Molecular Gas and Star Formation in NGC 3077

Abstract

We present high-resolution (∼25) CO(1–0) and CO(2–1) images of the central 1 kpc of NGC 3077 made with the Owens Valley Millimeter Array. CO emission is distributed in three major complexes, which resolve into at least seven giant molecular clouds (GMCs). Two complexes are associated with the central starburst. A third, more distant complex is not associated with strong star formation. The GMCs are ∼70 pc in size and contain ∼106 M⊙ of molecular gas. The Galactic conversion factor appears applicable to NGC 3077, consistent with its solar metallicity. Galactic rotation in NGC 3077 is detected for the first time in the molecular gas. The molecular gas counterrotates with respect to the large-scale H I tidal bridge. The molecular clouds closest to the starburst have supervirial line widths, possibly related to the turbulence generated by the starburst. The 2.6 mm radio continuum flux indicates that thermal bremsstrahlung dominates the emission from the starburst region below 5 cm and that the NLyc ≃ 3.7 × 1052 s-1, or ∼3000 O7 stars, corresponds to a star formation rate of 0.4 M⊙ yr-1. At this rate, the amount of molecular gas can sustain star formation for only ∼10 Myr; thus, NGC 3077 is a true starburst galaxy. The derived age of the starburst is consistent with the inferred ages of the superbubbles, which suggests that the burst is much younger than the age of the M81–M82–NGC 3077 interaction. We suggest that it is caused by gas that was pulled out of NGC 3077 during the interaction with M81, raining back down onto the galaxy.