AAS 199th meeting, Washington, DC, January 2002
Session 43. AGN - IR and Starbursts
Display, Tuesday, January 8, 2002, 9:20am-6:30pm, Monroe/Lincoln

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[43.03] Hubble Space Telescope Imaging of the Post-starburst Quasar UN J1025-0040: ``Post''-starburst No More?

M.A. Grabelsky (Rice University), M.S. Brotherton (National Optical Astronomy Observatory), G. Canalizo, W. Breugel (Lawrence Livermore National Laboratory), S.M. Croom (Anglo-Australian Telescope), A.V. Filippenko (Univerity of California-Berkeley), A. Stockton (University of Hawaii), R.J. Smith (Mt Stromlo and Siding Spring Observatories), B.J. Boyle (Anglo-Australian Telescope), L. Miller (Oxford University), T. Shanks (University of Durham)

We present new HST WFPC2 images of the post-starburst quasar UN J1025-0040, which contains both an AGN and a 400-Myr-old nuclear starburst of similar bolometric luminosity (~1011.6 solar luminosities). The F450W and F814W images fail to clearly separate the starburst and quasar components, but do show that the bulk of the stars (7 x 1010 solar masses) are contained within a central radius of about 600 parsecs. Equating the point-source light in each image with the AGN contribution, we determined the ratio of AGN-to-starburst light. This ratio is 68% 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.

Matthew Grabelsky acknowledges support from the NOAO/KPNO Research Experiences for Undergraduates program funded by the National Science Foundation. Support for proposal GO-08703 was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

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