First Results from COPSS: The CO Power Spectrum Survey
  We present constraints on the abundance of carbon-monoxide in the early
Universe from the CO Power Spectrum Survey (COPSS). We utilize a data set
collected between 2005 and 2008 using the Sunyaev-Zel'dovich Array (SZA), which
were previously used to measure arcminute-scale fluctuations of the CMB. This
data set features observations of 44 fields, covering an effective area of 1.7
square degrees, over a frequency range of 27 to 35 GHz. Using the technique of
intensity mapping, we are able to probe the CO(1-0) transition, with
sensitivity to spatial modes between $k=0.5{-}2\ h\,\textrm{Mpc}^{-1}$ over a
range in redshift of $z=2.3{-}3.3$, spanning a comoving volume of
$3.6\times10^{6}\ h^{-3}\,\textrm{Mpc}^{3}$. We demonstrate our ability to
mitigate foregrounds, and present estimates of the impact of continuum sources
on our measurement. We constrain the CO power spectrum to
$P_{\textrm{CO}}<2.6\times10^{4}\ \mu\textrm{K}^{2}
(h^{-1}\,\textrm{Mpc})^{3}$, or $\Delta^{2}_{\textrm{CO}}(k\! = \! 1 \
h\,\textrm{Mpc}^{-1})<1.3 \times10^{3}\ \mu\textrm{K}^{2}$, at $95\%$
confidence. This limit resides near optimistic predictions for the CO power
spectrum. Under the assumption that CO emission is proportional to halo mass
during bursts of active star formation, this corresponds to a limit on the
ratio of $\textrm{CO}(1{-}0)$ luminosity to host halo mass of
$A_{\textrm{CO}}<1.2\times10^{-5}\ L_{\odot}\ M_{\odot}^{-1}$. Further assuming
a Milky Way-like conversion factor between CO luminosity and molecular gas mass
($\alpha_{\textrm{CO}}=4.3\ M_{\odot}\ (\textrm{K}\ \textrm{km}\
\textrm{s}^{-1}\ \textrm{pc}^{-2})^{-1}$), we constrain the global density of
molecular gas to $\rho_{z\sim3}(M_{\textrm{H}_{2}})\leq 2.8 \times10^{8}\
M_{\odot}\ \textrm{Mpc}^{-3}$.
