The abundance and origin of cool gas in galaxy clusters in the TNG-Cluster simulation

TL;DR

This study investigates the abundance, distribution, and origin of cool gas with $T \\leq 10^{4.5}$ K in $z=0$ galaxy clusters by combining the TNG-Cluster and TNG300 simulations and employing Monte Carlo tracers to track gas histories. The analysis reveals that cool gas is present in nearly all clusters within $2R_{200c}$, though it constitutes a small mass fraction that declines with halo mass, while the total cool gas mass increases with mass and is largely bound to satellites. Most present-day cool gas enters clusters via the accretion of satellites and other halos, with about 65% from satellites and ~30% from other halos; in-situ cooling of hot ICM contributes notably in cool-core clusters. Gas cooling is a late-time, multi-cycle process, with final cooling typically occurring around $z \\sim 0.1$, after gas has previously spent time at higher temperatures, indicating a complex, multi-phase assembly of the intracluster medium. The findings illuminate the multi-channel origins of cluster cool gas and provide a quantitative framework to interpret observations of cool gas in nearby and high-redshift clusters.

Abstract

In addition to the hot intracluster medium, massive galaxy clusters host complex, multi-phase gaseous halos. In this work, we quantify the abundance, spatial distribution, and origin of the cool T < 10^4.5 K gas within clusters. To do so, we combine the TNG-Cluster and TNG300 cosmological magnetohydrodynamical simulations, yielding a sample of 632 simulated galaxy clusters at z=0 with masses M_200c ~ 10^14-15.4 solar masses. We find that cool gas is present in every cluster at z=0, although it constitutes only a small fraction of the total gas mass within twice the virial radius, ranging from ~10^-4 to a few per cent. The majority of cool gas resides in the cluster outskirts in infalling satellites and other halos. More rarely, cool gas can also be present in the central regions of clusters. More massive halos contain larger amounts (but not fractions) of cool gas ~10^10-12 solar masses, and we identify correlations between cluster cool gas fraction and several global halo and galaxy properties at fixed halo mass. Using Monte-Carlo Lagrangian tracer particles, we then track the origin of cool gas in present-day clusters. We find that the primary source is recent accretion at z < 0.1, predominantly in the form of pre-cooled gas carried by infalling satellite galaxies and other halos. However, in-situ cooling of the hot intracluster medium gas accreted at earlier epochs also contributes, especially in present-day cool-core clusters.

Figures (15)

• Figure 1: The fraction of each gas phase relative to the total gas mass (left) and the absolute gas mass for each phase (right) as functions of halo mass (x-axis) across all halos and clusters of TNG300 at $z=0$ . We divide gas into cool (first row, blue), warm (second row, purple), and hot (third row, red) components. The color indicates the number of halos per pixel. The black markers are the median in both gas fraction/gas mass and halo mass for mass bins of width 1 dex from $10^8$ to $10^{15} \,\rm M_\odot$. The error bars denote the 16th and 84th percentile along both axes. More massive clusters host larger amounts of cool gas within their FoF halos, even though fractionally the contribution of cool and warm gas to clusters decreases with total mass.
• Figure 2: Temperature distributions of gas in halos of TNG300, from $10^8$ to $10^{15} \,\rm M_\odot$ in mass bins of width 1 dex. The dotted lines show the average virial temperature in the corresponding mass bin. The shaded regions show the temperature regimes in the same colors as Figure \ref{['fig:results:mass_trends_tng300']}. Even for the very massive systems that are the focus of this paper (clusters with $> 10^{14} \,\rm M_\odot$, brown curve), a non negligible amount of cool gas resides within the ICM and cluster galaxies.
• Figure 3: Visualization of the cool gas distribution around the third most massive halo in TNG-Cluster at $z=0$. The field-of-view is $3 R_{\rm 200c}$ from side to side, as well as in the projection direction, which is random with respect to the cluster itself. The large white circle shows the $\sim 2$ Mpc virial radius of the cluster itself. Color indicates neutral hydrogen column density, a tracer of cool $\sim 10^4$ K gas. While tracer amounts of cool gas pervade the cluster environment (blue), the vast majority of cool gas mass is at higher column densities (green and orange). This material is predominantly localized in and around satellite subhalos, where the most massive 100 such subhalos are enclosed with white circles, with radii equal to their half mass radii.
• Figure 4: The abundance of cool gas in galaxy clusters at $z=0$ according to the TNG300 and TNG-Cluster simulations. Top: Cool ($T < 10^{4.5} K$) to total gas mass fraction (left) and cool gas mass (right) as a function of cluster mass $M_{200c}$. All gas within $2R_{200c}$ from the cluster center is considered, hence including both ICM as well as satellite gas. The circles (diamonds) show TNG300 (TNG-Cluster) halos at $z=0$, colored by the total star formation rate of the halo, a measure of the ongoing SF activity in both the central and satellite galaxies. The solid black lines show the running average of cool gas mass fraction and cool gas mass respectively, while the dashed black lines show the running average only for gas in satellites within $2R_{200c}$, regardless of host halo. Bottom: Pearson correlation coefficient between cool gas fraction within $2R_{200c}$ and a number of selected cluster properties (listed in detail in Table \ref{['tb:results:cluster_properties']}), in seven mass bins of 0.2 dex depicted by colored dots. The points have been given small horizontal shifts for visual clarity. The black crosses mark the mean correlation coefficient over all mass bins, including error bars for standard deviation. A large fraction of cool gas within $2R_{200c}$ is gravitationally bound to satellite galaxies, and its mass fraction weakly correlates with multiple properties of the cluster.
• Figure 5: As in Figure \ref{['fig:results:cool_gas_vs_cluster_mass']} but for the gas at the center of clusters. Namely, we show the Pearson correlation coefficients between cool gas fraction and halo and galaxy properties but here, instead of accounting for the cool gas within the entire $2 R_{200c}$ halo region, we consider it within the cluster core, $R < 0.05 R_{200c}$. The properties considered are described in Table \ref{['tb:results:cluster_properties']}. We split the sample into seven 0.2 dex mass bins (colored circles). The points have a small horizontal scatter for visual clarity. The black crosses mark the mean correlation coefficient over all mass bins, including error bars for standard deviation. Cool gas mass fraction within the central $5\%$ of the virial radius correlates strongly with star-formation rate, measured both in the central galaxy and integrated over all galaxies of the cluster.