An ALMA view of star formation efficiency suppression in early-type galaxies after gas-rich minor mergers

TL;DR

This study uses ALMA observations to investigate how gas-rich minor mergers in early-type galaxies lead to suppressed star formation efficiency due to dynamical stabilization of molecular gas, contrasting with typical merger-driven starbursts.

Contribution

It provides high-resolution ALMA data revealing the morphological and kinematic irregularities in molecular gas post-merger, linking these to star formation suppression in early-type galaxies.

Findings

Galaxies show irregular gas morphology and kinematics.

Star formation efficiency is two orders of magnitude below typical relations.

Dynamical effects stabilize gas, inhibiting gravitational collapse.

Abstract

Gas-rich minor mergers contribute significantly to the gas reservoir of early-type galaxies (ETGs) at low redshift, yet the star formation efficiency (SFE; the star formation rate divided by the molecular gas mass) appears to be strongly suppressed following some of these events, in contrast to the more well-known merger-driven starbursts. We present observations with the Atacama Large Millimeter/submillimeter Array (ALMA) of six ETGs, which have each recently undergone a gas-rich minor merger, as evidenced by their disturbed stellar morphologies. These galaxies were selected because they exhibit extremely low SFEs. We use the resolving power of ALMA to study the morphology and kinematics of the molecular gas. The majority of our galaxies exhibit spatial and kinematical irregularities, such as detached gas clouds, warps, and other asymmetries. These asymmetries support the interpretation that the suppression of the SFE is caused by dynamical effects stabilizing the gas against gravitational collapse. Through kinematic modelling we derive high velocity dispersions and Toomre Q stability parameters for the gas, but caution that such measurements in edge-on galaxies suffer from degeneracies. We estimate merger ages to be about 100~Myr based on the observed disturbances in the gas distribution. Furthermore, we determine that these galaxies lie, on average, two orders of magnitude below the Kennicutt-Schmidt relation for star-forming galaxies as well as below the relation for relaxed ETGs. We discuss potential dynamical processes responsible for this strong suppression of star formation surface density at fixed molecular gas surface density.