Evolution of cold gas at 2<z<5: a blind search for HI and OH absorption
  lines towards mid-infrared color selected radio-loud AGNs
  We present results from a spectroscopically blind search for associated and
intervening HI 21-cm and OH 18-cm absorption lines towards 88 AGNs at $2\le
z\le5$ using the upgraded Giant Metrewave Radio Telescope (uGMRT). The sample
of AGNs with 1.4 GHz spectral luminosity in the range, $10^{27 - 29.3}$ W/Hz,
is selected using mid-infrared colors and closely resembles the distribution of
the underlying quasar population. The search for associated or proximate
absorption, defined to be within 3000 km/s of the AGN redshift, led to one HI
21-cm absorption detection (M1540-1453; $z_{abs}$= 2.1139). This is only the
fourth known absorption at $z>2$. The detection rate ($1.6^{+3.8}_{-1.4}$%)
suggests low covering factor of cold neutral medium (CNM; T$\sim$100 K)
associated with these powerful AGNs. The intervening absorption line search,
with a sensitivity to detect CNM in damped Ly$\alpha$ systems (DLAs), has
comoving absorption path lengths of $\Delta$X = 130.1 and 167.7 for HI and OH,
respectively. The corresponding number of absorber per unit comoving path
lengths are $\le$0.014 and $\le$0.011, respectively. The former is at least 4.5
times lower than that of DLAs and consistent with the CNM cross-section
estimated using H$_2$ and CI absorbers at $z>2$. Our AGN sample is optically
fainter compared to the quasars used to search for DLAs in the past. In our
optical spectra obtained using the Southern African Large Telescope (SALT) and
the Nordic Optical Telescope (NOT), we detect 5 intervening (redshift
path$\sim9.3$) and 2 proximate DLAs. This is slightly excessive compared to the
statistics based on optically selected quasars. The non-detection of HI 21-cm
absorption from these DLAs suggests small CNM covering fraction around galaxies
at $z>2$.
