2006AJ....132.2289W

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 ∼4x6 kpc². Most of the H II regions are found at H I column densities above 1x10²¹/cm2 (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 2.3x10^–3 M_☉/yr 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_H2∼2x10⁶ 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 ∼3x10⁸ yr ago), a total mass of ∼7x10⁵ 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 ∼3x10⁸ 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.