Person: Rodriguez-Gomez, Vicente
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Rodriguez-Gomez
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Vicente
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Rodriguez-Gomez, Vicente
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Publication Modelling galactic conformity with the colour–halo age relation in the Illustris simulation(Oxford University Press (OUP), 2015) Bray, Aaron; Pillepich, Annalisa; Sales, Laura V.; Zhu, Emily; Genel, Shy; Rodriguez-Gomez, Vicente; Torrey, P; Nelson, Dylan; Vogelsberger, Mark; Springel, Volker; Eisenstein, Daniel; Hernquist, LarsComparisons between observational surveys and galaxy formation models find that dark matter haloes’ mass can largely explain their galaxies’ stellar mass. However, it remains uncertain whether additional environmental variables, known as assembly bias, are necessary to explain other galaxy properties. We use the Illustris simulation to investigate the role of assembly bias in producing galactic conformity by considering 18 000 galaxies with Mstellar > 2 × 109 M⊙. We find a significant signal of galactic conformity: out to distances of about 10 Mpc, the mean red fraction of galaxies around redder galaxies is higher than around bluer galaxies at fixed stellar mass. Dark matter haloes exhibit an analogous conformity signal, in which the fraction of haloes formed at earlier times (old haloes) is higher around old haloes than around younger ones at fixed halo mass. A plausible interpretation of galactic conformity is the combination of the halo conformity signal with the galaxy colour–halo age relation: at fixed stellar mass, particularly towards the low-mass end, Illustris’ galaxy colours correlate with halo age, with the reddest galaxies (often satellites) preferentially found in the oldest haloes. We explain the galactic conformity effect with a simple semi-empirical model, assigning stellar mass via halo mass (abundance matching) and galaxy colour via halo age (age matching). Regarding comparison to observations, we conclude that the adopted selection/isolation criteria, projection effects, and stacking techniques can have a significant impact on the measured amplitude of the conformity signal.Publication The stellar mass assembly of galaxies in the Illustris simulation: growth by mergers and the spatial distribution of accreted stars(Oxford University Press (OUP), 2016) Rodriguez-Gomez, Vicente; Pillepich, Annalisa; Sales, Laura V.; Genel, Shy; Vogelsberger, Mark; Zhu, Qirong; Wellons, Sarah; Nelson, Dylan; Torrey, P; Springel, Volker; Ma, Chung-Pei; Hernquist, LarsWe 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* = 109-1012 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 1011 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* ≈ 1012 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.Publication On the assembly of dwarf galaxies in clusters and their efficient formation of globular clusters(Oxford University Press (OUP), 2015) Mistani, Pouria A.; Sales, Laura V.; Pillepich, Annalisa; Sanchez-Janssen, Rubén; Vogelsberger, Mark; Nelson, Dylan; Rodriguez-Gomez, Vicente; Torrey, P; Hernquist, LarsGalaxy 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 × 108 < M*/M⊙ < 1010) 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.Publication The colours of satellite galaxies in the Illustris simulation(Oxford University Press (OUP), 2014) Sales, L. V.; Vogelsberger, M.; Genel, S.; Torrey, P; Nelson, D.; Rodriguez-Gomez, Vicente; Wang, W.; Pillepich, A.; Sijacki, D.; Springel, V.; Hernquist, LarsObservationally, 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* ≥ 1011 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.Publication Galactic Angular Momentum in the Illustris Simulation: Feedback and the Hubble Sequence(IOP Publishing, 2015) Genel, Shy; Fall, S. Michael; Hernquist, Lars; Vogelsberger, Mark; Snyder, Gregory F.; Rodriguez-Gomez, Vicente; Sijacki, Debora; Springel, VolkerWe study the stellar angular momentum of thousands of galaxies in the Illustris cosmological simulation, which captures gravitational and gas dynamics within galaxies, as well as feedback from stars and black holes. We find that the angular momentum of the simulated galaxies matches observations well, and in particular two distinct relations are found for late-type versus early-type galaxies. The relation for late-type galaxies corresponds to the value expected from full conservation of the specific angular momentum generated by cosmological tidal torques. The relation for early-type galaxies corresponds to retention of only ~30% of that, but we find that those early-type galaxies with low angular momentum at z = 0 nevertheless reside at high redshift on the late-type relation. Some of them abruptly lose angular momentum during major mergers. To gain further insight, we explore the scaling relations in simulations where the galaxy formation physics is modified with respect to the fiducial model. We find that galactic winds with high mass-loading factors are essential for obtaining the high angular momentum relation typical for late-type galaxies, while active galactic nucleus feedback largely operates in the opposite direction. Hence, feedback controls the stellar angular momentum of galaxies, and appears to be instrumental for establishing the Hubble sequence.Publication Galaxy Mergers and Some Consequences: The Cosmological Context(2016-05-18) Rodriguez-Gomez, Vicente; Eisenstein, Daniel; Conroy, Charles; Hernquist, Lars; Somerville, RachelThe last few years have seen enormous progress in the field of galaxy formation and evolution. The latest generation of hydrodynamic cosmological simulations (e.g., the "Illustris" simulation) has been able to produce reasonably realistic populations of galaxies by tracking the evolution of dark matter, gas, stars, and black holes over a cosmological volume "representative" of the large-scale density field. However, such increasingly sophisticated cosmological simulations require equally sophisticated analysis tools. The first part of my thesis work consisted in developing a method for connecting galaxies across cosmic time, which results in data structures known as merger trees. My algorithm, known as SUBLINK, improves upon previous methods by making galaxies less likely to become "lost" during close interactions, and has been benchmarked in a merger tree code comparison project, with favorable results. The second part of my thesis work, and the main topic of this dissertation, consists of a series of essential and increasingly complex applications of my merger trees: (1) measuring the merger rate of galaxies, (2) finding out how galaxies acquire their stellar mass, and (3) investigating the impact of mergers on galaxy morphology. I will show how my analysis tools, in combination with the Illustris cosmological simulation, have made quantitative and statistically robust contributions to the field of galaxy formation and evolution, where galaxy mergers are known to play a fundamental role.