Centrally concentrated molecular gas driving galactic-scale ionised gas outflows in star-forming galaxies

TL;DR

This study investigates how centrally concentrated molecular gas in star-forming galaxies influences the launching of galactic-scale ionised gas outflows, revealing that gas distribution rather than total gas content determines outflow capability.

Contribution

It provides high-resolution ALMA observations showing that molecular gas concentration in galaxy centers correlates with outflow presence, a novel insight into outflow mechanisms.

Findings

Outflow galaxies have more centrally concentrated molecular gas.

Global gas fractions are similar between outflow and control galaxies.

Centrally confined gas correlates with ionised gas outflows.

Abstract

We perform a joint-analysis of high spatial resolution molecular gas and star-formation rate (SFR) maps in main-sequence star-forming galaxies experiencing galactic-scale outflows of ionised gas. Our aim is to understand the mechanism that determines which galaxies are able to launch these intense winds. We observed CO(1-0) at 1" resolution with ALMA in 16 edge-on galaxies, which also have 2" spatial resolution optical integral field observations from the SAMI Galaxy Survey. Half the galaxies in the sample were previously identified as harbouring intense and large-scale outflows of ionised gas ("outflow-types"), the rest serve as control galaxies. The dataset is complemented by integrated CO(1-0) observations from the IRAM 30-m telescope to probe the total molecular gas reservoirs. We find that the galaxies powering outflows do not possess significantly different global gas fractions or star-formation efficiencies when compared with a control sample. However, the ALMA maps reveal that the molecular gas in the outflow-type galaxies is distributed more centrally than in the control galaxies. For our outflow-type objects, molecular gas and star-formation is largely confined within their inner effective radius ($\rm r_{eff}$), whereas in the control sample the distribution is more diffuse, extending far beyond $\rm r_{eff}$. We infer that outflows in normal star-forming galaxies may be caused by dynamical mechanisms that drive molecular gas into their central regions, which can result in locally-enhanced gas surface density and star-formation.