We present the methodology for “ blind ” millimetre-wave surveys for redshifted molecular absorption in the CO/HCO ^ { + } rotational lines .
The frequency range 30 - 50 GHz appears optimal for such surveys , providing sensitivity to absorbers at z \gtrsim 0.85 .
It is critical that the survey is “ blind ” , i.e .
based on a radio-selected sample , including sources without known redshifts .
We also report results from the first large survey of this kind , using the Q-band receiver on the Green Bank Telescope ( GBT ) to search for molecular absorption towards 36 sources , 3 without known redshifts , over the frequency range 39.6 - 49.5 GHz .
The GBT survey has a total redshift path of \Delta z \approx 24 , mostly at 0.81 < z < 1.91 , and a sensitivity sufficient to detect equivalent { H _ { 2 } } column densities \gtrsim 3 \times 10 ^ { 21 } cm ^ { -2 } in absorption at 5 \sigma significance ( using CO-to- { H _ { 2 } } and HCO ^ { + } -to- { H _ { 2 } } conversion factors of the Milky Way ) .
The survey yielded no confirmed detections of molecular absorption , yielding the 2 \sigma upper limit n ( z = 1.2 ) < 0.15 on the redshift number density of molecular gas at column densities N ( { H _ { 2 } } ) \gtrsim 3 \times 10 ^ { 21 } cm ^ { -2 } .