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