Spotting high-z molecular absorbers using neutral carbon: Results from a complete spectroscopic survey with the VLT

TL;DR

This study demonstrates that selecting absorbers based on neutral carbon lines effectively identifies high-redshift molecular gas, revealing high H2 and CO detection rates and insights into gas properties in distant galaxies.

Contribution

The paper introduces a complete spectroscopic survey targeting neutral carbon to efficiently detect high-redshift molecular absorbers, including new detections of H2 and CO.

Findings

H2 detected in all systems with measured lines, indicating self-shielded gas.

Seven new CO detections, with strong correlation between N(CO) and N(CI).

CI-selected absorbers probe gas deeper than the HI-H2 transition.

Abstract

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