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 luminous infrared galaxies (LIRGs). Data are from the Owens Valley and CARMA millimeter interferometers. Species detected at ≲ 4'' resolution include C 18 O, HCN, HCO + , HNC, CN, CH 3 OH, and, tentatively, HNCO. Along with existing CO, 13 CO, 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)/ 13 CO(1–0) and CO(1–0)/C 18 O(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). 13 CO/C 18 O abundance ratios are lower than in Galactic clouds, possibly because the C 18 O is enriched by massive star ejecta from the starburst. 13 CO is underabundant relative to CO. Extended emission from CH 3 OH 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.