First views of a nearby LIRG: Star formation and molecular gas in IRAS 04296+2923

We present a first look at the local luminous infrared galaxy (LIRG) IRAS 04296+2923. This barred spiral galaxy, overlooked because of its location behind the Taurus molecular cloud, is among the half dozen closest (D = 29 Mpc) 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 β images, Very Large Array continuum maps from λ = 20-1.3 cm, a subarcsecond Keck Long Wavelength Spectrometer image at 11.7 μm and Owens Valley Millimeter Array CO(1-0), ¹³CO(1-0), and 2.7 mm continuum images. The J-band image reveals a symmetric barred spiral galaxy. Two bright, compact mid-infrared and radio sources in the nucleus mark a starburst that is energetically equivalent to ∼ 10⁵ O7 stars, separated by <50 pc. This is probably a pair of young super star clusters, with estimated stellar masses of ∼10⁷ M⊙ each. The nuclear starburst is forming stars at the rate of ∼12 ± 6 M⊙ yr ^-1, or about half of the total star formation rate for the galaxy of ∼25 ± 10M⊙yr^-1. IRAS 04296+2923 is very bright in CO, and among the most gas-rich galaxies in the local universe. The ¹²CO luminosity of the inner half kpc is equivalent to that of the entire Milky Way. While the most intense CO emission is extended over a 15″ (2 kpc) diameter region, the nuclear starburst is confined to within 1″-2″ (150-250 pc) of the dynamical center. Based on masses obtained with ¹³CO, we find that the CO conversion factor in the nucleus is lower than the Galactic value, X_CO^Gal by a factor of three to four, typical of gas-rich spiral nuclei. The nuclear star formation efficiency (SFE) is ^nucM_gas/SFR^nuc = 2.7 × 10 ^-8yr^-1, corresponding to a gas consumption timescale, τ_SF^nuc ∼ 4 × 10⁷ yr. The SFE is 10 times lower in the disk, with τ_SF^disk∼ 3.3 × 10⁸ yr, The low absolute SFE 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. IRAS 04296+2923 appears to be beginning a 100 Myr period as an LIRG, during which it will turn much of its 6 × 10⁹ M⊙ of molecular gas into a nuclear cluster of stars.