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

¹ Instituto de Astrofísica de Andalucía, CSIC, Apdo. 3004, 18040 Granada, Spain
² Dept. de Física Teórica y del Cosmos, Universidad de Granada, Granada, Spain
³ Observatoire de Bordeaux, UMR 5804, CNRS/INSU, BP 89, 33270 Floirac, France
⁴ CNRS URA 2052 and CEA/DSM/DAPNIA Service d'Astrophysique, Saclay, 91191 Gif-sur-Yvette Cedex, France
⁵ INAOE, Apdo. Postal 51 & 216, Puebla, Pue 72000, Mexico
⁶ Cornell University, Astronomy Department, Ithaca, NY 14853, USA
⁷ Chercheur Associé, Observatoire de Paris, LERMA, 61 Av. de l'Observatoire, 75014 Paris, France

Received: 21 May 2004
Accepted: 7 July 2004

Abstract

We have mapped with the IRAM interferometer at Plateau de Bure (PdBI) the ¹²CO 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 ¹³CO emission of the same region was observed with the IRAM 30 m telescope and optical spectroscopy of several Hii regions in the area were obtained with the Calar Alto 3.5 m telescope.We recovered with the interferometer about 50% of the ¹²CO(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 Hα line emission. In SQ B, there is furthermore very good kinematical agreement between the CO, Hα 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 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 /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 ¹²CO/¹³CO > 25 is much higher than the values found in disks of spiral galaxies. A relatively low opacity for the ¹²CO gas is the most likely reason.