Ultraviolet emission and star formation in Stephan's Quintet

We present the first Galaxy Evolution Explorer (GALEX) UV images of the well-known interacting group of galaxies, Stephan's Quintet (SQ). We detect widespread UV emission throughout the group. However, there is no consistent coincidence between UV structure and emission in the optical, Hα, or H I. Excluding the foreground galaxy NGC 7320 (Sd), most of the UV emission is found in regions associated with the two spiral members of the group, NGC 7319 and NGC 7318b, and the intragroup medium starburst SQ-A. The extinction-corrected UV data are analyzed to investigate the overall star formation activity in SQ. We find that the total star formation rate (SFR) of SQ is 6.69 ± 0.65 M_⊙ yr^-1. Of this, 1.34 ± 0.16 M _⊙ yr^-1 is due to SQ-A. This is in excellent agreement with that derived from the extinction-corrected Hα luminosity of SQ-A. The SFR in regions related to NGC 7319 is 1.98 ± 0.58 M _⊙ yr^-1, most of which (68%) is contributed by the disk. The contribution from the "young tail" is only 15%. In the UV, the young tail is more extended (∼100 kpc) and shows a looplike structure, including the optical tail, the extragalactic H II regions recently discovered in Hα, and other UV emission regions discovered for the first time. The UV and optical colors of the "old tail" are consistent with a single stellar population of age t ≃ 10^8.5±0.4 yr. The UV emission associated with NGC 7318b is found in a very large (∼80 kpc) disk, with a net SFR of 3.37 ± 0.25 M_⊙ yr^-1. Several large UV emission regions are 30-40 kpc away from the nucleus of NGC 7318b. Although both NGC 7319 and NGC 7318b show peculiar UV morphology, their SFR is consistent with that of normal Sbc galaxies, indicating that the strength of star formation activity is not enhanced by interactions.