Simulating Galaxy Formation with the IllustrisTNG Model

TL;DR

The paper presents the IllustrisTNG framework, an updated, self-consistent galaxy formation model implemented in AREPO that combines magnetohydrodynamics, refined galactic winds, and a dual-mode black hole feedback scheme to address several tensions of the previous Illustris run. It integrates numerical upgrades (gradient estimation, time integration, passive scalars, and MHD) with new physics (isotropic winds with metallicity dependence, kinetic low-accretion BH feedback, and enriched yield tables) and validates the approach through a set of 25 Mpc/h cosmological boxes. The results show that magnetic fields influence massive halos, winds suppress low-mass galaxy growth to better match the observed stellar mass function, and BH feedback shapes the high-mass end and gas fractions, yielding improved SFR histories and SMHM relations. This work lays the groundwork for forthcoming large-volume IllustrisTNG simulations aimed at robust statistical comparisons with observations and broader explorations of galaxy evolution across environments and cosmic time.

Abstract

We introduce an updated physical model to simulate the formation and evolution of galaxies in cosmological, large-scale gravity+magnetohydrodynamical simulations with the moving mesh code AREPO. The overall framework builds upon the successes of the Illustris galaxy formation model, and includes prescriptions for star formation, stellar evolution, chemical enrichment, primordial and metal-line cooling of the gas, stellar feedback with galactic outflows, and black hole formation, growth and multi-mode feedback. In this paper we give a comprehensive description of the physical and numerical advances which form the core of the IllustrisTNG (The Next Generation) framework. We focus on the revised implementation of the galactic winds, of which we modify the directionality, velocity, thermal content, and energy scalings, and explore its effects on the galaxy population. As described in earlier works, the model also includes a new black hole driven kinetic feedback at low accretion rates, magnetohydrodynamics, and improvements to the numerical scheme. Using a suite of (25 Mpc $h^{-1}$)$^3$ cosmological boxes we assess the outcome of the new model at our fiducial resolution. The presence of a self-consistently amplified magnetic field is shown to have an important impact on the stellar content of $10^{12} M_{\rm sun}$ haloes and above. Finally, we demonstrate that the new galactic winds promise to solve key problems identified in Illustris in matching observational constraints and affecting the stellar content and sizes of the low mass end of the galaxy population.

Figures (16)

• Figure 1: Top: Fraction of mass returned to the ISM in a Hubble time per stellar mass formed at Solar metallicity. Two sets of bars compare results from the TNG model (wide bars) and the Illustris model (thin bars). Hydrogen and Helium dominate the stellar return in total mass. SNII dominate the metal return of essentially all elements considered here, but for Nitrogen and Iron, for which AGB and SNIa are, respectively, at par. Middle: ratio between the two models in linear scale to highlight the differences. These comparisons take into accounts all changes to the enrichment process. Namely, the different yield tables, the increased minimum mass for SNII, and the updated SNIa normalization factor. The differences from the yield tables alone are reported in the bottom panel, which account for most of the changes in metal return for SNII. Modifications from Illustris to TNG for SNIa are due to the more consistent normalization factor to the SNIa DTD, for all species but Magnesium.
• Figure 2: Qualitative inspection of the L25n512 volume run with the fiducial TNG model showing the large scale structure at $z=0$. Top row: column density of dark matter, gas and stellar mass. Bottom row: mass-weighted projected average of magnetic field strength, gas Mach number, and kinetic energy dissipation rate via shocks, to highlight the new diagnostic capabilities.
• Figure 3: Qualitative inspection of the L25n512 volume run with the fiducial TNG model: a random sample of fifteen $z=0$ galaxies selected to have halo mass greater than $10^{12}\,\rm M_{\odot}$. These include a mix of spheroid-type and disk-type systems, where the top fifteen panels are face-on, and the bottom fifteen panels are the same galaxies edge-on. Each is shown in projected stellar light combining three wide optical NIRCam filters (f200w, f115w, and f070w) from JWST. The TNG model still reproduces a diverse galaxy population, which is a basic requirement for any theoretical model for galaxy formation. Stellar masses are in $\rm M_{\odot}$ units.
• Figure 4: Quantitative properties of the fiducial model via galaxy population statistics at $z=0$ (unless otherwise stated). The black line shows the result of the fiducial TNG L25n512 simulation, while the red line shows the original Illustris model outcome on the same volume. We always give running medians (but for the cosmic star-formation rate density as a function of redshift - top left panel). Individual galaxies are shown as data points only for the fiducial TNG run L25n512. When aperture definitions are needed to measure e.g. stellar masses, they are denoted in each panel. Gray curves, shaded areas, and filled large symbols represent observational data or empirical constraints: Behroozi:2013Oesch:2015Baldry:2008Baldry:2012Bernardi:2013DSouza:2015Moster:2013Kormendy:2013McConnell:2013Giodini:2009Lovisari:2015Shen:2003. We note that the comparison to observational data is only intended as rough guideline, as we are not applying any observational mock post processing to our simulated galaxies.
• Figure 5: Galactic wind morphologies: gas mass density projection with overlaid gas velocity streamlines, with colour indicating velocity. We show a random selection of four galaxies from L25n512 having halo masses $\simeq 10^{11.5}\,\rm M_{\odot}$. Each is seen edge-on at $z=2$. Top row: gas patterns in the TNG fiducial model where wind particles are launched with random directions from the star forming gas cells (isotropic winds); Bottom row: matching galaxies simulated with the Illustris model, where the directionality of the winds is instead bipolar.