Growth and structure of multiphase gas in the cloud-crushing problem with cooling

TL;DR

This paper investigates the growth of dense gas in the cloud-crushing problem with radiative cooling, emphasizing the importance of mixed gas cooling time and simulation setup for accurate modeling of cloud growth.

Contribution

It demonstrates that the threshold for dense gas growth depends on the cooling time of mixed gas, clarifying previous conflicting results and providing guidelines for simulation design.

Findings

Growth threshold aligns with mixed gas cooling time

Simulations must be run long enough for late-time growth

Proper domain size prevents loss of mixed gas through boundaries

Abstract

We revisit the problem of the growth of dense/cold gas in the cloud-crushing setup with radiative cooling. The relative motion between the dense cloud and the diffuse medium produces a turbulent boundary layer of mixed gas with a short cooling time. This mixed gas may explain the ubiquity of the range of absorption/emission lines observed in various sources such as the circumgalactic medium and galactic/stellar/AGN outflows. Recently Gronke & Oh showed that the efficient radiative cooling of the mixed gas can lead to the continuous growth of the dense cloud. They presented a threshold cloud size for the growth of dense gas which was contradicted by the more recent works of Li et al. & Sparre et al. These thresholds are qualitatively different as the former is based on the cooling time of the mixed gas whereas the latter is based on the cooling time of the hot gas. Our simulations agree with the threshold based on the cooling time of the mixed gas. We argue that the radiative cloud-crushing simulations should be run long enough to allow for the late-time growth of the dense gas due to cooling of the mixed gas but not so long that the background gas cools catastrophically. Moreover, the simulation domain should be large enough that the mixed gas is not lost through the boundaries. While the mixing layer is roughly isobaric, the emissivity of the gas at different temperatures is fundamentally different from an isobaric single-phase steady cooling flow.