Recent Developments on the (H\scriptscriptstyle\rm I) Gas of Low-Redshift Galaxies Seen by the 21 cm Emission Lines

As a major interstellar medium, the atomic neutral hydrogen (H \scriptscriptstyle\rm I) plays an important role in the galaxy evolution. It provides the ingredient for star formation, and sensitively traces the internal processes and external perturbations influencing the galaxy. With the beginning of many new radio telescopes and surveys, H \scriptscriptstyle\rm I may make a more significant contribution to the understanding of galaxies in the near future. This review discusses the major development of the 21 cm emission-line H \scriptscriptstyle\rm I observations and studies in the past few years, including its scaling relations with other galaxy properties, its kinematics and structures, its role in environmental studies, and its constraints on hydrodynamical simulations. The local-Universe H \scriptscriptstyle\rm I scaling relations of stellar-mass-selected samples extend smoothly to 10^9M_\odot stellar mass, with a tentative evolution to the redshift of \sim 0.1. The development of measurement techniques enables better estimations of H \scriptscriptstyle\rm I non-circular motion, dispersion, and thickness, and new observations revealed extended or extra-planar H \scriptscriptstyle\rm I structures, both helpfully constraining the gas accretion, stellar feedback, and star formation processes of galaxy evolution models. H \scriptscriptstyle\rm I is very useful for tracing the satellite evolution in dense environments, the studies of which would benefit from ongoing blind H \scriptscriptstyle\rm I surveys. Though simulations still cannot fully reproduce H \scriptscriptstyle\rm I gas properties, they help to understand the role of possible factors in regulating H \scriptscriptstyle\rm I properties.