HI Properties of Field Galaxies at z ≈ 0.2–0.6: Insights into Declining Cosmic Star Formation

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 log Mstar/M 8 to log Mstar/M 11, with a median of log Mstar/M med 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, MH I ≈1010M☉. We find a high H I-to-stellar-mass ratio of MH I/Mstar ≈ 6 for low-mass galaxies with log Mstar/M 9.3 (>3.7σ). For galaxies with log Mstar/M 10.6, we find MH I/Mstar ≈ 0.3 (>4.7σ). In addition, the redshift evolution of H I mass, 〈MH 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.