First results from the IllustrisTNG simulations: the stellar mass content of groups and clusters of galaxies

TL;DR

This work introduces the IllustrisTNG simulations and provides a comprehensive census of stellar mass in massive galaxy groups and clusters at z ≤ 1, separating central galaxies, intra-cluster light, and satellites with fixed apertures. It demonstrates that halo mass strongly governs the whole stellar mass profile and that most stellar mass in the largest halos is accreted (ex-situ) rather than formed in situ, particularly in the outskirts. The authors offer analytic fits (sigmoid profiles) and a practical tool to recover full 3D stellar mass distributions from a single halo mass or central stellar mass, enabling direct comparisons with observations. They also assess resolution effects and present the stellar mass–halo mass relation and GSMF evolution up to z ≈ 4, highlighting both successes and caveats in matching data across mass scales and redshifts.

Abstract

The IllustrisTNG project is a new suite of cosmological magneto-hydrodynamical simulations of galaxy formation performed with the Arepo code and updated models for feedback physics. Here we introduce the first two simulations of the series, TNG100 and TNG300, and quantify the stellar mass content of about 4000 massive galaxy groups and clusters ($10^{13} \leq M_{\rm 200c}/M_{\rm sun} \leq 10^{15}$) at recent times ($z \leq 1$). The richest clusters have half of their total stellar mass bound to satellite galaxies, with the other half being associated with the central galaxy and the diffuse intra-cluster light. The exact ICL fraction depends sensitively on the definition of a central galaxy's mass and varies in our most massive clusters between 20 to 40% of the total stellar mass. Haloes of $5\times 10^{14}M_{\rm sun}$ and above have more diffuse stellar mass outside 100 kpc than within 100 kpc, with power-law slopes of the radial mass density distribution as shallow as the dark matter's ( $-3.5 < α_{\rm 3D} < -3$). Total halo mass is a very good predictor of stellar mass, and vice versa: at $z=0$, the 3D stellar mass measured within 30 kpc scales as $\propto (M_{\rm 500c})^{0.49}$ with a $\sim 0.12$ dex scatter. This is possibly too steep in comparison to the available observational constraints, even though the abundance of TNG less massive galaxies ($< 10^{11}M_{\rm sun}$ in stars) is in good agreement with the measured galaxy stellar mass functions at recent epochs. The 3D sizes of massive galaxies fall too on a tight ($\sim$0.16 dex scatter) power-law relation with halo mass, with $r^{\rm stars}_{\rm 0.5} \propto (M_{\rm 500c})^{0.53}$. Even more fundamentally, halo mass alone is a good predictor for the whole stellar mass profiles beyond the inner few kpc, and we show how on average these can be precisely recovered given a single mass measurement of the galaxy or its halo.

Figures (16)

• Figure 1: The IllustrisTNG Simulations: $z=0$ visual representation of the scope and spatial volumes encompassed by the TNG100 and TNG300 runs presented in this paper. The background represents the DM density field across the $\sim$ 300 Mpc volume of TNG300, while the upper right inset shows the distribution of stellar mass across the entire $\sim$ 100 Mpc volume of TNG100, each projected through a slice a third of the box in depth. Panels on the left show two examples of galaxy-galaxy interactions, and two examples of fine-grained structure of the extended stellar haloes -- shells, tidal tails, and luminous satellites -- around two massive ellipticals at $z=0$, in projected stellar mass density. The bottom right insets show the stellar light on scales of 60 kpc per side (face-on) of two randomly-selected $z=0$ galaxies with a stellar mass larger than $10^{11}{\rm M}_{\odot}$, from the high-resolution TNG100 box.
• Figure 2: Demographics of the haloes and galaxies in the TNG100 and TNG300 simulations. Top: cumulative halo mass functions at different redshifts, in absolute numbers from the $100^3$ (TNG100) and $300^3$ Mpc$^3$ (TNG300) simulated volumes. The size of the TNG massive end is compared to the C-Eagle/Hydrangea Barnes:2017Bahe:2017, the Rhapsody-G Hahn:2017 and the Trieste Ragone:2013 zoom-in projects: currently the only simulations at comparable resolution (expressed in terms of the baryonic resolution element mass, $m$). Vertical grey bands denote the mass estimates of the local Fornax, Virgo, Perseus, and Coma clusters Weinmann:2011. Bottom: Richness of haloes in the rescaled rTNG300, i.e. average number of member galaxies (including the central) within $R_{\rm 200c}$ as a function of halo mass (bottom axis) or central stellar mass (top axis).
• Figure 3: Stellar mass density projections of the 20 most massive objects in the TNG300 simulation, spanning total halo masses from $1.5\times 10^{15}{\rm M}_{\odot}$ to about $4\times 10^{14}{\rm M}_{\odot}$. The surface mass densities range from 0.1 to $10^{10} {\rm M}_{\odot}$ kpc$^{-2}$ and the stamps measure $R_{\rm 200c}$ on a side. The most massive galaxies in the Universe occupy the centers of these massive systems and are surrounded by hundreds (even thousands) of less luminous satellite galaxies as well as by a cloud of diffuse stellar material extending to very large distances: the intra-cluster light.
• Figure 4: As in Figure \ref{['fig:stamps1']}, but for the 20 most massive objects in the TNG100 simulation: as the simulation volume is smaller, here we see a sample of slightly less massive and less rich clusters, with total halo masses between $9\times 10^{13}{\rm M}_{\odot}$ and about $4\times 10^{14}{\rm M}_{\odot}$. The diffuse stellar envelopes exhibit a diversity of shapes and morphologies, including subtle phase-space features like shells and stellar streams.
• Figure 5: Top: stellar mass density projections of three typical central galaxies and their surrounding satellites, from low (left) to high (right) halo masses (in ${\rm M}_{\odot}$). Bottom: thin grey curves depict profiles of the diffuse (i.e. excluding satellites) stellar mass of individual objects, thick blue curves are average stacked profiles in the labeled mass bin ($\pm0.01, 0.05, 0.2$ dex from left to right, in $\rm{log}_{\rm10}M_{\rm 200c}$). In the $y$-labels, $M^{\rm stars}_{200c} = M_{\rm stars}(< R_{\rm 200c})$. The stellar content of small haloes extends to much smaller distances than that of more massive haloes, even when the distances are renormalized by the virial radius.