2000A&A...361..500H

Using the IRAM 30m radio telescope we have mapped the tidal arm feature south-east of NGC 3077 where we recently detected molecular gas in the CO (J=1-0) and (2-1) transitions. We find that the molecular gas is much more extended than previously thought (several kpc). The CO emission can be separated into at least 3 distinct complexes with equivalent radii between 250pc and 700pc and all well confined over a narrow range in velocity - the newly detected complexes therefore range among the largest molecular complexes in the local universe. For one complex we have also obtained a CO (3-2) spectrum using the KOSMA 3m radio telescope; utilizing an LVG model we find that the kinetic temperature for this complex must be about 10K, and the H₂ volume density between 600 and 10000cm^–3. Mass estimates based on virialization yield a total mass for the complexes of order 2-4x10⁷M_☉, i.e. more than the estimated molecular mass within NGC 3077 itself. This implies that interactions between galaxies can efficiently remove heavy elements and molecules from a galaxy and enrich the intergalactic medium. A comparison of the distribution of HI and CO shows no clear correlation. However, CO is only found in regions where the HI column density exceeds 1.1x10²¹cm^–2. HI masses for the molecular complexes mapped are of the same order as the corresponding molecular masses. Because the intergalactic pressure is most likely too low to confine the complexes we conclude that they are gravitationally bound. Since the tidal arm with its molecular complexes has all the ingredients to form stars in the future, we are thus presumably witnessing the birth of a dwarf galaxy. This process could be important for the formation of dwarf galaxies especially at larger look-back times in the universe where galaxy interactions may have been more frequent.