Molecular and ionized gas in the tidal tail in Stephan's Quintet

We have mapped with the IRAM interferometer at Plateau de Bure (PdBI) the (CO)-C-12 emission towards intergalactic star forming regions located in the tidal tail stemming from NGC 7319, in the Stephan's Quintet compact group of galaxies. The (CO)-C-13 emission of the same region was observed with the IRAM 30 m telescope and optical spectroscopy of several H.. regions in the area were obtained with the Calar Alto 3.5 m telescope. We recovered with the interferometer about 50% of the (CO)-C-12(1-0) total emission that had been earlier measured with single dish observations (Lisenfeld 2002, A&A, 394, 823), indicating that about half of the molecular gas is distributed on spatial scales larger than about 10-15 (corresponding to 4-6 kpc) to which PdBI is not sensitive. We find two main areas of CO emission: (i) an elongated region towards the area known as SQ B where a Tidal Dwarf Galaxy could currently be forming and (ii) a barely resolved area at the tip of the optical tidal arm. Both regions follow dust lanes visible on HST images and their CO peak coincides spatially exactly with the maximum of the Ha line emission. In SQ B, there is furthermore very good kinematical agreement between the CO, Ha and HI components. We conclude from these coincidences that the gaseous matter found in quantities in the area is physically associated to the optical tidal tail and thus that the intergalactic atomic hydrogen there was expelled from NGC 7319. Its origin had previously been much debated. Furthermore, the relatively high oxygen abundances ( about solar) estimated from the optical spectra of the H.. regions imply that the gas feeding the star formation originated from the inner regions of the parent galaxy. In SQ B, we derive from different tracers a star formation rate, corrected for dust extinction-which is important in the area-of 0.5 M-circle dot/yr, i. e. one of the highest values so far measured outside galaxies. The inferred molecular gas consumption time of 0.5 Gyr lies in the range of values found for spiral and starburst galaxies. On the other hand, the ratio of (CO)-C-12/(CO)-C-13 > 25 is much higher than the values found in disks of spiral galaxies. A relatively low opacity for the (CO)-C-12 gas is the most likely reason.