H I Properties of Field Galaxies at $\boldsymbol{z\approx 0.2}$-0.6: Insights into Declining Cosmic Star Formation

TL;DR

This study delivers the first statistically significant detection of H I $21\,$cm emission from field galaxies at intermediate redshift ($0.24\le z\le0.63$) by stacking $\approx$6000 galaxies across 11 CUBS-MALS fields, using MeerKAT data to measure mean neutral gas content. The analysis reveals $\langle M_{\mathrm{H\,I}}\rangle\approx1.3\times10^{10}\ M_\odot$ for the full sample, with $M_{\mathrm{H\,I}}/M_\star\approx6$ for low-mass and $\approx0.3$ for high-mass galaxies, and a redshift evolution that tracks the cosmic star formation history via the Kennicutt–Schmidt relation. Importantly, HI signals extend well beyond the stellar bodies, with significant HI detected out to radius $>75$ kpc, indicating substantial extended HI halos. The results underscore the impact of aperture and sample selection on inferred gas fractions and support a scenario in which the declining supply of neutral gas contributes to the downturn in cosmic star formation, while highlighting the need to account for extended gas reservoirs in interpreting H I measurements at intermediate redshift.

Abstract

We report statistically significant detection of H I 21-cm emission from intermediate-redshift ($z\approx0.2$-0.6) galaxies. By leveraging multi-sightline galaxy survey data from the Cosmic Ultraviolet Baryon Survey (CUBS) and deep radio observations from the MeerKAT Absorption Line Survey (MALS), we have established a sample of $\approx6000$ spectroscopically identified galaxies in 11 distinct fields to constrain the neutral gas content at intermediate redshifts. The galaxies sample a broad range in stellar mass -- $8\lesssim\log{M_\rm{star}/\rm{M}_\odot}\lesssim11$ with a median of $\langle\log{M_\rm{star}/\rm{M}_\odot}\rangle_\rm{med}\approx10$ -- and a wide range in redshift -- $0.24\lesssim z\lesssim0.63$ with a median of $\langle z\rangle_\rm{med}=0.44$. Our detected emission-line signal exceeds $4\,σ$ significance in the stacked spectra of all subsamples, and the observed total H I 21-cm line flux translates to a H I mass $M_\rm{H\;I}\approx10^{10}\rm{M}_\odot$. We find a high H I-to-stellar mass ratio of $M_\mathrm{H\;I}/M_\rm{star}\approx6$ for low-mass galaxies with $\langle\log{M_\rm{star}/\rm{M}_\odot}\rangle \approx9.3$ ($>3.7\,σ$). For galaxies with $\langle\log{M_\rm{star}/\rm{M}_\odot}\rangle\approx10.6$, we find $M_\mathrm{H\;I}/M_\rm{star}\approx0.3$ ($>4.7\,σ$). Additionally, the redshift evolution of H I mass in both low- and high-mass field galaxies, inferred from the stacked emission-line signal, aligns well with the expectation from the cosmic star formation history. This suggests that the overall decline in the cosmic star formation activity across the general galaxy population may be connected to a decreasing supply of neutral hydrogen. Finally, our analysis has revealed significant 21-cm signals at distances greater than 75 kpc from these intermediate-redshift galaxies, indicating a substantial reservoir of H I gas in their extended surroundings.

Figures (6)

• Figure 1: Summary of galaxy properties in the CUBS-MALS galaxy sample. The data points are color-coded according to redshift, with galaxies at $z<0.44$ shown in blue and higher-redshift galaxies in red. The redshift distribution of stellar mass is presented in the left panels. In the bottom panel, the blue and red lines indicate the corresponding redshift coverage of the $L$ and UHF bands, respectively. Hatched regions demarcate bands of RFI. Except where truncated by RFI or the samples' boundary, the bins in the histogram of redshifts in the top panel bridge multiples of $\Delta z=1/30$. The dashed orange and solid green stairs represent the moving weighted median and weighted mean, respectively (see §\ref{['sec:stacking']} for details), within each bin of redshift. The stellar mass versus the observed $g-r$ color is displayed in the middle panels with the $g-r=1.1$ color threshold marked by a dashed line. In the histogram of stellar mass in the right panel, the bins span multiples of $0.2\;\mathrm{dex}$. The CUBS-MALS galaxy sample spans a broad range in stellar mass, from $\log\,\hbox{${M}_{\rm star}$}/\hbox{${\rm M}_\odot$}\approx 8$ to $\log\,\hbox{${M}_{\rm star}$}/\hbox{${\rm M}_\odot$}\approx 11$, with a median of $\langle\log\,\hbox{${M}_{\rm star}$}/\hbox{${\rm M}_\odot$}\rangle_{\rm med}\approx 10$ and a wide range in redshift from $z\approx 0.24$ to $z\approx 0.63$ with a median of $\langle\,z\rangle_{\rm med}=0.44$. The observed $g-r$ color correlates well with ${M}_{\rm star}$ but with a large scatter.
• Figure 2: Weighted mean ( left) and weighted median ( right) spectra of the H1 21 cm emission line extracted using a physical aperture of 300 kpc in diameter around CUBS-MALS galaxies at $0.24<z<0.63$. The stacked spectra of CUBS galaxies are shown in thick green, while the reference spectra are shown in thin orange (see § \ref{['sec:stacking']} for details). The gray band marks the RMS fluctuations in the error spectrum. The signal associated with the full sample is presented in the top panel, where the signals extracted for individual subsamples are presented in the bottom four rows. The dotted vertical lines mark the velocity window for extracting the total H1 mass for each sample. The 21 cm line is confidently detected in all samples with high-$z$ galaxies displaying the strongest signal strength in both low- and high-mass galaxies (bottom two panels). The mean stacks for 150 kpc- and 450 kpc-diameter apertures are shown in Appendix \ref{['sec:Stacks-app']}.
• Figure 3: Cumulative mean H1 mass with increasing aperture diameter. Doubling the extraction aperture diameter, from 150 kpc to 300 kpc, increases the extracted total H1 21-cm line flux by more than a factor of two, indicating a significant amount of neutral gas located beyond 75 kpc from the central galaxies, while further increasing the aperture diameter to 450 kpc results in a substantial increase in noise without a corresponding gain in signal (see also Figure \ref{['Fig:Stacks-app']}).
• Figure 4: Variation of mean H1 mass with redshift (panel a) and galaxy stellar mass (panel b). Data points and vertical error bars mark the weighted mean H1 mass measurements and the associated uncertainties, color-coded according to redshift following what is presented in Figure \ref{['fig:RedshiftStellarMass']}. The horizontal bars indicate the dispersion of each subsample. For comparison, the solid green curve in panel (a) shows the anticipated redshift evolution of neutral gas content inferred from the cosmic star-formation rate density Madau2014, assuming a Kennicutt--Schmidt relation Kennicutt1998. The solid green line and the associated shaded band in panel (b) show the scaling relation and uncertainties from the combined MIGHTEE-H1+CHILES study by Bianchetti2025NewData. The gray dashed lines mark fixed ${M}_{\mathrm{H}\;\textsc{i}}$/${M}_{\rm star}$ ratios to facilitate visual calibrations.
• Figure A.1: Weighted mean spectral stacks for apertures with diameters of 150kpc ( left) and 450kpc ( right). The remaining details are the same as in Figure \ref{['Fig:Stacks']}.