Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Volume 420, Issue 1, Pages 878-900Publisher
OXFORD UNIV PRESS
DOI: 10.1111/j.1365-2966.2011.20101.x
Keywords
galaxies: active; galaxies: ISM; galaxies: Seyfert
Categories
Funding
- Dean's Leadership Board faculty fellowship
- Leverhulme Trust
- STFC
- Microsoft
- US National Science Foundation
- NASA [PF9-00069, NAS8-03060]
- Alfred P. Sloan Foundation
- National Science Foundation
- US Department of Energy
- Japanese Monbukagakusho
- Max Planck Society
- University of Chicago
- Fermilab
- Institute for Advanced Study
- Japan Participation Group
- Johns Hopkins University
- Los Alamos National Laboratory
- Max-Planck-Institute for Astronomy (MPIA)
- Max-Planck-Institute for Astrophysics (MPA)
- New Mexico State University
- Princeton University
- US Naval Observatory
- University of Washington
- Science and Technology Facilities Council [PP/E003427/1] Funding Source: researchfish
- STFC [PP/E003427/1] Funding Source: UKRI
- Direct For Education and Human Resources
- Division Of Research On Learning [0941610] Funding Source: National Science Foundation
- Direct For Mathematical & Physical Scien
- Division Of Astronomical Sciences [1004872] Funding Source: National Science Foundation
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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 x 10(5) 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 approximate to 10(5) 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 r(proj) > 10 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 r = 37 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 degrees to 112 degrees. 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 > 12; 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 x 10(5) years) for the length of individual bright phases.
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