H I 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 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 ≈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 -- 8logMₛtar/M_11 with a median of logMₛtar/M_ₘed≈10 -- and a wide range in redshift -- 0.24 z0.63 with a median of zₘed=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 MH\;I≈10¹⁰M_. We find a high H I-to-stellar mass ratio of MH\;I/Mₛtar≈6 for low-mass galaxies with logMₛtar/M_ ≈9.3 ($>3.7\,σ$). For galaxies with logMₛtar/M_≈10.6, we find MH\;I/Mₛtar≈0.3 ($>4.7\,σ$). 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.