Published in The Astronomical Journal, Volume 144, Issue 2, August 1, 2012.
The definitive version is available at https://doi.org/10.1088/0004-6256/144/2/66.
We present Hubble Space Telescope imaging and spectroscopy, along with supporting Galaxy Evolution Explorer and ground-based data, for the extended high-ionization cloud known as Hanny's Voorwerp, near the spiral galaxy IC 2497. Wide Field Camera 3 images show complex dust absorption near the nucleus of IC 2497. The galaxy core in these data is, within the errors, coincident with the very long baseline interferometry core component marking the active nucleus. Space Telescope Imaging Spectrograph (STIS) optical spectra show the active galactic nucleus (AGN) to be a type 2 Seyfert galaxy of rather low luminosity. The derived ionization parameter log U = –3.5 is in accordance with the weak X-ray emission from the AGN. We find no high-ionization gas near the nucleus, adding to the evidence that the AGN is currently at a low radiative output (perhaps with the central black hole having switched to a mode dominated by kinetic energy). The nucleus is accompanied by an expanding ring of ionized gas 500 pc in projected diameter on the side opposite Hanny's Voorwerp. Where sampled by the STIS slit, this ring has Doppler offset 300 km s–1 from the nucleus, implying a kinematic age × 105 years. Narrowband [O III] and Hα+[N II] Advanced Camera for Surveys images show fine structure in Hanny's Voorwerp, including limb-brightened sections suggesting modest interaction with a galactic outflow and small areas where Hα is strong. We identify these latter regions as regions ionized by recent star formation, in contrast to the AGN ionization of the entire cloud. These candidate "normal" H II regions contain blue continuum objects, whose colors are consistent with young stellar populations; they appear only in a 2 kpc region toward IC 2497 in projection, perhaps meaning that the star formation was triggered by compression from a narrow outflow. The ionization-sensitive ratio [O III]/Hα shows broad bands across the object at a skew angle to the galaxy nucleus, and no discernible pattern near the prominent "hole" in the ionized gas. The independence of ionization and surface brightness (SB) suggests that there is substantial spatial structure which remains unresolved, to such an extent that the SB samples the number of denser filaments rather than the characteristic density in emission regions; this might be a typical feature of gas in tidal tails, currently measurable only when such gas is highly ionized. These results fit with our picture of an ionization echo from an AGN whose ionizing luminosity has dropped by a factor >100 (and possibly much more) within the last (1-2) × 105 years; we suggest a tentative sequence of events in IC 2497 and discuss implications of such rapid fluctuations in luminosity for our understanding of AGN demographics.
2012 IOP Science.
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