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 (H2).
"Anti-inverted" H2CO line ratios thus provide an accurate measurement of the H2
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 meterwave H2CO lines in galaxies during the peak
of cosmic star formation may contaminate HI 21 cm tomography of the Epoch of
Reionization.
