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 z = 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 halos 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 z = 0, for the stellar as well as the gaseous components, and their appearance in HST 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.

We present the full public release of all data from the TNG100 and TNG300 simulations of the Illus-trisTNG project. IllustrisTNG is a suite of large volume, cosmological, gravo-magnetohydrodynamical simulations run with the moving-mesh code Arepo. TNG includes a comprehensive model for galaxy formation physics, and each TNG simulation selfconsistently solves for the coupled evolution of dark matter, cosmic gas, luminous stars, and supermassive blackholes from early time to the present day, z = 0. Each of the flagship runs -TNG50, TNG100, and TNG300 -are accompanied by halo/subhalo catalogs, merger trees, lower-resolution and darkmatter only counterparts, all available with 100 snapshots. We discuss scientific and numerical cautions and caveats relevant when using TNG.The data volume now directly accessible online is ∼750 TB, including 1200 full volume snapshots and ∼80,000 high time-resolution subbox snapshots. This will increase to ∼1.1 PB with the future release of TNG50. Data access and analysis examples are available in IDL, Python, and Matlab. We describe improvements and new functionality in the webbased API, including on-demand visualization and analysis of galaxies and halos, exploratory plotting of scaling relations and other relationships between galactic and halo properties, and a new Jupyter-Lab interface. This provides an online, browserbased, near-native data analysis platform enabling user computation with local access to TNG data, alleviating the need to download large datasets.

We have constructed merger trees for galaxies in the Illustris Simulation by directly tracking the baryonic content of subhalos. 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.

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We present the full public release of all data from the Illustris simulation project. Illustris is a suite of large volume, cosmological hydrodynamical simulations run with the moving-mesh code Arepo and including a comprehensive set of physical models critical for following the formation and evolution of galaxies across cosmic time. Each simulates a volume of (106.5 Mpc) 3 and self-consistently evolves five different types of resolution elements from a starting redshift of z = 127 to the present day, z = 0. These components are: dark matter particles, gas cells, passive gas tracers, stars and stellar wind particles, and supermassive black holes. This data release includes the snapshots at all 136 available redshifts, halo and subhalo catalogs at each snapshot, and two distinct merger trees. Six primary realizations of the Illustris volume are released, including the flagship Illustris-1 run. These include three resolution levels with the fiducial "full" baryonic physics model, and a dark matter only analog for each. In addition, we provide four distinct, high time resolution, smaller volume "subboxes". The total data volume is ∼265 TB, including ∼800 full volume snapshots and ∼30,000 subbox snapshots. We describe the released data products as well as tools we have developed for their analysis. All data may be directly downloaded in its native HDF5 format. Additionally, we release a comprehensive, web-based API which allows programmatic access to search and data processing tasks. In both cases we provide example scripts and a getting-started guide in several languages: currently, IDL, Python, and Matlab. This paper addresses scientific issues relevant for the interpretation of the simulations, serves as a pointer to published and on-line documentation of the project, describes planned future additional data releases, and discusses technical aspects of the release.

We have generated synthetic images of ∼27,000 galaxies from the IllustrisTNG and the original Illustris hydrodynamic cosmological simulations, designed to match Pan-STARRS observations of log 10 (M * /M ) ≈ 9.8-11.3 galaxies at z ≈ 0.05. Most of our synthetic images were created with the SKIRT radiative transfer code, including the effects of dust attenuation and scattering, and performing the radiative transfer directly on the Voronoi mesh used by the simulations themselves. We have analysed both our synthetic and real Pan-STARRS images with the newly developed statmorph code, which calculates non-parametric morphological diagnostics -including the Gini-M 20 and concentration-asymmetry-smoothness (CAS) statistics -and performs 2D Sérsic fits. Overall, we find that the optical morphologies of IllustrisTNG galaxies are in good agreement with observations, and represent a substantial improvement compared to the original Illustris simulation. In particular, the locus of the Gini-M 20 diagram is consistent with that inferred from observations, while the median trends with stellar mass of all the morphological, size and shape parameters considered in this work lie within the ∼1σ scatter of the observational trends. However, the IllustrisTNG model has some difficulty with more stringent tests, such as producing a strong morphology-colour relation. This results in a somewhat higher fraction of red discs and blue spheroids compared to observations. Similarly, the morphology-size relation is problematic: while observations show that discs tend to be larger than spheroids at a fixed stellar mass, such a trend is not present in IllustrisTNG.

We 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 AGN 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.

We use the Illustris Simulations to gain insight into the build-up of the outer, low-surface brightness regions which surround galaxies. We characterise 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 analyze a sample of ∼5,000 galaxies resolved with more than 5 × 10 4 particles each, and spanning a variety of morphologies and halo masses (3×10 11M 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 disk-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 analogs 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 indexes for M31 and the Milky Way, appear to favour a massive M31, and a Milky Way characterised by a much quieter accretion history over the past 10 Gyrs than its companion.