Extended Star Formation and Molecular Gas in the Tidal Arms near NGC 3077

Abstract

We report the detection of ongoing star formation in the prominent tidal arms near NGC 3077 (member of the M81 triplet). In total, 36 faint compact H II regions are identified, covering an area of ~4 × 6 kpc2. Most of the H II regions are found at H I column densities above 1 × 1021 cm-2 (on scales of 200 pc), well within the range of threshold columns measured in normal galaxies. The H II luminosity function resembles those derived for other low-mass dwarf galaxies in the same group; we derive a total star formation rate of 2.3 × 10-3 M⊙ yr-1 in the tidal feature. We also present new high-resolution imaging of the molecular gas distribution in the tidal arm using CO observations obtained with the Owens Valley Radio Observatory interferometer. We recover about one-sixth of the CO flux (or M ~ 2 × 106 M⊙, assuming a Galactic conversion factor) originally detected in the IRAM 30 m single-dish observations, indicating the presence of a diffuse molecular gas component in the tidal arm. The brightest CO peak in the interferometer map (comprising half of the detected CO flux) is coincident with one of the brightest H II regions in the feature. Assuming a constant star formation rate since the creation of the tidal feature (presumably ~3 × 108 yr ago), a total mass of ~7 × 105 M⊙ has been transformed from gas into stars. Over this period the star formation in the tidal arm has resulted in an additional enrichment of ΔZ > 0.002. The reservoir of atomic and molecular gas in the tidal arm is ~3 × 108 M⊙, allowing star formation to continue at its present rate for a Hubble time. Such widespread, low-level star formation would be difficult to image around more distant galaxies but may be detectable through intervening absorption in quasar spectra.