A Formaldehyde Deep Field
  Formaldehyde (H2CO) is often observed at centimeter wavelengths as an
absorption line against the cosmic microwave background (CMB). This is possible
when energy level populations are cooled to the point where line excitation
temperatures fall below the local CMB temperature. Collisions with molecular
hydrogen "pump" this anti-maser excitation, and the cm line ratios of H2CO
provide a measurement of the local H2 density. H2CO absorption of CMB light
provides all of the benefits of absorption lines (no distance dimming) but none
of the drawbacks: the CMB provides uniform illumination of all molecular gas in
galaxies (no pencil beam sampling), and all galaxies lie in front of the CMB -
no fortuitous alignments with background light sources are needed. A
formaldehyde deep field (FDF) would therefore provide a blind, mass-limited
survey of molecular gas across the history of star formation and galaxy
evolution. Moreover, the combination of column density and number density
measurements may provide geometric distances in large galaxy samples and at
higher redshifts than can be done using the Sunyaev Zel'dovich effect in galaxy
clusters. We present a fiducial FDF that would span redshifts z=0-7 and provide
H2CO line ratios to measure n(H2) for z > 0.45.
