Multiphase gas in the circumgalactic medium: relative role of $t_{\rm cool}/t_{\rm ff}$ and density fluctuations

Abstract: We perform a suite of simulations with realistic gravity and thermal balance in shells to quantify the role of the ratio of cooling time to the free-fall time ($t_{\rm cool}/t_{\rm ff}$) and the amplitude of density perturbations ($\delta \rho/\rho$) in the production of multiphase gas in the circumgalactic medium (CGM). Previous idealized simulations, focussing on small amplitude perturbations in the intracluster medium (ICM), found that cold gas can condense out of the hot ICM in global thermal balance when the background $t_{\rm cool}/t_{\rm ff} \lesssim 10$. Recent observations suggest the presence of cold gas even when the background profiles have somewhat large values of ${t_{\rm cool}/t_{\rm ff}}$. This partly motivates a better understanding of additional factors such as large density perturbations that can enhance the propensity for cooling and condensation even when the background ${t_{\rm cool}/t_{\rm ff}}$ is high. Such large density contrasts can be seeded by galaxy wakes or dense cosmological filaments. From our simulations, we introduce a condensation curve in the $(\delta \rho/\rho)$ - min$(t_{\rm cool}/t_{\rm ff})$ space, that defines the threshold for condensation of multiphase gas in the CGM. We show that this condensation curve corresponds to ${(t_{\rm cool}/t_{\rm ff})}{\rm blob} \lesssim 10$ applied to the overdense blob instead of the background for which $t{\rm cool}/t_{\rm ff}$ can be higher. We also study the modification in the condensation curve by varying entropy stratification. Steeper (positive) entropy gradients shift the condensation curve to higher amplitudes of perturbations (i.e., make condensation difficult). A constant entropy core, applicable to the CGM in smaller halos, shows condensation over a larger range of radii as compared to the steeper entropy profiles in the ICM.