COLDz: Karl G. Jansky Very Large Array discovery of a gas-rich galaxy in
  COSMOS
  The broad spectral bandwidth at mm and cm-wavelengths provided by the recent
upgrades to the Karl G. Jansky Very Large Array (VLA) has made it possible to
conduct unbiased searches for molecular CO line emission at redshifts, z >
1.31. We present the discovery of a gas-rich, star-forming galaxy at z = 2.48,
through the detection of CO(1-0) line emission in the COLDz survey, through a
sensitive, Ka-band (31 to 39 GHz) VLA survey of a 6.5 square arcminute region
of the COSMOS field. We argue that the broad line (FWHM ~570 +/- 80 km/s) is
most likely to be CO(1-0) at z=2.48, as the integrated emission is spatially
coincident with an infrared-detected galaxy with a photometric redshift
estimate of z = 3.2 +/- 0.4. The CO(1-0) line luminosity is L'_CO = (2.2 +/-
0.3) x 10^{10} K km/s pc^2, suggesting a cold molecular gas mass of M_gas ~ (2
- 8)x10^{10}M_solar depending on the assumed value of the molecular gas mass to
CO luminosity ratio alpha_CO. The estimated infrared luminosity from the
(rest-frame) far-infrared spectral energy distribution (SED) is L_IR =
2.5x10^{12} L_solar and the star-formation rate is ~250 M_solar/yr, with the
SED shape indicating substantial dust obscuration of the stellar light. The
infrared to CO line luminosity ratio is ~114+/-19 L_solar/(K km/s pc^2),
similar to galaxies with similar SFRs selected at UV/optical to radio
wavelengths. This discovery confirms the potential for molecular emission line
surveys as a route to study populations of gas-rich galaxies in the future.
