CHILES. XI. Resolved H I Morphologies and the Evolution of the H 2 /H I Ratio over the Last Five Billion Years

Abstract We present the neutral gas morphology of four galaxies from z = 0.22 to 0.47 obtained with the COSMOS H i Large Extragalactic Survey (CHILES). The H i is resolved at the highest redshift with the 7 . ″ 5 beam of CHILES and 43 kpc linear scale, with all four galaxies having extended H I . Three are massive galaxies ( M * > 3 × 10 10 M ⊙ ), with H i masses of (1.6–6.7) × 10 10 M ⊙ and active star formation (3–30 M ⊙ yr −1 ). The morphology and kinematics of the galaxies vary from regular to disturbed, including an asymmetric H i disk surrounding the fourth smaller galaxy ( M * ∼ 10 9 M ⊙ ). CO (1–0) observations of the sample, obtained with the Large Millimeter Telescope, confirm the redshifts of three of the four galaxies, and we derive H 2 masses of (0.4–5.2) × 10 10 M ⊙ . JWST imaging with four combined NIRCam filters reveals disturbed stellar components with compact knots in two of the galaxies. We combine our new higher-redshift galaxies with previously published observations to conduct a more complete study of H i and H 2 evolution in the redshift range 0–0.5. With our H i flux-limited observations compared to similar lower-redshift galaxies with high stellar mass ( M * > 10 10 M ⊙ ), the results show that the mean H 2 /H i ratio at the highest redshift is 10.3 ± 3.4, larger than the mean H 2 /H i ratio in the local Universe.