On the existence, rareness and uniqueness of quenched HI-rich galaxies in the local Universe

Using data from ALFALFA, xGASS, HI-MaNGA and SDSS, we identify a sample of 47"red but HI-rich"(RR) galaxies with $NUV-r > 5$ and unusually highHI-to-stellar mass ratios. We compare the optical properties and localenvironments between the RR galaxies and a control sample of "red andHI-normal"(RN) galaxies that are matched in stellar mass and color. The twosamples are similar in the optical properties typical of massive red (quenched)galaxies in the local Universe. The RR sample tends to be associated withslightly lower density environments and has lower clustering amplitudes andsmaller neighbor counts at scales from several $\times$100kpc to a few Mpc. Theresults are consistent with that the RR galaxies preferentially locate at thecenter of low-mass halos, with a median halo mass $\sim 10^{12}h^{-1}M_{\odot}$compared to $\sim 10^{12.5}h^{-1}M_{\odot}$ for the RN sample. This result isconfirmed by the SDSS group catalog which reveals a central fraction of 89% forthe RR sample, compared to $\sim 60\%$ for the RN sample. If assumed to followthe HI size-mass relation of normal galaxies, the RR galaxies have an averageHI-to-optical radius ratio of $R_{HI}/R_{90}\sim 4$, four times the averageratio for the RN sample. We compare our RR sample with similar samples inprevious studies, and quantify the population of RR galaxies using the SDSScomplete sample. We conclude that the RR galaxies form a unique but rarepopulation, accounting for only a small fraction of the massive quiescentgalaxy population. We discuss the formation scenarios of the RR galaxies.