Recent Developments on the HI Gas of Low-Redshift Galaxies Seen by the 21cm Emission Lines

TL;DR

This review summarizes recent advances in 21cm HI emission-line studies of low-redshift galaxies, highlighting new observational techniques, key findings on galaxy properties, and future prospects with upcoming radio surveys.

Contribution

It provides a comprehensive overview of recent observational developments, analysis methods, and the role of HI in galaxy evolution, including insights from new surveys and simulations.

Findings

HI scaling relations extend to stellar masses of 10^9 M_sun.

New techniques improve estimates of HI kinematics and structure.

Extended and extra-planar HI structures constrain galaxy evolution models.

Abstract

As a major interstellar medium, the atomic neutral hydrogen (HI) 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, HI may make a more significant contribution to the understanding of galaxies in the near future. This review discusses the major development of the $21\,\text{cm}$ emission-line HI 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 HI scaling relations of stellar-mass--selected samples extend smoothly to $10^9\,\text{M}_\odot$ stellar mass, with a tentative evolution to the redshift of ${\sim}0.1$. The development of measurement techniques enables better estimations of HI non-circular motion, dispersion, and thickness, and new observations revealed extended or extra-planar HI structures, both helpfully constraining the gas accretion, stellar feedback, and star formation processes of galaxy evolution models. HI is very useful for tracing the on-going satellite evolution in dense environments, the studies of which would benefit from ongoing blind HI surveys. Though simulations still cannot fully reproduce HI gas properties, they help to understand the role of possible factors in regulating HI properties. We also discuss possible future progress with new observations at FAST.