Ultraviolet Emission from Stellar Populations within Tidal Tails: Catching the Youngest Galaxies in Formation?

New Galaxy Evolution Explorer (GALEX) observations have detected significant far-UV (FUV; 1530 Å) and near-UV (NUV; 2310 Å) emission from stellar substructures within the tidal tails of four ongoing galaxy mergers. The UV-bright regions are optically faint and are coincident with H I density enhancements. FUV emission is detected at any location where the H I surface density exceeds ~2 M☉ pc-2, and it is often detected in the absence of visible wavelength emission. UV luminosities of the brighter regions of the tidal tails imply masses of 106 to ~109 M☉ in young stars in the tails, and H I luminosities imply similar H I masses. UV-optical colors of the tidal tails indicate stellar populations as young as a few megayears, and in all cases ages under 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. This could be because the star formation occurs progressively along the tails, or because the star formation has been inhibited near the galaxy/tail interface. 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.