ALBERT

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: 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: All gravitationally bound clusters expand, due to both gas loss from their most massive members and binary heating. All are eventually disrupted tidally, either by passing molecular clouds or the gravitational potential of their host galaxies. However, their interior evolution can follow two very different paths. Only clusters of sufficiently large initial population and size undergo the combined interior contraction and exterior expansion that leads eventually to core collapse. In all other systems, core collapse is frustrated by binary heating. These clusters globally expand for their entire lives, up to the point of tidal disruption. Using a suite of direct N -body calculations, we trace the ‘collapse line’ in r v - N space that separates these two paths. Here, r v and N are the cluster's initial virial radius and population, respectively. For realistic starting radii, the dividing N -value is from 10 4 to over 10 5 . We also show that there exists a minimum population, N min , for core collapse. Clusters with N  〈 N min tidally disrupt before core collapse occurs. At the Sun's Galactocentric radius, R G  = 8.5 kpc, we find N min 300. The minimum population scales with Galactocentric radius as $R_{\rm G}^{-9/8}$ . The position of an observed cluster relative to the collapse line can be used to predict its future evolution. Using a small sample of open clusters, we find that most lie below the collapse line, and thus will never undergo core collapse. Most globular clusters, on the other hand, lie well above the line. In such a case, the cluster may or may not go through core collapse, depending on its initial size. We show how an accurate age determination can help settle this issue.

Description: Pulsar timing arrays (PTAs) are placing increasingly stringent constraints on the strain amplitude of continuous gravitational waves emitted by supermassive black hole binaries on subparsec scales. In this paper, we incorporate independent information about the dynamical masses M bh of supermassive black holes in specific galaxies at known distances and use this additional information to further constrain whether or not those galaxies could host a detectable supermassive black hole binary. We estimate the strain amplitudes from individual binaries as a function of binary mass ratio for two samples of nearby galaxies: (1) those with direct dynamical measurements of M bh in the literature, and (2) the 116 most massive early-type galaxies (and thus likely hosts of the most massive black holes) within 108 Mpc from the MASSIVE Survey. Our exploratory analysis shows that the current PTA upper limits on continuous waves (as a function of angular position in the sky) can already constrain the mass ratios of hypothetical black hole binaries in many galaxies in our samples. The constraints are stronger for galaxies with larger M bh and at smaller distances. For the black holes with M bh 5 x 10 9 M at the centres of NGC 1600, NGC 4889, NGC 4486 (M87), and NGC 4649 (M60), any binary companion in orbit within the PTA frequency bands would have to have a mass ratio of a few per cent or less.

Description: We present spatially resolved two-dimensional stellar kinematics for the 41 most massive early-type galaxies (ETGs; M K –25.7 mag, stellar mass M * 10 11.8 M ) of the volume-limited ( D 〈 108 Mpc) MASSIVE survey. For each galaxy, we obtain high-quality spectra in the wavelength range of 3650–5850 Å from the 246-fibre Mitchell integral-field spectrograph at McDonald Observatory, covering a 107 arcsec x 107 arcsec field of view (often reaching 2 to 3 effective radii). We measure the 2D spatial distribution of each galaxy's angular momentum ( and fast or slow rotator status), velocity dispersion (), and higher order non-Gaussian velocity features (Gauss–Hermite moments h 3 to h 6 ). Our sample contains a high fraction (~80 per cent) of slow and non-rotators with 0.2. When combined with the lower mass ETGs in the ATLAS 3D survey, we find the fraction of slow rotators to increase dramatically with galaxy mass, reaching ~50 per cent at M K ~ –25.5 mag and ~90 per cent at M K –26 mag. All of our fast rotators show a clear anticorrelation between h 3 and V /, and the slope of the anticorrelation is steeper in more round galaxies. The radial profiles of show a clear luminosity and environmental dependence: the 12 most luminous galaxies in our sample ( M K –26 mag) are all brightest cluster/group galaxies (except NGC 4874) and all have rising or nearly flat profiles, whereas five of the seven ‘isolated’ galaxies are all fainter than M K = –25.8 mag and have falling . All of our galaxies have positive average h 4 ; the most luminous galaxies have average h 4 ~ 0.05, while less luminous galaxies have a range of values between 0 and 0.05. Most of our galaxies show positive radial gradients in h 4 , and those galaxies also tend to have rising profiles. We discuss the implications for the relationship among dynamical mass, , h 4 , and velocity anisotropy for these massive galaxies.

Description: In this work, we present CO(1–0) and CO(2–1) observations of a pilot sample of 15 early-type galaxies (ETGs) drawn from the MASSIVE galaxy survey, a volume-limited integral-field spectroscopic study of the most massive ETGs ( M * 10 11.5 M ) within 108 Mpc. These objects were selected because they showed signs of an interstellar medium and/or star formation. A large amount of gas (〉2  x  10 8 M ) is present in 10 out of 15 objects, and these galaxies have gas fractions higher than expected based on extrapolation from lower mass samples. We tentatively interpret this as evidence that stellar mass-loss and hot halo cooling may be starting to play a role in fuelling the most massive galaxies. These MASSIVE ETGs seem to have lower star formation efficiencies (SFE = SFR/ M H2 ) than spiral galaxies, but the SFEs derived are consistent with being drawn from the same distribution found in other lower mass ETG samples. This suggests that the SFE is not simply a function of stellar mass, but that local, internal processes are more important for regulating star formation. Finally, we used the CO line profiles to investigate the high-mass end of the Tully–Fisher relation (TFR). We find that there is a break in the slope of the TFR for ETGs at high masses (consistent with previous studies). The strength of this break correlates with the stellar velocity dispersion of the host galaxies, suggesting it is caused by additional baryonic mass being present in the centre of massive ETGs. We speculate on the root cause of this change and its implications for galaxy formation theories.

Description: We have constructed merger trees for galaxies in the Illustris simulation by directly tracking the baryonic content of subhaloes. These merger trees are used to calculate the galaxy–galaxy merger rate as a function of descendant stellar mass, progenitor stellar mass ratio, and redshift. We demonstrate that the most appropriate definition for the mass ratio of a galaxy–galaxy merger consists in taking both progenitor masses at the time when the secondary progenitor reaches its maximum stellar mass. Additionally, we avoid effects from ‘orphaned’ galaxies by allowing some objects to ‘skip’ a snapshot when finding a descendant, and by only considering mergers which show a well-defined ‘infall’ moment. Adopting these definitions, we obtain well-converged predictions for the galaxy–galaxy merger rate with the following main features, which are qualitatively similar to the halo–halo merger rate except for the last one: a strong correlation with redshift that evolves as ~(1 + z ) 2.4–2.8 , a power law with respect to mass ratio, and an increasing dependence on descendant stellar mass, which steepens significantly for descendant stellar masses greater than ~2 10 11 M . These trends are consistent with observational constraints for medium-sized galaxies ( M * 10 10 M ), but in tension with some recent observations of the close pair fraction for massive galaxies ( M * 10 11 M ), which report a nearly constant or decreasing evolution with redshift. Finally, we provide a fitting function for the galaxy–galaxy merger rate which is accurate over a wide range of stellar masses, progenitor mass ratios, and redshifts.