Have we missed QSOs at low-to-intermediate redshifts becasue they no longer appear stellar? The search for low-luminosity AGN has been extended, using variability, to objects with non-stellar (extended) light profiles. So far, we have roughly 50 candidates, which are presumably all intermediate redshift (z<1) galaxies. To date we only have spectroscopy of the three brightes candirtteas which all show evidence for the presence of AGN.
The primary scientific questions we are trying to answer with these surveys are:
Collaborators: R. Kron (U. Chicago/Fermilab), S. Majewski (U. Virginia), J. Munn (US Naval Obs, Flagstaff), D. Trevese (U. Rome, "La Sapienza"), J. Semtanka (U. Chicago), D. Koo (UC Santa Cruz/Lick Observatory)
Fig. 1:
Color density vs. distance (from the stellar locus) parameters for all
objects in our survey. The color-selected candidates are all
non-crosses objects to the upper right of the boundaries )the crosses
represent objects with BJ-RF>1 and
RF-IN>1, which are likely M dwarfs). The other
symbols have the following meanings. Diamonds: broad-lined objects
(i.e. quasars); box + dot: narrow-lined objects; box:
spectroscopically a galaxy; circle: spectroscopically a star; dot: all
others (objects with poor or non-existent spectra).
Fig. 2: BJ,RF,IN color-color diagrams
for candidates in one of our fields (Lynx-2) with
20<RF<21.5. The left panel shows the objects selected
according to the previous 1, again excluding the objects with
BJ-RF>1 and
RF-IN>1. The symbols are as in the previous
figure. The central panel shows the compliment, namely all other
objects. The right panel gives the distribution of resolved images
(i.e. galaxies), for comparison. In all panels, the locus giving the
adopted stellar mean color is drawn for reference.
The
derived luminosity fuinction from our sample alone, binned as a
function of redshift (the bin center is marked). The density is given
per unit magnitude per Gpc3.
"QSO Counts: A Complete Survey of Stellar Objects to B=23," Koo, D.C,
Kron, R.G. 1982, A&A , 105 , 107
"Quasars to B > 22.5 in Selected Area 57 - A Catalog of Multicolor
Photometry, Variability, and Astrometry," Koo, D.C., Kron, R.G. and
Cudworth, K.M. 1986, PASP , 98 , 285
"Spectroscopic survey of QSOs to B = 22.5 - The luminosity function,"
Koo, D.C., Kron, R.G. 1988, ApJ , 325 , 92
``A Survey for Faint Variable Objects in $SA$ 57,'' Trevese, D.,
Pittella, G., Koo D.C., Kron, R.G., and Bershady, M.A. 1989,
A.J., 98, 108
"Quasar Colors: Statistics and Cosmological Evolution," Giallongo, E.,
Trevese, D. 1990, ApJ , 353 , 24
``A Search For Faint Variable Galactic Nuclei In $SA$ 57,'' Bershady,
M.A., Trevese, D., and Kron, R.G. 1991, in Variability of
Active Galactic Nuclei, eds. H.R. Miller and P.J. Wiita,
(Cambridge University Press), p. 67
``The Faint Quasar Luminosity Function,'' Kron, R.G., Bershady,
M.A., Munn, J.A., Smetanka, J.J., Majewski, S.R., and Koo,
D.C. 1991, in Workshop on the Space Distribution of Quasars,
ed. D. Crampton (Provo: Astronomical Society of the Pacific), vol. 21,
p. 32
``A Variability and Proper Motion QSO Survey to $B_J$=22.5,''
Majewski, S.R., Munn, J.A., Kron, R.G., Bershady, M.A., Smetanka,
and Koo, D.C. 1991, in Workshop on the Space Distribution of
Quasars, ed. D. Crampton (Provo: Astronomical Society of the
Pacific), vol. 21, p. 55
``Emission-Line Properties of Faint Quasars and Compact Galaxies,''
Smetanka, J.J., Bershady, M.A., Kron, R.G., Majewski, S.R., and
Munn, J.A. 1991, in Workshop on the Space Distribution of
Quasars, ed. D. Crampton (Provo: Astronomical Society of the
Pacific), vol. 21, p. 100
``A Deep QSO Survey,'' Majewski, S.R., Bershady, M.A., Kron, R.G.,
Munn, J.A., Smetanka, J.J., and Koo, D.C. 1991, in Workshop on
the Space Distribution of Quasars, ed. D. Crampton (Provo:
Astronomical Society of the Pacific), vol. 21, p. 393
``Survey Incompleteness and the Evolution of the QSO Luminosity
Function,'' Majewski, S.R., Munn, J.A., Kron, R.G., Bershady,
M.A., Smetanka, J.J., and Koo, D. C. 1992, in The Evolution of
Galaxies and Their Environment Environment, eds. D. Hollenbach, H.
Thronson, and J. M. Shull (NASA Conference Publication 3190: Ames
Research Center), p. 7
``The Ensemble Variability Properties of Faint QSOs,'' Trevese, D.,
Kron, R.G., Majewski, S.R., Bershady, M.A. and Koo, D.C. 1994,
ApJ, 433, 494
"Optical Variability of Quasars," Trevese, D., Bunone, A., Nanni, D.,
Bershady, M. A., Kron, R. G. 1997, Mem.S.A.It., 68, 251
"A Search for Galaxies with a Variable Nucleus," Trevese, D.,
Bershady, M. A., Kron, R. G. 1997, to appear in proceedings of
ESO/IAC Workshop on Quasar Hosts, (Puerto de la Cruz, Tenerife,
Spain)
"Variable Extended Objects in SA 57," Bershady, M. A., Trevese, D.,
Kron, R. G. 1997, submitted to ApJ
Selecting QSOs by variability and zero proper motion
(under construction) Space density of QSOs selected by color
Redshift vs. RF magnitude (Hubble) diagram for all objects
with good spectroscopic information, plotted such that for
q0=1, objects of fixed absolute magnitude and fnu
proportional to nu-alpha, alpha=1 are on diagonal
lines. Solid diamonds: broad-lined objects in the candidate sample;
open diamonds: all othger broad-lined objects; box+dot: narrow-lined
objects; box: galaxy. 
Selecting faint AGN at intermediate redshift
(under construction)
References: Papers directly related to the above
surveys
"Stars with Zero Proper Motion and the Number of of faint QSOs,"
Kron, R.G. and Chiu, L.T.G. 1981, PASP , 93 , 397
last updated: Feb. 12, 1996 (mab@astro.psu.edu)