The State of the Molecular Gas in Post-Starburst Galaxies
  The molecular gas in galaxies traces both the fuel for star formation and the
processes that can either enhance or suppress star formation. Observations of
the molecular gas state can thus point to when and why galaxies stop forming
stars. In this study, we present ALMA observations of the molecular gas in
galaxies evolving through the post-starburst phase. These galaxies have low
current star formation rates, regardless of the SFR tracer used, yet their
optical spectra show evidence for recent bursts of star formation that have
ended within the last 600 Myr. We present CO (3--2) observations for three
post-starburst galaxies, and dense gas HCN/HCO$^+$/HNC (1--0) observations for
six (four new) post-starburst galaxies. The post-starbursts have low excitation
as traced by the CO spectral line energy distribution (SLED) up to CO (3--2),
more similar to early-type than starburst galaxies. The low excitation
indicates that lower density rather than high temperatures may suppress star
formation during the post-starburst phase. One galaxy displays a blueshifted
molecular gas outflow traced by CO (3--2). MaNGA observations show that the
ionized gas velocity is disturbed relative to the stellar velocity field, with
a blueshifted component aligned with the molecular gas outflow, suggestive of a
multiphase outflow. Low ratios of HCO$^+$/CO, indicating low fractions of dense
molecular gas relative to the total molecular gas, are seen throughout
post-starburst phase, except for the youngest post-starburst galaxy considered
here.
