Interacting Galaxies and Tidal tails
Neff et al.  report the detection of significant FUV and NUV emission from stellar substructures within the tidal tails of four on-going galaxy mergers: ARP 157, ARP 295, NGC 5713/19 and NGC 7769/71. The UV-bright regions are optically faint and are coincident with HI density enhancements. FUV emission is detected at any location where the HI surface density exceeds ~ 2 M pc-2, and is often detected in the absence of any visible wavelength emission. One example of UV tails and bridges, taken from their paper, is shown in Figure 8. UV luminosities of the brighter regions of the tidal tails imply masses of 106 M up to ~ 109 M in young stars in the tails, and HI luminosities imply similar HI masses. UV-optical colors of the tidal tails indicate stellar populations as young as a few Myr, and in all cases ages < 400 Myr. Most of the young stars in the tails formed in single bursts rather than resulting from continuous star formation, and they formed in situ as the tails evolved. Star formation appears to be older near the parent galaxies and younger at increasing distances from the parent galaxy. The youngest stellar concentrations, usually near the ends of long tidal tails, have masses comparable to confirmed tidal dwarf galaxies and may be newly forming galaxies undergoing their first burst of star formation.
Figure 8. An example of the tidal tails discussed in Neff et al. . In this case, the interacting system Arp 295 nicely illustrates the UV clumps, tails and bridges being found by GALEX in the outer regions of merging systems.
A more detailed study of GALEX observations of an interacting galaxy, the Antennae, has been undertaken by Hibbard et al. . The tidal UV morphology seen there is remarkably similar to the tidal neutral hydrogen gas, with a close correspondence between regions of bright UV emission and high HI column density. There are interesting features in the UV morphology of the inner regions. The FUV light in the south-western half of the disk of NGC 4039 is sharply truncated, coincident with a similar truncation in the HI disk. This gas may have been removed by the X-ray loops recently imaged in this system by Chandra, inhibiting subsequent star formation. On a larger scale, there is a FUV "halo", possibly related to shocked gas in the X-ray halo.
While it has long been known that tails are generally blue, the GALEX observation provide color baselines to determine how much of the UV radiation comes from stars younger than the dynamical age of the tails. The preliminary analysis by Hibbard et al.  suggests that most of the stars within the tidal tails pre-date the tail formation period. The population in the northern tail appears older than that in the southern tail. Examining individual UV-bright regions within the tails they find regions of more recent star formation, which occur at regions of higher HI column density, and extend well beyond the previously identified tail star forming regions. Such tails are providing promising laboratories for the study of star formation outside of the usual disk environment, and thereby testing general theories of star and cluster formation.
I want to thank Cristina Popescu and Richard Tuffs for the opportunity to report at their meeting on early results from the GALEX mission. And I thank all of my colleagues on the GALEX science team for their contributions to this interim review in specific and to the overall scientific success of the mission to date. I have shamelessly begged, borrowed and stolen images, plots, ideas and text from the many papers so far published by the Team; I hope that proper attribution is evident in this overview. A complete collection of papers resulting from the first round of publishing by the GALEX Team are slated to appear in a dedicated volume of the Astrophysical Journal (Letters) early in 2005; and they can also be found in their entirety at http://www.galex.caltech.edu/PUBLICATIONS/.
The entire GALEX Team gratefully acknowledges NASA's support for construction, operation, and science analysis for the GALEX mission, developed in corporation with the Centre National d'Etudes Spatiales of France and the Korean Ministry of Science and Technology. The grism, imaging window, and uncoated aspheric corrector were supplied by France. We acknowledge the dedicated team of engineers, technicians, and administrative staff from JPL/Caltech, Orbital Sciences Corporation, University of California, Berkeley, Laboratory Astrophysique Marseille, and the other institutions who made this mission possible. BFM was supported by the Observatories of Carnegie Institution of Washington and by the NASA/IPAC Extragalactic Database (NED).