CHILES

X. Molecular and atomic gas at intermediate redshift

¹ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory 43992 Onsala, Sweden
² ASTRON, the Netherlands Institute for Radio Astronomy Postbus 2 7990 AA Dwingeloo, The Netherlands
³ National Radio Astronomy Observatory (NRAO) 520 Edgemont Road Charlottesville VA 22903, USA
⁴ Department of Physics, Montana State University P.O. Box 173840 Bozeman MT 59717, USA
⁵ Department of Astronomy, Columbia University 550 West 120th Street New York NY 10027, USA
⁶ Department of Astronomy, University of Massachusetts Amherst MA 01003, USA
⁷ National Radio Astronomy Observatory P.O. Box O Socorro NM 87801, USA
⁸ International Centre for Radio Astronomy Research (ICRAR), University of Western Australia 35 Stirling Hwy Crawley WA 6009, Australia
⁹ Department of Astronomy, Yonsei University 50 Yonsei-ro Seodaemun-gu Seoul 03722, Republic of Korea
¹⁰ Department of Physics, Virginia Tech 850 West Campus Drive Blacksburg VA 24061, USA
¹¹ Department of Astronomy, University of Cape Town Private Bag X3 Rondebosch 7701, South Africa

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 June 2025
Accepted: 1 October 2025

Abstract

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(M_H2/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.