COLDz: Karl G. Jansky Very Large Array Discovery of a Gas-rich Galaxy in COSMOS

Abstract

The broad spectral bandwidth at millimeter and centimeter 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 J = 1–0 line emission in the COLDz survey and through a sensitive, Ka-band (31–39 GHz) VLA survey of a 6.5 arcmin^2 region of the COSMOS field. We argue that the broad line (FWHM ~ 570  ±  80 km s^−1) is most likely to be CO J = 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_phot = 3.2 ± 0.4. The CO J = 1–0 line luminosity is L'_CO = (2.2±0.3)× 10^10  K km s^−1 pc^2, suggesting a cold molecular gas mass of M_gas ~ (2–8) × 10^10 M_☉ depending on the assumed value of the molecular gas mass to CO luminosity ratio α_CO. The estimated infrared luminosity from the (rest-frame) far-infrared spectral energy distribution (SED) is L_IR = 2.5 × 10^12 L^☉ and the star formation rate is ~250 M^☉ yr^−1, with the SED shape indicating substantial dust obscuration of the stellar light. The infrared to CO line luminosity ratio is ~114  ±  19 L_☉/(K km s^−1 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.