Hubble Space TelescopeImaging of the Poststarburst Quasar UN J1025−0040: Evidence for Recent Star Formation

We present new Hubble Space Telescope (HST) WFPC2 images of thepost-starburst quasar UN J1025-0040, which contains both an active galacticnucleus (AGN) and a 400-Myr-old nuclear starburst of similar bolometricluminosity (10^{11.6} solar luminosities). The F450W and F814W images resolvethe AGN from the starburst and show that the bulk of the star light (6 x10^{10} solar masses) is contained within a central radius of about 600parsecs, and lacks clear morphological structures at this scale. Equating thepoint-source light in each image with the AGN contribution, we determined theratio of AGN-to-starburst light. This ratio is 69% in the red F814W image,consistent with our previous spectral analysis, but about 50% in the blue F450Wimage whereas we had predicted 76%. The HST images are consistent with previousphotometry, ruling out variability (a fading AGN) as a cause for this result.We can explain the new data if there is a previously unknown young stellarpopulation present, 40 Myr or younger, with as much as 10% of the mass of thedominant 400-Myr-old population. This younger starburst may represent thetrigger for the current nuclear activity. The multiple starburst ages seen inUN J1025-0040 and its companion galaxy indicate a complex interaction andstar-formation history.Comment: 10 pages, 6 figures (3 in color), accepted by PAS