A blind Green Bank Telescope millimetre-wave survey for redshifted molecular absorption

Abstract

We present the methodology for ``blind'' millimetre-wave surveys for redshifted molecular absorption in the CO/HCO<sup>+</sup> rotational lines. The frequency range 30−50 GHz appears optimal for such surveys, providing sensitivity to absorbers at z≳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 Δz≈24, mostly at 0.81<z<1.91, and a sensitivity sufficient to detect equivalent H2 column densities ≳3×1021 cm^-2 in absorption at 5σ significance (using the CO-to-H₂ and HCO⁺ -to-H2 conversion factors of the Milky Way). The survey yielded no confirmed detections of molecular absorption, yielding the 2σ upper limit n(z=1.2)<0.15 on the redshift number density of molecular gas at column densities N(H₂)≳3×1021 cm^-2.