Spatially Resolved Chemistry in Nearby Galaxies III. Dense Molecular Gas in the Inner Disk of the LIRG IRAS 04296+2923

TL;DR

This study maps dense molecular gas in the inner disk of the nearby LIRG IRAS 04296+2923, revealing unique chemical properties, star formation activity, and early-stage starburst characteristics through millimeter interferometry observations.

Contribution

First detailed spatially resolved chemical analysis of dense molecular gas in IRAS 04296+2923, highlighting unusual isotopic ratios and starburst indicators.

Findings

Dense gas fuels high star formation rate in the nucleus.

Unusual CO isotopic line ratios suggest enrichment by starburst ejecta.

Starburst in IRAS 04296+2923 is in an early development stage.

Abstract

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.