Jansky Very Large Array Detections of CO(1-0) Emission in H I-absorption-selected Galaxies at z ≳ 2

Abstract

We report a Karl G. Jansky Very Large Array search for redshifted CO(1-0) emission from three H I-absorption-selected galaxies at z ≈ 2, identified earlier in their CO(3-2) or CO(4-3) emission. We detect CO(1-0) emission from DLA B1228-113 at z ≈ 2.1933 and DLA J0918+1636 at z ≈ 2.5848; these are the first detections of CO(1-0) emission in high-z H I-selected galaxies. We obtain high molecular gas masses, M _mol ≈ 10¹¹ × (α _CO/4.36) M _⊙, for the two objects with CO(1-0) detections, which are a factor of ≈1.5-2 lower than earlier estimates. We determine the excitation of the mid-J CO rotational levels relative to the J = 1 level, r _J1, in H I-selected galaxies for the first time, obtaining r ₃₁ = 1.00 ± 0.20 and r ₄₁ = 1.03 ± 0.23 for DLA J0918+1636, and r ₃₁ = 0.86 ± 0.21 for DLA B1228-113. These values are consistent with thermal excitation of the J = 3 and J = 4 levels. The excitation of the J = 3 level in the H I-selected galaxies is similar to that seen in massive main-sequence and submillimeter galaxies at z≳2, but higher than that in main-sequence galaxies at z ≈ 1.5; the higher excitation of the galaxies at z ≳ 2 is likely to be due to their higher star formation rate (SFR) surface density. We use Hubble Space Telescope Wide Field Camera 3 imaging to detect the rest-frame near-ultraviolet (NUV) emission of DLA B1228-113, obtaining an NUV SFR of 4.44 ± 0.47 M _⊙ yr^-1, significantly lower than that obtained from the total infrared luminosity, indicating significant dust extinction in the z ≈ 2.1933 galaxy.