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

We report the detection of ongoing star formation in the prominent tidal armsnear NGC 3077 (member of the M 81 triplet). In total, 36 faint compact HIIregions were identified, covering an area of ~4x6 kpc^2. Most of the HIIregions are found at HI column densities above 1x10^21 cm^-2 (on scales of 200pc), well within the range of threshold columns measured in normal galaxies.The HII luminosity function resembles the ones derived for other low-mass dwarfgalaxies in the same group; we derive a total star formation rate of 2.6x10^-3M_sun/yr in the tidal feature. We also present new high-resolution imaging ofthe molecular gas distribution in the tidal arm using CO observations obtainedwith the OVRO interferometer. We recover about one sixth of the CO flux (orM_H2~2x10^6 M_sun, assuming a Galactic conversion factor) originally detectedin the IRAM 30m single dish observations, indicating the presence of a diffusemolecular gas component in the tidal arm. The brightest CO peak in theinterferometer map (comprising half of the detected CO flux) is coincident withone of the brightest HII regions in the feature. Assuming a constant starformation rate since the creation of the tidal feature (presumably ~3x10^8years ago), a total mass of ~7x10^5 M_sun has been transformed from gas intostars. Over this period, the star formation in the tidal arm has resulted in anadditional enrichment of Delta(Z)>0.002. The reservoir of atomic and moleculargas in the tidal arm is ~3x10^8 M_sun, allowing star formation to continue atits present rate for a Hubble time. Such wide-spread, low-level star formationwould be difficult to image around more distant galaxies but may be detectablethrough intervening absorption in quasar spectra.Comment: Accepted for publication in the Astronomical Journa