Origins of molecular clouds in early-type galaxies

TL;DR

This study investigates the connection between hot atmospheres and cold molecular gas in early-type galaxies, revealing power-law relations and the conditions under which molecular gas forms from hot atmospheres.

Contribution

It provides new temperature, density, and mass profiles for early-type galaxies and demonstrates the correlation between hot atmospheres and molecular gas, highlighting the role of cooling times.

Findings

Molecular gas mass correlates with atmospheric gas mass and density.

Early-type galaxies have molecular gas when cooling times are below ~1 Gyr.

No correlation between $t_{c}/t_{ff}$ ratio and molecular gas presence.

Abstract

We analyze $Chandra$ observations of the hot atmospheres of 40 early spiral and elliptical galaxies. Using new temperature, density, cooling time, and mass profiles, we explore relationships between their hot atmospheres and cold molecular gas. Molecular gas mass correlates with atmospheric gas mass and density over four decades from central galaxies in clusters to normal giant ellipticals and early spirals. The mass and density relations follow power laws: $M_{\rm mol} \propto M_{\rm X}^{1.4\pm0.1}$ and $M_{\rm mol} \propto n_{\rm e}^{1.8\pm0.3}$, respectively, at 10 kpc. The ratio of molecular gas to atmospheric gas within a 10 kpc radius lies between $3\%$ and $10\%$ for early-type galaxies and between $3\%$ and $50\%$ for central galaxies in clusters. Early-type galaxies have detectable levels of molecular gas when their atmospheric cooling times falls below $\sim \rm Gyr$ at a radius of 10 kpc. A similar trend is found in central cluster galaxies. We find no relationship between the ratio of the cooling time to free fall time, $t_{\rm c}/t_{\rm ff}$, and the presence or absence of molecular clouds in early-type galaxies. The data are consistent with much of the molecular gas in early-type galaxies having condensed from their hot atmospheres.