A blind green bank telescope millimeter-wave survey for redshifted molecular absorption

We present the methodology for "blind" millimeter-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 ≳ 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 toward 36 sources, 3 without known redshifts, over the frequency range 39.6-49.5 GHz. The GBT survey has a total redshift path of Δz24, 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 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.