Imaging of ultraluminous infrared galaxies in the near-ultraviolet

We present the first ground-based U' (lambda(central) = 3410 Angstrom, Delta(lambda) = 320 Angstrom) images of ultraluminous infrared galaxies (ULIGs). Two samples were observed: (1) "warm" ULIGs with mid-infrared colors characteristic of active galactic nuclei (f(25 mu m)/f(60 mu m) > 0.2), which are believed to represent a critical transition phase between cooler ULIGs and optically selected QSOs according to a previously proposed evolutionary model, and (2) the complementary "cool" ULIGs (f(25 mu m)/f(60 mu m) < 0.2), which in the evolutionary model are the progenitors of warm ULIGs and which have many characteristics associated with active star formation. Although in some cases there is also emission identified with an active galactic nucleus, the U' emission originates primarily in massive young stars and as such allows a direct examination of the sites of recent high-mass star formation. Strong U' emission (median total M-U = -20.8) is seen in all systems, and in some cases the extended tidal features (both the smooth stellar distribution and compact star-forming features) contribute up to 60%-80% of the total flux. The star-forming regions in both samples are found to have ages based on spectral synthesis models in the range 10-100 Myr, and most differences in color between them can be attributed to the effects of dust reddening. Additionally, it is found that star formation in compact knots in the tidal tails is most prominent in those ULIGs that have double nuclei, suggesting that the star formation rate in the tails peaks prior to the actual coalescence of the galaxy nuclei and diminishes quickly thereafter. Similar to results at other wavelengths, the observed star formation at U' can account for only a small fraction of the known bolometric luminosity of the ULIGs. Azimuthally averaged radial light profiles at U' are characterized by a SBrsic law with index n = 2, which is intermediate between an exponential disk and an r(-1/4) law and closely resembles the latter at large radii. The implications of this near-ultraviolet imaging for optical/near-infrared observations of high-redshift counterparts of ULIGs are discussed.