CHILES XI: Resolved HI morphologies and the evolution of the H2/HI ratio over the last five billion years

TL;DR

This study investigates the morphology and gas ratios of four galaxies at intermediate redshifts, revealing that the H2/HI ratio was significantly higher five billion years ago, indicating evolution in galaxy gas composition.

Contribution

First resolved HI morphologies at z=0.22-0.47 with detailed analysis of gas ratios and galaxy evolution over five billion years.

Findings

H2/HI ratio was 10.3 times higher at z~0.3 than today

Galaxies show varied HI morphologies from regular to disturbed

Confirmed H2 masses range from 0.4 to 5.2 x 10^10 solar masses

Abstract

We present the neutral gas morphology of four galaxies from z = 0.22 to 0.47 obtained with the COSMOS HI Large Extragalactic Survey (CHILES). The HI is resolved at the highest redshift with the 7.5 arcsec beam of CHILES and 43 kpc linear scale, with all four galaxies having extended HI. Three are massive galaxies (Mstellar > 3 e10 Mo), with HI masses of 1.6 - 6.7 e10 Mo, and active star formation (3 - 30 Mo/yr). The morphology and kinematics of the galaxies vary from regular to disturbed, including an asymmetric HI disk surrounding the fourth smaller galaxy (Mstellar ~ e9 Mo). CO(1-0) observations of the sample, obtained with the LMT, confirm the redshifts of three of the four galaxies and we derive H2 masses of 0.4 - 5.2 e10 Mo. 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 HI and H2 evolution in the redshift range 0 - 0.5. With our HI flux-limited observations compared to similar lower redshift galaxies with high stellar mass (Mstellar > e10 Mo), the results show the mean H2/HI ratio at the highest redshift is 10.3 +- 3.4 larger than the mean H2/HI ratio in the local Universe.