A Search for Intrinsic HI 21 cm and OH 18 cm Absorption Toward Compact
  Radio Sources
  We present the results of a large search for intrinsic HI 21 cm and OH 18 cm
absorption in 145 compact radio sources in the redshift range 0.02< z <3.8 with
the Green Bank Telescope. We re-detect HI 21 cm absorption toward six known
absorption systems but detect no new HI or OH absorption in 102
interference-free sources. 79 sources have not previously been observed for HI
21 cm absorption. We recover a mean optical depth limit of
$\tau_{3\sigma}<0.023$ for all the non-detections in the survey. Our results do
not support the high intrinsic absorption rates found by previous studies in
compact radio sources at low redshift. Our results do, however, support the
hypothesis proposed by Curran et al. (2008) that high ultraviolet (UV)
luminosity active galactic nuclei (AGN) do not show intrinsic HI 21 cm
absorption, confirming a threshold of $L_{\rm UV} = 10^{23}$ W Hz$^{-1}$, above
which our intrinsic absorption fraction is zero (54 sources). The exact nature
of the UV luminosity effect on HI absorption systems remains ambiguous. We
additionally find no statistical correlation between the 1.4 GHz radio
luminosity or the source size and the 21~cm absorption detection rate. We
attribute the lack of intrinsic absorption in our survey to the UV luminosity
effect caused by an optical selection bias and a decreased column density
sensitivity with increasing redshift due to lower radio continuum flux
densities, high radio frequency interference, and higher telescope system
temperatures at low frequencies.
