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