We report on a survey for associated H i 21-cm and OH 18-cm absorption with the Giant Metrewave Radio Telescope at redshifts z \approx 0.2 - 0.4 .
Although the low redshift selection ensures that our targets are below the critical ultra-violet luminosity ( L _ { UV } \sim 10 ^ { 23 } W Hz ^ { -1 } ) , which is hypothesised to ionise all of the neutral gas in the host galaxy , we do not obtain any detections in the six sources searched .
Analysing these in context of the previous surveys , in addition to the anti-correlation with the ultra-violet luminosity ( ionising photon rate ) , we find a correlation between the strength of the absorption and the blue – near-infrared colour , as well as the radio-band turnover frequency .
We believe that these are due to the photo-ionisation of the neutral gas , an obscured sight-line being more conducive to the presence of cold gas and the compact radio emission being better intercepted by the absorbing gas , maximising the flux coverage , respectively .
Regarding the photo-ionisation , the compilation of the previous surveys increases the significance of the critical ionising photon rate , above which all of the gas in the host galaxy is hypothesised to be ionised ( Q _ { \text { H { \sc i } } } \approx 3 \times 10 ^ { 56 } sec ^ { -1 } ) , to > 5 \sigma .
This reaffirms that this is an ubiquitous effect , which has profound implications for the detection of neutral gas in these objects with the Square Kilometre Array .