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

TL;DR

This study directly images Hi in four galaxies at $0.22<z<0.47$ with CHILES, combining Hi morphology and kinematics with CO-derived H$_2$ masses from the LMT and resolved stellar morphologies from JWST. By placing these high-$z$ detections in the broader CHILES sample (0<z<0.5) and comparing to local surveys, the authors show that the H$_2$/Hi ratio increases markedly with redshift for high-$M_{*}$ systems (mean $10.3\pm3.4$ times the local value at the highest redshift), while Hi fractions decline and total gas fractions stay roughly constant. The results illuminate the cosmic evolution of cold gas reservoirs and their linkage to star formation, demonstrating that distant, Hi-rich disks can host significant molecular reservoirs and enhanced star formation relative to the local Universe. These findings rely on direct detections rather than stacking, and they highlight the importance of consistent CO conversion factors when comparing across redshifts, with JWST providing crucial context for interpreting gas dynamics and stellar morphology.

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.

Figures (14)

• Figure 1: Characterization of the locations of Hi detections within the CHILES image cubes. Top: The number of unflagged visibilities used in imaging the CHILES data. Lower: Kurtosis values of the CHILES data, following the processing outlined in Luber2025a. A kurtosis value of three indicates perfectly Gaussian data. The kurtosis values indicate that the noise deviates from Gaussianity in frequency ranges 964-1060 MHz and 1100-1200 MHz. The four Hi detections (turquoise lines) are in frequency ranges where little flagging is done (top) and where the noise is closer to Gaussian (bottom).
• Figure 2: Relation between the predicted Hi mass and the detected Hi mass. The pink circles are CHILES galaxies from $z$ = 0 -- 0.12 BlueBird2020Hess2019Hess2025, the purple circle is the CHILES galaxy at $z$ = 0.38 Fernandez2016, and the orange circles are new galaxies presented in this work. The diagonal line indicates equal masses, with the dex scatter of the Catinella2012 predicted mass relation shown in the shaded region.
• Figure 3: CO (1--0) spectra obtained using the Redshift Search Receiver (RSR) on the LMT. The shaded gray area indicates the area of detection. The CO line center frequency is shown (blue dashed line) along with the CO-rest-frequency equivalents for the Hi line center (purple dashed line) and the optical redshift (green dashed line). Panel a: CO(1--0) detection for Galaxy 1432541 at 78.3 GHz ($z$ = 0.4729). Panel b: CO(1--0) non-detection for galaxy 1199583. Panel c: CO(1--0) detection for galaxy 1203414 at 91.7 GHz ($z$ = 0.2577). Panel d: CO(1--0) detection for galaxy 1421092 at 94.5 GHz ($z$ = 0.2202).
• Figure 4: [Oii] a long-slit spectrum for galaxy 1432441 obtained using SALT. The shaded gray area indicates the area of detection. The [Oii] doublet center wavelength is shown (blue dashed line), which coincides with the observed CO(1-0) line center for this galaxy interpreted as $z = 0.473$.
• Figure 5: Near-IR JWST images. These images are made by combining four filters (1.15, 1.50, 2.77, 4.40 $\mu m$) from the COSMOS-Web NIRCam survey Casey2023. The filters are assigned to the red, green, and blue channels as (R,G,B) = (F444W + F277W, F277W + F150W, F150W + F115W). RGB images are constructed according to the Trilogy9 prescription Coe2012. There is no JWST image for Galaxy 1421092 as it falls out of the COSMOS-Web FOV. Panel a: JWST image for galaxy 1432541 at $z$ = 0.47. Panel b: JWST image for galaxy 1199583 at $z$ = 0.29. Panel c: JWST image for galaxy 1203414 at $z$ = 0.26.