2.2. Optical Emission Lines, Variability, and Zero Proper Motion Selection
There have been a wide variety of AGN surveys relying on
low-resolution optical spectra. These techniques select AGN on the
strength of the UV/optical emission lines (see
Osmer & Hewett 1991
for a summary of this technique, and
Salzer et al. 2000
for the latest application with the Kitt Peak National Observatory
International Spectroscopic Survey).
While this method essentially
has no "false" detections, its completeness is very difficult to
evaluate, since the signal-to-noise depends on the equivalent widths
of the lines. Since the lines observed in the optical depend on the
redshifts of the objects (e.g., from
H to
Ly
) and
vary in intrinsic strength by over an order of magnitude, it is very
difficult to evaluate the completeness of the sample. Furthermore, the
effects of redshifting make the effective band smaller by (1 +
z), such that at z = 3, only 1400 Å in the rest-frame
are covered by the total ground-based optical wavelength band (3200 -
8900 Å). The limit of this technique
(Ho, Filippenko, &
Sargent 1995)
is to obtain high
signal-to-noise spectra of the nucleus of every object in a complete
sample. However, even this method runs into the difficulty of removing
absorption lines due to stars in the relatively low-luminosity
AGN spectra and requires extreme care.
Variability can also be used as a survey technique (Usher & Mitchell 1978; Brunzendorf & Meusinger 2002; Dobrzycki et al. 2003), since on long enough timescales, virtually all AGN are variable (Veron & Hawkins 1995; Giveon et al. 1999). This method requires a large number of observations and seems to produce a lower areal density of objects than other techniques. However, this method also finds large numbers of variable stars and supernovae and must be combined with other selection criteria.
Similarly, since AGN are at great distances compared to bright stars, the absence of proper motion can be a guide to AGN selection. However, this technique has not been widely used and fails at faint magnitudes, where most of the stars also do not show proper motion. This technique will be resurrected when large solid angle astrometric data sets are available.