Selection Effects in the SDSS Quasar Sample: The Filter Gap Footprint
  In the Sloan Digital Sky Survey (SDSS) quasars are targeted using colors and
anything that can cause the identifying characteristics of the colors to
disappear can create problems in the source selection process. Quasar spectra
contain strong emission lines that can seriously affect the colors in
photometric systems in which the transmission characteristics vary abruptly and
significantly with redshift. When a strong line crosses a gap between two
filter passbands the color effects induced by the line change abruptly, and
there is also a dimming in apparent brightness compared to those redshifts
where the strong line is inside a filter passband where the transmission is
high. The strong emission lines in quasars, combined with the varying
detectability introduced by the transmission pattern of the five filters, will
result in a filter-gap footprint being imprinted on the N(z) distribution, with
more quasars being missed when a strong line falls in a filter gap. It is shown
here that a periodicity of Delta(z)~0.6 is imprinted on the redshift-number
distribution by this selection effect. Because this effect cannot be rigorously
corrected for, astronomers need to be aware of it in any investigation that
uses the SDSS N(z) distribution. Its presence also means that the SDSS quasar
data cannot be used either to confirm or to rule out the Delta(z)~0.6 redshift
period reported previously in other, unrelated quasar data.
