The CO Luminosity Density at High-z (COLDz) Survey: A Sensitive, Large
  Area Blind Search for Low-J CO Emission from Cold Gas in the Early Universe
  with the Karl G. Jansky Very Large Array
  We describe the CO Luminosity Density at High-z (COLDz) survey, the first
spectral line deep field targeting CO(1-0) emission from galaxies at
$z=1.95-2.85$ and CO(2-1) at $z=4.91-6.70$. The main goal of COLDz is to
constrain the cosmic density of molecular gas at the peak epoch of cosmic star
formation. By targeting both a wide ($\sim$51 arcmin$^2$) and a deep area
($\sim$9 arcmin$^2$), the survey is designed to robustly constrain the bright
end and the characteristic luminosity of the CO(1-0) luminosity function. An
extensive analysis of the reliability of our line candidates, and new
techniques provide detailed completeness and statistical corrections as
necessary to determine the best constraints to date on the CO luminosity
function. Our blind search for CO(1-0) uniformly selects starbursts and massive
Main Sequence galaxies based on their cold molecular gas masses. Our search
also detects CO(2-1) line emission from optically dark, dusty star-forming
galaxies at $z>5$. We find a range of spatial sizes for the CO-traced gas
reservoirs up to $\sim40$ kpc, suggesting that spatially extended cold
molecular gas reservoirs may be common in massive, gas-rich galaxies at
$z\sim2$. Through CO line stacking, we constrain the gas mass fraction in
previously known typical star-forming galaxies at $z=2$-3. The stacked CO
detection suggests lower molecular gas mass fractions than expected for massive
Main Sequence galaxies by a factor of $\sim3-6$. We find total CO line
brightness at $\sim34\,$GHz of $0.45\pm0.2\,\mu$K, which constrains future line
intensity mapping and CMB experiments.
