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

Abstract

We have mapped with the IRAM interferometer at Plateau de Bure (PdBI) the 12 CO 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 13 CO emission of the same region was observed with the IRAM 30m telescope and optical spectroscopy of several Hii regions in the area were obtained with the Calar Alto 3.5m telescope.We recovered with the interferometer about 50% of the 12 CO(1-0) total emission that had been earlier measured with single dish observations (Lisenfeld et al. 2002), 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 (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 Hline emission. In SQ B, there is furthermore very good kinematical agreement between the CO, Hand 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 Hii 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⊙/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 12 CO/ 13 CO > 25 is much higher than the values found in disks of spiral galaxies. A relatively low opacity for the 12 CO gas is the most likely reason.