Survival of higher overdensity cold gas in a turbulent, multiphase medium

TL;DR

The paper investigates the survivability of high-overdensity cold gas clouds ($\chi\sim1000$) in a turbulent, hot intracluster medium (ICM). It uses 3D Athena++ simulations across turbulent Mach numbers $\mathcal{M}$ in $(0.25,0.75)$ and cooling-to-crushing ratios $\alpha = t_{\text{cool,mix}}/t_{\text{cc}}$ in $[0.001,10]$, tracking cold gas mass with $T<2T_{\rm floor}$. The key result is that clouds can survive only when $\alpha\lesssim 0.01$ (near $\mathcal{M}\approx 0.5$), a ~1 dex shift from the $\chi=100$ case, implying higher overdensity clouds need faster cooling to persist; this finding is robust to resolution and stochastic turbulence seeds and suggests alternative survival criteria such as $t_{\text{grow}}/t_{\text{cc}}$ may be relevant. The work refines our understanding of cold-gas growth and survival in cluster environments, with implications for multiphase baryon cycling in the ICM and the interpretation of cold gas reservoirs in turbulent halos.

Abstract

Cold gas clouds embedded in a hot, turbulent medium are expected to be short-lived due to disruptive hydrodynamic instabilities. However, radiative cooling might allow such clouds to survive and grow. We present 3D \texttt{Athena++} simulations of clouds with a density contrast of $χ= 1000$, exploring turbulent Mach numbers $\mathcal{M}\in (0.25, 0.75)$ and cloud radii chosen to span cooling-to-crushing ratios $α\in [0.001, 10]$. We find a shift in the survival boundary, with cloud survival occurring only when the cooling-to-cloud-crushing ratio ($t_{\text{cool,mix}} / t_{\text{cc}}$) $\lesssim 0.01$, which is lower than the expected boundary of $\sim 1$. This result shows that it is more difficult for higher over-density cold clouds to survive in a turbulent, hot medium, and suggests another `survival criterion'.

Figures (1)

• Figure 1: a) Density projections at 3 different times. Top: $\alpha=1$, cold cloud is destroyed; Bottom: $\alpha=0.01$ cold cloud grows. b) Cold cloud gas mass normalised with initial mass, $m_{\text{initial}}$ vs time normalised by $t_{\text{eddy}}$. Solid lines:$128^3$ cells; dashed lines: $256^3$ cells at $\mathcal{M} \sim 0.5$. $\textbf{c)}$ Plot of $\mathcal{M}$ vs $\alpha$, points at $\mathcal{M}\sim0.5$ were offset for visual clarity. The grey line shows the established survival boundary for clouds for $\chi$ of 100 from gronketurb2022, the black line represents the observed survival boundary for $\chi=1000$.