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

N. Kanekar, A. Gupta, C. L. Carilli, J. T. Stocke, K. W. Willett

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8 Scopus citations


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.

Original languageEnglish (US)
Article number56
JournalAstrophysical Journal
Issue number1
StatePublished - Feb 10 2014


  • galaxies: high-redshift
  • molecular processes
  • quasars: absorption lines


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