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Title:
The Galaxy Zoo survey for giant AGN-ionized clouds: past and present black hole accretion events
Authors:
Keel, William C.; Chojnowski, S. Drew; Bennert, Vardha N.; Schawinski, Kevin; Lintott, Chris J.; Lynn, Stuart; Pancoast, Anna; Harris, Chelsea; Nierenberg, A. M.; Sonnenfeld, Alessandro; Proctor, Richard
Affiliation:
AA(Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL 35487, USA; SARA Observatory, Physics and Space Sciences Department, Florida Institute of Technology, Melbourne, FL 32901, USA), AB(SARA Observatory, Physics and Space Sciences Department, Florida Institute of Technology, Melbourne, FL 32901, USA; Texas Christian University, Forth Worth, TX 76129, USA), AC(Department of Physics, University of California, Santa Barbara, CA 93106, USA; Department of Physics, California Polytechnic State University, San Luis Obispo, CA 93407, USA), AD(Department of Physics, Yale University, New Haven, CT 06511, USA; Yale Center for Astronomy and Astrophysics, Yale University, PO Box 208121, New Haven, CT 06520, USA), AE(Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH; Adler Planetarium, 1300 S. Lakeshore Drive, Chicago, IL 60605, USA), AF(Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH), AG(Department of Physics, University of California, Santa Barbara, CA 93106, USA), AH(Department of Physics, University of California, Santa Barbara, CA 93106, USA), AI(Department of Physics, University of California, Santa Barbara, CA 93106, USA), AJ(Department of Physics, University of California, Santa Barbara, CA 93106, USA), AK(Waveney Consulting, Wimborne, Dorset BH21 3QY)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 420, Issue 1, pp. 878-900. (MNRAS Homepage)
Publication Date:
02/2012
Origin:
WILEY
Astronomy Keywords:
galaxies: active, galaxies: ISM, galaxies: Seyfert
Abstract Copyright:
© 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS
DOI:
10.1111/j.1365-2966.2011.20101.x
Bibliographic Code:
2012MNRAS.420..878K

Abstract

Some active galactic nuclei (AGN) are surrounded by extended emission-line regions (EELRs), which trace both the illumination pattern of escaping radiation and its history over the light travel time from the AGN to the gas. From a new set of such EELRs, we present evidence that the AGN in many Seyfert galaxies undergo luminous episodes 0.2-2 ? years in duration. Motivated by the discovery of the spectacular nebula known as Hanny's Voorwerp, ionized by a powerful AGN which has apparently faded dramatically within ? years, Galaxy Zoo volunteers have carried out both targeted and serendipitous searches for similar emission-line clouds around low-redshift galaxies. We present the resulting list of candidates and describe spectroscopy identifying 19 galaxies with AGN-ionized regions at projected radii ? kpc. This search recovered known EELRs (such as Mrk 78, Mrk 266 and NGC 5252) and identified additional previously unknown cases, one with detected emission to ? kpc. One new Sy 2 was identified. At least 14/19 are in interacting or merging systems, suggesting that tidal tails are a prime source of distant gas out of the galaxy plane to be ionized by an AGN. We see a mix of one- and two-sided structures, with observed cone angles from 23? to 112?. We consider the energy balance in the ionized clouds, with lower and upper bounds on ionizing luminosity from recombination and ionization-parameter arguments, and estimate the luminosity of the core from the far-infrared data. The implied ratio of ionizing radiation seen by the clouds to that emitted by the nucleus, on the assumption of a non-variable nuclear source, ranges from 0.02 to ?; 7/19 exceed unity. Small values fit well with a heavily obscured AGN in which only a small fraction of the ionizing output escapes to be traced by surrounding gas. However, large values may require that the AGN has faded over tens of thousands of years, giving us several examples of systems in which such dramatic long-period variation has occurred; this is the only current technique for addressing these time-scales in AGN history. The relative numbers of faded and non-faded objects we infer, and the projected extents of the ionized regions, give our estimate (0.2-2 ? years) for the length of individual bright phases.
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