2010AJ....140.1294M

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, or about half of the total star formation rate for the galaxy of ∼25±10 M_☉/yr. 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 ^Gal_CO by a factor of three to four, typical of gas-rich spiral nuclei. The nuclear star formation efficiency (SFE) is ^nucM_gas/SFR^nuc = 2.7x10^–8/yr, corresponding to a gas consumption timescale, τ^nuc _SF∼ 4x10⁷ yr. The SFE is 10 times lower in the disk, with τ^disk_SF∼ 3.3x10⁸ 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 6x10⁹ M_☉ of molecular gas into a nuclear cluster of stars.