Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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.

11
The categories of this paper are: hep-lat
16
The categories of this paper are: stat
2
The categories of this paper are: math
0
The categories of this paper are: hep-ph
8
The categories of this paper are: hep-ex
24
The categories of this paper are: dg-ga
3
The categories of this paper are: physics
37
The categories of this paper are: supr-con
4
The categories of this paper are: cond-mat
7
The categories of this paper are: hep-th
9
The categories of this paper are: nlin
25
The categories of this paper are: patt-sol
30
The categories of this paper are: q-alg
34
The categories of this paper are: chem-ph
26
The categories of this paper are: bayes-an
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The categories of this paper are: funct-an