminosity function. The scatter in the simulated BTF is smaller than observed, even when considering all simulated galaxies and not just rotationally supported ones. The simulations predict that the BTF should become increasingly steep at the faint end, although the velocity scatter at fixed mass should remain small. Observed galaxies with rotation speeds below ~40 km s –1 seem to deviate from this prediction. We discuss observational biases and modelling uncertainties that may help to explain this disagreement in the context of CDM models of dwarf galaxy formation.