HST/WFPC2 observations of warm ultraluminous infrared galaxies

We present new high-resolution B-and I-band images of a nearly complete sample of nine "warm" (f(25)/f(60) > 0.2), ultraluminous infrared galaxies (ULIGs) obtained with the Wide Field Planetary Camera of the Hubble Space Telescope (HST). The HST images clearly reveal the presence of tidal tails and other features associated with merging galaxies. All of the warm ULIGs show evidence of complex structures such as dust lanes and spiral features in their inner few kiloparsecs. Additionally, they show compact, blue "knots" of star formation (between 4 and 31 knots per object) that appear similar to those seen in more nearby merger systems. Spectral synthesis modeling is used to estimate mean upper age limits and masses: the median upper age limit for the knots in individual galaxies is similar to 3 x 10(8) yr (ranging from similar to 10(7) to 1 x 10(9) yr), and the range of knot masses is similar to 10(5)-10(9) M.. We also argue that these starburst knots cannot be significant contributors to the extremely high bolometric luminosity of these galaxies. Additionally, each object contains one or two knots whose luminosity and color are implausible in terms of star formation; we identify these as putative active nuclei. These observations are consistent with the hypothesis that warm ULIGs may represent a critical transition stage in the evolution of ULIGs into optical quasi-stellar objects.