FORMALDEHYDE SILHOUETTES AGAINST THE COSMIC MICROWAVE BACKGROUND: A MASS-LIMITED, DISTANCE-INDEPENDENT, EXTINCTION-FREE TRACER OF STAR FORMATION ACROSS THE EPOCH OF GALAXY EVOLUTION

We examine the absorption of cosmic microwave background (CMB) photons by formaldehyde (H2CO) over cosmic time. The K-doublet rotational transitions of H2CO become "refrigerated"-their excitation temperatures are driven below the CMB temperature-via collisional pumping by molecular hydrogen (H-2). "Anti-inverted" H2CO line ratios thus provide an accurate measurement of the H-2 density in molecular clouds. Using a radiative transfer model, we demonstrate that H2CO centimeter wavelength line excitation and detectability are nearly independent of redshift or gas kinetic temperature. Since the H2CO K-doublet lines absorb CMB light, and since the CMB lies behind every galaxy and provides an exceptionally uniform extended illumination source, H2CO 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 H2CO 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 H2CO lines in galaxies during the peak of cosmic star formation may contaminate H I 21 cm tomography of the Epoch of Reionization.