2012ApJ...749L..33D

We examine the absorption of cosmic microwave background (CMB) photons by formaldehyde (H₂CO) over cosmic time. The K-doublet rotational transitions of H₂ CO become "refrigerated"–their excitation temperatures are driven below the CMB temperature–via collisional pumping by molecular hydrogen (H₂). "Anti-inverted" H₂ CO line ratios thus provide an accurate measurement of the H₂ density in molecular clouds. Using a radiative transfer model, we demonstrate that H₂ CO centimeter wavelength line excitation and detectability are nearly independent of redshift or gas kinetic temperature. Since the H₂ CO K-doublet lines absorb CMB light, and since the CMB lies behind every galaxy and provides an exceptionally uniform extended illumination source, H₂ CO is a distance-independent, extinction-free molecular gas mass-limited tracer of dense gas in galaxies. A Formaldehyde Deep Field could map the history of cosmic star formation in a uniquely unbiased fashion and may be possible with large bandwidth wide-field radio interferometers whereby the silhouettes of star-forming galaxies would be detected across the epoch of galaxy evolution. We also examine the possibility that H₂ CO lines may provide a standardizable galaxy ruler for cosmology similar to the Sunyaev-Zel'dovich effect in galaxy clusters but applicable to much higher redshifts and larger samples. Finally, we explore how anti-inverted meter-wave H₂ CO lines in galaxies during the peak of cosmic star formation may contaminate H I 21 cm tomography of the Epoch of Reionization.