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The distribution of redshifts among the DXRBS FSRQs identified so far (Figure
3) is quite similar to that of the 1 Jy and S4 samples. Not much more can be
said at present. A more thorough comparison will have to await completion of
our survey and the convolution of the redshift distribution with the WGACAT
sky coverage. As has
already been mentioned by Hook et al. (1996), selecting flat-spectrum
sources is an efficient way of finding high-redshift, radio-loud
quasars. We would therefore expect that the majority
of the FSRQs would be at high redshifts (z > 1), a suspicion which
the data confirm. The mean redshift of our FSRQ sample is , and the high-redshift tail extends to
. Our redshift
distribution cannot, however, be directly compared to that of Hook et
al. (1996) because those authors required that the sources be
particularly ``red,'' a criterion we do not impose.
While a significant fraction (7 of 33) of the BL Lacs so far included in our
sample still lack redshifts, it is apparent that the redshift distribution for
the DXRBS BL Lacs (Figure 4) is dominated by objects at z<0.4, and therefore
more similar to that of the Slew Survey (Perlman
et al. 1996a; or the EMSS survey: Stocke et al. 1991) than the 1 Jy
sample (Stickel et al. 1991), which is less peaked at low redshifts and has a
much more prominent high-z tail than either the DXRBS
or Slew Survey samples.
As above, a direct comparison between these distributions is not
possible at present, given also the different survey methods and selection
criteria. We only mention here that the difference between the DXRBS and 1 Jy
redshift distributions appears to be striking considering that
the radio flux limit of DXRBS is a factor 20 lower so that if anything, one
would expect a higher fraction of high-z sources, rather than the
dearth of high-z sources that we observe. The flatter distribution of the 1
Jy sample might be related to the fact that it misses some low
redshift objects hidden within bright hosts (which might be misidentified as
radio galaxies; see §7) and might also be contaminated by
misidentified quasars at high redshifts (e.g., 1 Jy 1308+326), as suggested by
Marchã & Browne (1995) and Perlman et al. (1996b). Some 1 Jy BL Lacs might
be gravitationally lensed if superposed by chance onto a lower-redshift galaxy
(as originally suggested by Ostriker & Vietri 1985). This last possibility
now appears quite likely from the results of Stocke & Rector (1997), which
show that the 1 Jy BL Lacs have a excess of Mg II absorbers
along their sight lines.