Spotting high-z molecular absorbers using neutral carbon: Results from a
  complete spectroscopic survey with the VLT
  While molecular quasar absorption systems provide unique probes of the
physical and chemical properties of the gas as well as original constraints on
fundamental physics and cosmology, their detection remains challenging. Here we
present the results from a complete survey for molecular gas in thirty-nine
absorption systems selected solely upon the detection of neutral carbon lines
in SDSS spectra, without any prior knowledge of the atomic or molecular gas
content. H2 is found in all twelve systems (including seven new detections)
where the corresponding lines are covered by the instrument setups and measured
to have log N(H2)>=18, indicating a self-shielded regime. We also report seven
CO detections (7/39) down to log N(CO)~13.5, including a new one, and put
stringent constraints on N(CO) for the remaining 32 systems. N(CO) and N(CI)
are found to be strongly correlated with N(CO)/N(CI)~1/10. This suggests that
the CI-selected absorber population is probing gas deeper than the HI-H2
transition in which a substantial fraction of the total hydrogen in the cloud
is in the form of H2. We conclude that targeting CI-bearing absorbers is a very
efficient way to find high-metallicity molecular absorbers. However, probing
the molecular content in lower metallicity regimes as well as high column
density neutral gas remains to be undertaken to unravel the processes of gas
conversion in normal high-z galaxies
