A search for HI and OH absorption in z > 3 CO emitters
  We present the results of a survey for HI 21-cm and OH 18-cm absorption in seven strong CO emitters at z > 3. Despite reaching limits comparable to those required to detect 21-cm absorption at lower redshifts, we do not detect either transition in any of the objects searched. We believe that this is due to the high redshift selection causing all of our targets to have ultra-violet luminosities above the critical value, where all of the atomic gas in the host galaxy disk is suspected to be ionised. However, not only are all of our targets bright in CO emission, but detection of CO above the critical UV luminosity is generally not uncommon. This suggests that the molecular gas is shielded from the radiation or is physically remote from the source of the continuum emission, as it appears to be from CO observations of high redshift radio galaxies. 
S. J. Curran, J. R. Allison, M. T. Whiting, E. M. Sadler, F. Combes,   M. B. Pracy, C. Bignell and R. Athreya
astro-ph.GA astro-ph.CO