SPATIALLY RESOLVED CHEMISTRY IN NEARBY GALAXIES. III. DENSE MOLECULAR GAS IN THE INNER DISK OF THE LIRG IRAS 04296+2923

摘要

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 luminous infrared galaxies (LIRGs). Data are from the Owens Valley and CARMA millimeter interferometers. Species detected at 4'' resolution include C18O, HCN, HCO+, HNC, CN, CH3OH, 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 ☉?yr–1 and is traced by lines of HCN, HCO+, HNC, and CN. A correlation between HCN and star formation rate is observed on sub-kiloparsec 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)/C18O(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/C18O abundance ratios are lower than in Galactic clouds, possibly because the C18O is enriched by massive star ejecta from the starburst. 13CO is underabundant relative to CO. Extended emission from CH3OH 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.