First Views of a Nearby LIRG: Star Formation and Molecular Gas in IRAS 04296+2923

Abstract

We present a first look at the local LIRG, IRAS04296+2923. This barred spiral, overlooked because of its location in the Galactic plane, is among the half dozen closest LIRGs. More IR-luminous than either M82 or the Antennae, it may be the best local example of a nuclear starburst caused by bar-mediated secular evolution. We present Palomar J and Pa beta images, VLA maps from 20-1.3cm, a Keck LWS image at 11.7mic and OVRO CO(1-0) and ^13CO(1-0), and 2.7 mm continuum images. The J-band image shows a symmetric barred spiral. Two bright, compact mid-IR/radio sources in the nucleus comprise a starburst that is equivalent to 10^5 O7 stars, probably a pair of young super star clusters separated by 30pc. The nuclear starburst is forming stars at the rate of ~12Msun/yr, half of the total star formation rate for the galaxy of ~25Msun/yr. IRAS04296 is bright in CO, and among the most gas-rich galaxies in the local universe. The CO luminosity of the inner half kpc is equivalent to that of the entire Milky Way. While the most intense CO emission extends over a 15"(2 kpc) region, the nuclear starburst is confined to ~1-2"(150-250 pc) of the dynamical center. From ^13CO, we find that the CO conversion factor in the nucleus is higher than the Galactic value by a factor 3-4, typical of gas-rich spiral nuclei. The nuclear star formation efficiency is M_gas/SFR^nuc = 2.7x10^-8 yr^-1, corresponding to gas consumption timescale, tau_SF^nuc~4x10^7 yrs. The star formation efficiency is ten times lower in the disk, tau_SF^disk~3.3x10^8 yrs. The low absolute star formation efficiency in the disk implies that the molecular gas is not completely consumed before it drifts into the nucleus, and is capable of fueling a sustained nuclear starburst. IRAS04296 is beginning a 100Myr period as a LIRG, during which it will turn much of its 6x10^9Msun of molecular gas into a nuclear cluster of stars. (abridged)