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E. J. Hooper (McDonald Obs.), R. A. Finn (U. Mass.), C. D. Impey (Steward Obs.)
One of the enduring questions in AGN research is what triggers and sustains accretion onto the central supermassive black hole. What roles do mechanisms wholly internal to the host galaxy, nearby companions, associated groups of galaxies, and larger clusters play in stimulating AGN activity? A long-standing conventional wisdom holds that external environments, particularly galaxy clusters, play a significant role in triggering and fueling AGN activity. However, recent results, including our own, indicate that the situation may be more complicated. We find little evidence, based on R-band imaging, for large cluster-scale environments around either radio-loud or radio-quiet quasars.
Out of a sample of 33 quasar fields drawn from the optically selected Large Bright Quasar Survey (LBQS), only two quasars appear to lie in environments as rich as Abell class 0 galaxy clusters, and the error bars on one of these are large. The remaining quasars are in environments consistent with those of field galaxies. Most of the quasars in the sample have redshifts in the range 0.4 < z < 0.5. Similar results hold for a subsample of 16 LBQS quasars imaged with HST.
Given the sparse environments, we have reached the limits of what single-band imaging can tell us. While there are some tantalizing suggestions of possible galaxy concentrations around several of the quasars, they do not stand out above the field galaxies in a simple number count analysis. We have recently completed multislit spectroscopic follow-up of about half of the fields in order to check for any small galaxy groups which may attend the quasars and to place more stringent limits on the environments of all of the fields.
The first two authors gratefully acknowledge support of NSF Astronomy & Astrophysics Postdoctoral Fellowships.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.