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

Abstract

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 (~10^11.6 L_solar). The F450W and F814W images resolve the AGN from the starburst and show that the bulk of the starlight (~6×10¹⁰ M_solar) 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.