Molecular and atomic gas in dust lane early-type galaxies - I: Low
  star-formation efficiencies in minor merger remnants
  In this work we present IRAM-30m telescope observations of a sample of
bulge-dominated galaxies with large dust lanes, which have had a recent minor
merger. We find these galaxies are very gas rich, with H2 masses between 4x10^8
and 2x10^10 Msun. We use these molecular gas masses, combined with atomic gas
masses from an accompanying paper, to calculate gas-to-dust and gas-to-stellar
mass ratios. The gas-to-dust ratios of our sample objects vary widely (between
~50 and 750), suggesting many objects have low gas-phase metallicities, and
thus that the gas has been accreted through a recent merger with a lower mass
companion. We calculate the implied minor companion masses and gas fractions,
finding a median predicted stellar mass ratio of ~40:1. The minor companion
likely had masses between ~10^7 - 10^10 Msun. The implied merger mass ratios
are consistent with the expectation for low redshift gas-rich mergers from
simulations. We then go on to present evidence that (no matter which
star-formation rate indicator is used) our sample objects have very low
star-formation efficiencies (star-formation rate per unit gas mass), lower even
than the early-type galaxies from ATLAS3D which already show a suppression.
This suggests that minor mergers can actually suppress star-formation activity.
We discuss mechanisms that could cause such a suppression, include dynamical
effects induced by the minor merger.
