The Cosmic Baryon Cycle in IllustrisTNG: flows of mass, energy, and metals

TL;DR

This study uses IllustrisTNG100 to quantify mass, energy, and metal inflows and outflows around galaxies across a broad halo-mass and redshift range, distinguishing SN- and AGN-dominated feedback. By measuring flows in halo- and ISM-scale shells and defining loading factors, the work reveals that halos transition from SN-dominated to kinetic-AGN-dominated feedback as they grow, with strong halo-scale preventative feedback emerging at intermediate to high masses and at lower redshifts. The results show net halo inflows at high redshift, but near the onset of kinetic AGN feedback inflows and outflows balance, contributing to galaxy quenching and CGM evolution; ISM inflows show recycling signatures (galactic fountains) in low to intermediate-mass halos. These detailed flow catalogs, including metal and energy budgets, provide essential constraints for semi-analytic models and gas-regulator frameworks, improving our understanding of how feedback regulates galaxy growth and quenching in the cosmic context.

Abstract

We measure and analyze the inflows and outflows of mass, energy, and metals through the interstellar medium (ISM) and circumgalactic medium (CGM) of galaxies in the IllustrisTNG100 simulations. We identify the dominant feedback mechanism in bins of halo virial mass and redshift by computing the integrated energy input from SNe and the ``kinetic'' and ``thermal'' mode of AGN feedback. We measure all quantities in a shell at the virial radius (``halo scale'') and one chosen to be approximately at the interface of the CGM and the interstellar medium (ISM; ``ISM scale''). We find that galaxies have strong net positive inflows on halo scales, and weaker but still net positive inflows on ISM scales, at $z\gtrsim 2$. At later times, partially due to the onset of kinetic AGN feedback in massive halos, inflows and outflows nearly balance one another, leading to the familiar effects of the slow-down of galaxy growth and the onset of quenching. Halos dominated by SN feedback show only weak evidence of preventative feedback on halo scales, and we see excess ISM scale accretion indicative of rapid gas recycling. Wind mass loadings decrease with increasing halo mass, and with increasing redshift, while energy loadings are nearly independent of both mass and redshift. The detailed catalogs of these mass, metal, and energy inflow and outflow rates on galaxy and halo scales can be used to guide empirical and semi-analytic models, and provide deeper insight into how galaxy growth and quenching is regulated in the IllustrisTNG simulations.

Figures (17)

• Figure 1: The distribution of TNG100 halos analyzed in this work as a function of halo virial mass, colored by redshift. In total, 9522 TNG100 halos are analyzed across 10 redshifts and in virial mass bins of 0.3 dex, starting from $10^{10}~M_{\odot}$ and up to the most massive halo in each snapshot.
• Figure 2: The instantaneous star formation rates in the central galaxies of all halos from four of the redshifts analyzed in this work (top left: $z=0$; top right: $z=0.2$; bottom left: $z=1$; bottom right: $z=6$), as a function of halo virial mass. Halos whose central galaxies are star forming are marked in blue, and halos whose central galaxies are quenched are marked in red (classification into star forming or quenched is based on the method introduced in Karmakar2023). At each redshift, the median is plotted using a dashed black line.
• Figure 3: Instantaneous energy injection rates from the different feedback mechanisms in the TNG100 simulation, for all halos and redshifts analyzed in this work. Left panel: SNe feedback, based on the subgrid recipe described in Pillepich2018. Central panel: thermal / high-accretion mode AGN feedback. Right panel: kinetic / low-accretion mode AGN feedback. Both AGN energy injection rates are based on the subgrid recipe described in Weinberger2017. In all panels, marker colors range from blue ($z=0$) to red ($z=10$). SMBH generally begin their life in the thermal mode, and eventually transition to the kinetic mode.
• Figure 4: Fraction of halos, in redshift and virial mass bins, for which AGN are the dominant feedback energy injection mechanism. Fractions range from 0 (blue --- all halos in bin dominated by SNe feedback) to 1 (red --- all halos in bin dominated by AGN feedback). Bins with no halos are colored in black. We use a dotted black line to mark the lowest mass bins, per redshift, where the fraction of AGN dominated halos is greater than 0.5. Left panel: combination of kinetic + thermal feedback. Right panel: only kinetic feedback. In both cases, soon after AGN begin injecting energy into their surroundings, they become the dominant feedback energy injection mechanism.
• Figure 5: Gas mass outflow rates (top row) and inflow rates (bottom row), estimated at the halo scale (left column) and at the ISM scale (right column), all normalized by the halo virial mass and presented as a function of the halo virial mass. The gas mass flow rates are calculated from $z=10$ (red markers with error bars) to $z=0$ (blue markers with error bars). For each redshift, we mark halos dominated by SNe feedback with open circles, halos dominated by thermal AGN feedback with full circles, and halos dominated by kinetic AGN feedback with full triangles.