AGN-stimulated Cooling of Hot Gas in Elliptical Galaxies

TL;DR

This study shows that weak AGN feedback in elliptical galaxies can stimulate hot gas cooling, leading to a persistent cold gas phase that explains observed cold gas and residual star formation, with irregular cold gas dynamics and low dust content.

Contribution

It demonstrates that AGN activity can promote cooling and cold gas formation in ellipticals, challenging the view that AGN solely heats the ISM and providing new insights into cold gas origins.

Findings

Cold gas mass ranges from 10^4 to over 5×10^7 M_sun.

Cooling occurs when t_cool/t_ff ≤ 70, less restrictive than previous thresholds.

Cold gas often forms outflows with low dust content.

Abstract

We study the impact of relatively weak AGN feedback on the interstellar medium of intermediate and massive elliptical galaxies. We find that the AGN activity, while globally heating the ISM, naturally stimulates some degree of hot gas cooling on scales of several kpc. This process generates the persistent presence of a cold ISM phase, with mass ranging between 10$^4$ and $\gtrsim$ 5 $\times$ 10$^7$ M$_\odot$, where the latter value is appropriate for group centered, massive galaxies. Widespread cooling occurs where the ratio of cooling to free-fall time before the activation of the AGN feedback satisfies $t_{cool}/t_{ff} \lesssim 70$, that is we find a less restrictive threshold than commonly quoted in the literature. This process helps explaining the body of observations of cold gas (both ionized and neutral/molecular) in Ellipticals and, perhaps, the residual star formation detected in many early-type galaxies. The amount and distribution of the off-center cold gas vary irregularly with time. The cold ISM velocity field is irregular, initially sharing the (outflowing) turbulent hot gas motion. Typical velocity dispersions of the cold gas lie in the range 100-200 km/s. Freshly generated cold gas often forms a cold outflow and can appear kinematically misaligned with respect to the stars. We also follow the dust evolution in the hot and cold gas. We find that the internally generated cold ISM has a very low dust content, with representative values of the dust-to-gas ratio of 10$^{-4}$- 10$^{-5}$. Therefore, this cold gas can escape detection in the traditional dust-absorption maps.