Spatially Resolved Chemistry in Nearby Galaxies III. Dense Molecular Gas
  in the Inner Disk of the LIRG IRAS 04296+2923
  We present a survey of 3 mm molecular lines in IRAS 04296+2923, one of the
brightest known molecular-line emitting galaxies, and one of the closest LIRGs.
Data are from the Owens Valley and CARMA millimeter interferometers. Species
detected at ~<4" resolution include C^18O, HCN, HCO+, HNC, CN, CH_3OH and,
tentatively, HNCO. Along with existing CO, ^13CO and radio continuum data,
these lines constrain the chemical properties of the inner disk. Dense
molecular gas in the nucleus fuels a star formation rate ~>10 M_sun/yr and is
traced by lines of HCN, HCO+, HNC, and CN. A correlation between HCN and star
formation rate is observed on sub-kpc scales, consistent with global relations.
Toward the nucleus, CN abundances are similar to those of HCN, indicating
emission comes from a collection (~40-50) of moderate visual extinction,
photon-dominated region clouds. The CO isotopic line ratios are unusual:
CO(1-0)/^13CO(1-0) and CO(1-0)/C^18O(1-0) line ratios are large toward the
starburst, as is commonly observed in LIRGs, but farther out in the disk these
ratios are remarkably low (~<3). ^13CO/C^18O abundance ratios are lower than in
Galactic clouds, possibly because the C^18O is enriched by massive star ejecta
from the starburst. ^13CO is underabundant relative to CO. Extended emission
from CH_3OH indicates that dynamical shocks pervade both the nucleus and the
inner disk. The unusual CO isotopologue ratios, the CO/HCN intensity ratio
versus L_IR, the HCN/CN abundance ratio and the gas consumption time versus
inflow rate, all indicate that the starburst in IRAS 04296+2923 is in an early
stage of development.
