Cold Molecular Outflows in the Local Universe

TL;DR

This study analyzes molecular outflows in 45 local galaxies, revealing their relation to galaxy properties, energetics, and potential impact on galaxy evolution, with implications for understanding feedback mechanisms.

Contribution

It provides an empirical relation linking outflow rates to SFR, AGN luminosity, and stellar mass, and discusses the energetics and effectiveness of outflows in galaxy quenching.

Findings

Outflow rates correlate tightly with SFR, $L_{\rm AGN}$, and stellar mass.

Outflow kinetic power varies from 0.1 to 5% of $L_{\rm AGN}$.

Less than 5% of outflowing gas escapes the galaxy.

Abstract

We study molecular outflows in a sample of 45 local galaxies, both star forming and AGN, primarily by using CO data from the ALMA archive and from the literature. For a subsample we also compare the molecular outflow with the ionized and neutral atomic phases. We infer an empirical analytical function relating the outflow rate simultaneously to the SFR, $L_{\rm AGN}$, and galaxy stellar mass; this relation is much tighter than the relations with the individual quantities. The outflow kinetic power shows a larger scatter than in previous, more biased studies, spanning from 0.1 to 5 per cent of $L_{\rm AGN}$, while the momentum rate ranges from 1 to 30 times $L_{\rm AGN}/c$, indicating that these outflows can be both energy-driven, but with a broad range of coupling efficiencies with the ISM, and radiation pressure-driven. For about 10 per cent of the objects the outflow energetics significantly exceed the maximum theoretical values; we interpret these as 'fossil outflows' resulting from activity of a past strong AGN, which has now faded. We estimate that, in the stellar mass range probed here ($>$ 10$^{10}~\rm M_{\odot}$), less than 5 per cent of the outflowing gas escapes the galaxy. The molecular gas depletion time associated with the outflow can be as short as a few million years in powerful AGN, however, the total gas (H$_2$+HI) depletion times are much longer. Altogether, our findings suggest that even AGN-driven outflows might be relatively ineffective in clearing galaxies of their entire gas content, although they are likely capable of clearing and quenching the central region.