2025ApJ...993L..18D

We report statistically significant detection of H I 21 cm emission from intermediate-redshift (z ≈ 0.2-0.6) galaxies. By leveraging multisightline galaxy survey data from the Cosmic Ultraviolet Baryon Survey and deep radio observations from the MeerKAT Absorption Line Survey, we have established a sample of ≈6000 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, from $\mathrm{log}\,{M}_{\mathrm{star}}/{M}_{\odot }\approx 8$ to $\mathrm{log}\,{M}_{\mathrm{star}}/{M}_{\odot }\approx 11$, with a median of ${\langle \mathrm{log}\,{M}_{\mathrm{star}}/{M}_{\odot }\rangle }_{\mathrm{med}}\approx 10$ and a wide range in redshift from z ≈ 0.24 to z ≈ 0.63 with a median of 〈 z〉_med = 0.44. While no individual galaxies show detectable H I emission, the emission line signal is detected in the stacked spectra of all subsamples at greater than 4σ significance. The observed total H I 21 cm line flux translates to a H I mass, M_H I_ ≈10¹⁰M_⊙. We find a high H I-to-stellar-mass ratio of M_H I_/M_star ≈ 6 for low-mass galaxies with $\langle \mathrm{log}\,{M}_{\mathrm{star}}/{M}_{\odot }\rangle \approx 9.3$ (>3.7σ). For galaxies with $\langle \mathrm{log}\,{M}_{\mathrm{star}}/{M}_{\odot }\rangle \approx 10.6$, we find M_H I_/M_star ≈ 0.3 (>4.7σ). In addition, the redshift evolution of H I mass, 〈M_H I_〉, 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.