2024ApJ...963...86L

Using data from ALFALFA, xGASS, H I-MaNGA, and the Sloan Digital Sky Survey (SDSS), we identify a sample of 47 “red but H I-rich” (RR) galaxies with near-UV (NUV) - r > 5 and unusually high H I-to-stellar mass ratios. We compare the optical properties and local environments between the RR galaxies and a control sample of “red and H I-normal” (RN) galaxies that are matched in stellar mass and color. The two samples are similar in the optical properties typical of massive red (quenched) galaxies in the local Universe. The RR sample tends to be associated with slightly lower-density environments and has lower clustering amplitudes and smaller neighbor counts at scales from several hundred kiloparsecs to a few megaparsecs. The results are consistent with the RR galaxies being preferentially located at the center of low-mass halos, with a median halo mass ∼10¹²h^–1M_⊙ compared to ∼10^12.5h^–1M_⊙ for the RN sample. This result is confirmed by the SDSS group catalog, which reveals a central fraction of 89% for the RR sample, compared to ∼60% for the RN sample. If assumed to follow the H I size-mass relation of normal galaxies, the RR galaxies have an average H I-to-optical radius ratio of R_HI/R₉₀ ∼ 4, four times the average ratio for the RN sample. We compare our RR sample with similar samples in previous studies, and quantify the population of RR galaxies using the SDSS complete sample. We conclude that the RR galaxies form a unique but rare population, accounting for only a small fraction of the massive quiescent galaxy population. We discuss the formation scenarios of the RR galaxies.