Enhanced Dense Gas Fraction in Ultra-Luminous Infrared Galaxies
  We present a detailed analysis of the relation between infrared luminosity
and molecular line luminosity, for a variety of molecular transitions, using a
sample of 34 nearby galaxies spanning a broad range of infrared luminosities
(10^{10} < L_{IR} < 10^{12.5} L_sun). We show that the power-law index of the
relation is sensitive to the critical density of the molecular gas tracer used,
and that the dominant driver in observed molecular line ratios in galaxies is
the gas density. As most nearby ultraluminous infrared galaxies (ULIRGs)
exhibit strong signatures of active galactic nuclei (AGN) in their center, we
revisit previous claims questioning the reliability of HCN as a probe of the
dense gas responsible for star formation in the presence of AGN. We find that
the enhanced HCN(1-0)/CO(1-0) luminosity ratio observed in ULIRGs can be
successfully reproduced using numerical models with fixed chemical abundances
and without AGN-induced chemistry effects. We extend this analysis to a total
of ten molecular line ratios by combining the following transitions: CO(1-0),
HCO+(1-0), HCO+(3-2), HCN(1-0), and HCN(3-2). Our results suggest that AGNs
reside in systems with higher dense gas fraction, and that chemistry or other
effects associated with their hard radiation field may not dominate (NGC 1068
is one exception). Galaxy merger could be the underlying cause of increased
dense gas fraction and the evolutionary stage of such mergers may be another
determinant of the HCN/CO luminosity ratio.
