Hubble Space Telescope Imaging of the Post-Starburst Quasar UN J1025−0040: Evidence for Recent Star Formation

We present new Hubble Space Telescope (HST) WFPC2 images of the poststarburst quasar UN J1025−0040, which contains both an active galactic nucleus (AGN) and a 400-Myr-old nuclear starburst of similar bolometric luminosity ( ∼ 10 11.6 L⊙). The F450W and F814W images resolve the AGN from the starburst and show that the bulk of the star light ( ∼ 6 × 10 10 M⊙) is contained within a central radius of about 600 parsecs, and lacks clear morphological structures at this scale. Equating the point-source light in each image with the AGN contribution, we determined the ratio of AGN-to-starburst light. This ratio is 69 % in the red F814W image, consistent with our previous spectral analysis, but ≤ 50 % in the blue F450W image whereas we had predicted 76%. The HST images are consistent with previous photometry, 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 stellar population present, 40 Myr or younger, with as much as 10 % of the mass of the dominant 400-Myr-old population. This younger