TL;DR
This paper discusses the discovery of extended HI disks in nearby spiral galaxies, their warped structures, and the implications for dark matter research using tilted ring models and velocity dispersion observations.
Contribution
It highlights the presence of very extended HI disks in spiral galaxies and explores methods to analyze their rotation curves and dark matter content.
Findings
Extended HI disks reach 2-2.5 times the Holmberg radius.
Warped galaxy disks can be modeled with tilted ring models.
Velocity dispersion observations may help resolve disk-halo degeneracy.
Abstract
In this short write-up, I will concentrate on a few topics of interest. In the 1970s I found very extended HI disks in galaxies such as NGC 5055 and NGC 2841, out to 2 - 2.5 times the Holmberg radius. Since these galaxies are warped, a "tilted ring model" allows rotation curves to be derived, and evidence for dark matter to be found. The evaluation of the amount of dark matter is hampered by a disk-halo degeneracy, which can possibly be broken by observations of velocity dispersions in both the MgI region and the CaII region.
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