Cold Molecular Gas in Merger Remnants. II. The properties of dense
  molecular gas
  We present the 3 mm wavelength spectra of 28 local galaxy merger remnants
obtained with the Large Millimeter Telescope. Fifteen molecular lines from 13
different molecular species and isotopologues were identified, and 21 out of 28
sources were detected in one or more molecular lines. On average, the line
ratios of the dense gas tracers, such as HCN (1-0) and HCO$^{+}$(1-0), to
$^{13}$CO (1-0) are 3-4 times higher in ultra/luminous infrared galaxies
(U/LIRGs) than in non-LIRGs in our sample. These high line ratios could be
explained by the deficiency of $^{13}$CO and high dense gas fractions suggested
by high HCN (1-0)/$^{12}$CO (1-0) ratios. We calculate the IR-to-HCN (1-0)
luminosity ratio as a proxy of the dense gas star formation efficiency. There
is no correlation between the IR/HCN ratio and the IR luminosity, while the
IR/HCN ratio varies from source to source (1.1-6.5) $\times 10^{3}$
$L_{\odot}$/(K km s$^{-1}$ pc$^{2}$). Compared with the control sample, we find
that the average IR/HCN ratio of the merger remnants is higher by a factor of
2-3 than those of the early/mid-stage mergers and non-merging LIRGs, and it is
comparable to that of the late-stage mergers. The IR-to-$^{12}$CO (1-0) ratios
show a similar trend to the IR/HCN ratios. These results suggest that star
formation efficiency is enhanced by the merging process and maintained at high
levels even after the final coalescence. The dynamical interactions and mergers
could change the star formation mode and continue to impact the star formation
properties of the gas in the post-merger phase.
