CHILES X: Molecular and atomic gas at intermediate redshift

TL;DR

This paper presents the first spatially resolved CO(1-0) observations of HI-detected galaxies beyond the local universe, using 14 CHILES galaxies at z ≈ 0.12. Five galaxies show resolved CO with rotating-disk kinematics, and stacking of the non-detections yields a mean molecular mass of $\log(M_{H_2}/M_\odot)=8.46$ at $\log(M_*/M_\odot)=9.35$. The study finds HI to be more extended than CO, while CO emission aligns with regions of high stellar-mass density and dust-obscured star formation traced by 1.4 GHz continuum, and it places CHILES results in the broader context of local and intermediate-redshift gas surveys. Comparison with literature suggests mild evolution in molecular and atomic gas reservoirs with redshift, but substantial galaxy-to-galaxy scatter and environmental effects—particularly in clusters—limit definitive evolutionary inferences within this redshift range. The work also emphasizes the challenges posed by radio frequency interference for HI studies at these frequencies and foreshadows the role of SKA-era observations in expanding our view of gas evolution in galaxies.

Abstract

We present ALMA CO observations of 14 HI-detected galaxies from the CHILES survey found in a cosmic over-density at z~0.12. This is the largest collection of spatially resolved CO + HI observations beyond the local Universe (z>0.05) to date. While the HI-detected parent sample spans a range of stellar masses, star formation rates (SFR), and environments, we only directly detect CO in the highest stellar mass galaxies, log(M_*/M_Sun)>10.0, with SFRs greater than ~2 M_Sun/yr. The detected CO has the kinematic signature of a rotating disk, consistent with the HI. We stack the CO non-detections and find a mean H_2 mass of log(M_H2/M_Sun) = 8.46 in galaxies with a mean stellar mass of log(M_*/M_Sun) = 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. We interpret the redder colors as the molecular gas being coincident with dusty regions of obscured star formation. The 14 HI detections show a range of morphologies, but the HI 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 H_2-to-HI gas ratio with redshift in HI flux-limited samples. We show that the scatter in the HI, and HI-to-stellar mass ratio is too great to conclusively measure evolution below z=0.2, and is even extremely difficult below z=0.4. Detections from CHILES are likely to be the only individual galaxies detected in HI 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 contemporary HI surveys.

Figures (20)

• Figure 1: H i contours overlaid on DECaLS griz false-color images. Galaxies are ordered top-to-bottom, left-to-right as they appear in Table \ref{['tab:observations']}. Top row: CHILES galaxies detected in CO (1-0) by ALMA. H i column densities are $2^n\times[2.6, 2.1, 2.8, 1.8, 2.0]\times10^{20}$cm$^{-2}$$(n=0,1,2...)$. Middle and bottom row: CHILES galaxies that are undetected in CO (1-0). H i column densities are as above: $2^n\times[2.1, 3.2, 2.0, 1.8]$, and $[1.8, 2.6, 1.7, 1.3]\times10^{20}$cm$^{-2}$$(n=0,1,2...)$. There is a clear dichotomy between the two sets of galaxies: CO detections are noticeably redder, while CO non-detections are significantly bluer. See text for discussion. Note that the top left-most galaxy (0969208) has a star superimposed to the north of the nucleus which appears white. The red area to the south of the star is from the background galaxy.
• Figure 2: Molecular and atomic gas properties of CHILES $z=0.12$ galaxies
• Figure 3: Left: HST/ACS F814W images overlaid with H i contours (purple to teal) as in Fig \ref{['fig:decals']} and CO (1-0) contours (white to red). CO contours correspond to H$_2$ column densities $2^n\times[3.6, 2.6, 2.3, 2.3, 3.9]\times10^{20}$cm$^{-2}$$(n=0,1,2,...)$ Center: CHILES Con Pol 1.4 GHz continuum images, overlaid with CO contours (white to dark red). Right: Spitzer 3.6 $\mu$m images, converted to bins of stellar mass surface density overlaid with CO contours (black to white). Galaxies are presented in the same order top-to-bottom that they are left-to-right in Figure \ref{['fig:decals']}.
• Figure 4: H i (top) and CO (bottom) intensity weighted velocity (moment 1) maps for the five CO detections. The kinematic major axis is indicated by the dashed line. The position angle, systemic velocity, profile width at 20% and 50% of the peak ($W_{20}$ and $W_{50}$), as calculated by SoFiA-2, are written inset at the bottom of the figures. The H i and CO for each galaxy are plotted on the same scale.
• Figure 5: Stacked images for the CO detections (top row) and CO non-detections (bottom row). From left to right, we stacked HST ACS mosaic images in gray scale, HI total intensity (moment 0) maps in blue, and CO maps collapsed over the channels indicated in Figure \ref{['fig:stack_spectra']} in orange. In each image the concentric circles represent [2, 4, 6] arcsec radius apertures over which we extracted the CO spectra. Inset in the lower right corresponds to the stacked non-detections, smoothed to 4 arcsec resolution (see text). Consistently across all images, the CO-detected galaxies in the top row appear spatially larger on the sky.