HI in the Outskirts of Nearby Galaxies

TL;DR

This paper reviews the role of HI observations in understanding dark matter, galaxy structure, and gas accretion in nearby galaxies, highlighting recent advances and ongoing challenges in the field.

Contribution

It provides a comprehensive overview of high-resolution HI imaging, its impact on dark matter studies, and discusses current issues like the disk-halo degeneracy and gas accretion processes.

Findings

HI extends beyond optical images, tracing dark matter.

Discrepancies exist between dynamical methods for dark matter.

HI observations help address the core-cusp problem in dwarf galaxies.

Abstract

The HI in disk galaxies frequently extends beyond the optical image, and can trace the dark matter there. I briefly highlight the history of high spatial resolution HI imaging, the contribution it made to the dark matter problem, and the current tension between several dynamical methods to break the disk-halo degeneracy. I then turn to the flaring problem, which could in principle probe the shape of the dark halo. Instead, however, a lot of attention is now devoted to understanding the role of gas accretion via galactic fountains. The current $\rm \Lambda$ cold dark matter theory has problems on galactic scales, such as the core-cusp problem, which can be addressed with HI observations of dwarf galaxies. For a similar range in rotation velocities, galaxies of type Sd have thin disks, while those of type Im are much thicker. After a few comments on modified Newtonian dynamics and on irregular galaxies, I close with statistics on the HI extent of galaxies.