CHILES X. Molecular and atomic gas at intermediate redshift

Journal article, 2026

We present ALMA CO observations of 14 H I-detected galaxies from the COSMOS H I Large Extragalactic Survey (CHILES) found in a cosmic over-density at z ∼ 0.12. This is the largest collection of spatially resolved CO + H I observations beyond the local Universe (z > 0.05) to date. While the H I-detected parent sample spans a range of stellar masses, star formation rates (SFRs), and environments, we only directly detect CO in the highest stellar mass galaxies, log(M*/M⊙) > 10.0, with SFRs greater than ∼2 M⊙ yr⁻¹. The detected CO has the kinematic signature of a rotating disk, consistent with the H I. We stacked the CO non-detections and find a mean H2 mass of log(MH2/M ⊙) = 8.46 in galaxies with a mean stellar mass of log(M*/M⊙) = 9.35. In addition to high stellar masses and SFRs, the systems detected in CO are spatially larger, have redder overall colors, and exhibit broader (stacked) line widths. The CO emission is spatially coincident with both the highest stellar mass surface density and star forming region of the galaxies, as revealed by the 1.4 GHz continuum emission from CHILES Con Pol. We interpret the redder colors as the molecular gas being coincident with dusty regions of obscured star formation. The 14 H I detections show a range of morphologies, but the H I reservoir is always more extended than the CO. Finally, we compare with samples in the literature and find mild evidence for evolution in the molecular gas reservoir and H2-to-H I gas ratio with redshift in H I flux-limited samples. We also show that the scatter in the H I, and H I-to-stellar mass ratio is too great to conclusively measure evolution below z = 0.2, and would be even extremely difficult below z = 0.4. Detections from CHILES are likely to be the only individual galaxies detected in H I between 0.1 < z < 0.23 for the foreseeable future due to the severity of satellite radio frequency interference, and its preferential impact on short baselines which dominate the observations of contemporary H I surveys.