Computer Science – Graphics
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21714208k&link_type=abstract
American Astronomical Society, AAS Meeting #217, #142.08; Bulletin of the American Astronomical Society, Vol. 43, 2011
Computer Science
Graphics
Scientific paper
Perhaps the signature discovery of the Galaxy Zoo citizen-science project has been Hanny's Voorwerp, high-ionization cloud extending 45 kpc from the spiral galaxy IC 2497. It must be ionized by a luminous AGN, either deeply obscured or having dimmed dramatically within 200,000 years. We explore this system using HST imaging and spectroscopy. The disk of IC 2497 is warped, with complex dust absorption near the nucleus; the near-IR peak coincides closely with the VLBI core marking the AGN. STIS spectra show the AGN as a low-luminosity LINER, with ionization parameter log U= -3.5, matching its weak X-ray emission. The nucleus is accompanied by an expanding loop of ionized gas 500 pc in diameter, opposite Hanny's Voorwerp. The loop's Doppler span 300 km/s implies kinematic age < 700,000 years. We find no high-ionization gas near the core, further evidence that the AGN is seen at a low radiative output (perhaps now dominated by kinetic energy). [O III] and Hα +[N II] ACS images show fine structure in Hanny's Voorwerp, including limb-brightened sections suggesting modest interaction with a galactic outflow. We identify small regions ionized by recent star formation, unlike the AGN ionization of the overall cloud. These H II regions contain blue continuum objects, consistent with young stellar populations; these occur where projected closest to IC 2497, perhaps meaning that the star formation was triggered by compression from an outflow. The ionization-sensitive [O III]/Hα ratio shows broad bands across the object, and no discernible pattern with emission-line structures or near the prominent "hole" in the ionized gas. These results fit with our picture of an ionization echo from an AGN whose ionizing luminosity has dropped by a factor >100 within the last 200,000 years. Such rapid fluctuations in luminosity could alter our understanding of AGN demographics. Supported by NASA/STScI.
Bennert V.
Chojnowski Drew S.
Galaxy Zoo Team
Keel William C.
Lintott Chris
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