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We examine the absorption of cosmic microwave background (CMB) photons byformaldehyde (H2CO) over cosmic time. The K-doublet rotational transitions ofH2CO become "refrigerated" - their excitation temperatures are driven below theCMB temperature - via collisional pumping by molecular hydrogen (H2)."Anti-inverted" H2CO line ratios thus provide an accurate measurement of the H2density in molecular clouds. Using a radiative transfer model, we demonstratethat H2CO centimeter wavelength line excitation and detectability are nearlyindependent of redshift or gas kinetic temperature. Since the H2CO K-doubletlines absorb CMB light, and since the CMB lies behind every galaxy and providesan exceptionally uniform extended illumination source, H2CO is adistance-independent, extinction-free molecular gas mass-limited tracer ofdense gas in galaxies. A Formaldehyde Deep Field could map the history ofcosmic star formation in a uniquely unbiased fashion and may be possible withlarge bandwidth wide-field radio interferometers whereby the silhouettes ofstar-forming galaxies would be detected across the epoch of galaxy evolution.We also examine the possibility that H2CO lines may provide a standardizablegalaxy ruler for cosmology similar to the Sunyaev-Zel'dovich effect in galaxyclusters but applicable to much higher redshifts and larger samples. Finally,we explore how anti-inverted meterwave H2CO lines in galaxies during the peakof cosmic star formation may contaminate HI 21 cm tomography of the Epoch ofReionization.

the astrophysical journal. letters

Formaldehyde Silhouettes Against the Cosmic Microwave Background: A  Mass-Limited, Distance-Independent, Extinction-Free Tracer of Star Formation  Across the Epoch of Galaxy Evolution