An Intensity Mapping Detection of Aggregate CO Line Emission at 3 mm
  We present a detection of molecular gas emission at $z\sim1-5$ using the
technique of line intensity mapping. We make use of a pair of 3 mm
interferometric data sets, the first from the ALMA Spectroscopic Survey in the
Hubble Ultra Deep Field (ASPECS), and the second from a series of Atacama
Compact Array (ACA) observations conducted between 2016 and 2018, targeting the
COSMOS field. At 100 GHz, we measure non-zero power at 97.8% and 99.9%
confidence in the ACA and ALMA data sets, respectively. In the joint result, we
reject the zero-power hypothesis at 99.99% confidence, finding
$\tilde{I}^{2}_{s}(\nu)=770\pm210\ \mu\textrm{K}^2\ \textrm{Hz}\ \textrm{sr}$.
After accounting for sample variance effects, the estimated spectral shot power
is $\tilde{I}^{2}_{s}(\nu)=1010_{-390}^{+550}\ \mu\textrm{K}^2\ \textrm{Hz}\
\textrm{sr}$. We derive a model for the various line species our measurement is
expected to be sensitive to, and estimate the shot power to be
$120_{-40}^{+80}\ \mu\textrm{K}^2\ h^{-3}\,\textrm{Mpc}^{3}$,
$200^{+120}_{-70}\ \mu\textrm{K}^2\ h^{-3}\,\textrm{Mpc}^{3}$, and
$90^{+70}_{-40}\ \mu\textrm{K}^2\ h^{-3}\,\textrm{Mpc}^{3}$ for CO(2-1) at
$z=1.3$, CO(3-2) at $z=2.5$, and CO(4-3) at $z=3.6$, respectively. Using line
ratios appropriate for high-redshift galaxies, we find these results to be in
good agreement with those from the CO Power Spectrum Survey (COPSS). Adopting
$\alpha_{\rm CO}=3.6\ M_{\odot}\ (\textrm{K}\ \textrm{km}\ \textrm{s}^{-1}\
\textrm{pc}^{2})^{-1}$, we estimate a cosmic molecular gas density of
$\rho_{\textrm{H}_2}(z)\sim 10^{8}\ M_{\odot}\ \textrm{Mpc}^{-3}$ between
$z=1-3$.
