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

We present new Hubble Space Telescope (HST) Wide Field Planetary Camera 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 (∼1011.6 L⊙). The F450W and F814W images resolve the AGN from the starburst and show that the bulk of the starlight (∼ 6 × 1010 M⊙) is contained within a central radius of about 600 pc 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 starburst may represent the trigger for the current nuclear activity. The multiple starburst ages seen in UN J1025−0040 and its companion galaxy indicate a complex interaction and star formation history.