Disk Stability and Neutral Hydrogen as a Tracer of Dark Matter

TL;DR

This paper demonstrates that the distribution of neutral hydrogen in galaxies traces dark matter over large radii because the ISM disk self-regulates its stability, reflecting the underlying dark matter potential.

Contribution

It provides a theoretical framework linking ISM stability, surface density profiles, and dark matter distribution, supported by observational data from 20 galaxies.

Findings

ISM disks maintain a constant Toomre Q parameter across radii.

The surface density of HI follows a 1/R decline, correlating with the potential.

HI traces dark matter due to disk self-regulation in a dark matter dominated potential.

Abstract

We derive the projected surface mass distribution Sigma_M for spherically symmetric mass distributions having an arbitrary rotation curve. For a galaxy with a flat rotation curve and an ISM disk having a constant Toomre stability parameter, Q, the ISM surface mass density Sigma_g as well as Sigma_M both fall off as 1/R. We use published data on a sample of 20 well studied galaxies to show that ISM disks do maintain a constant Q over radii usually encompassing more than 50% of the HI mass. The power law slope in Sigma_g covers a range of exponents and is well correlated with the slope in the epicyclic frequency. This implies that the ISM disk is responding to the potential, and hence that secular evolution is important for setting the structure of ISM disks. We show that the gas to total mass ratio should be anti-correlated with the maximum rotational velocity, and that the sample falls on the expected relationship. A very steep fall off in Sigma_g is required at the outermost radii to keep the mass and angular momentum content finite for typical rotation curve shapes, and is observed. The observation that HI traces dark matter over a significant range of radii in galaxies is thus due to the disks stabilising themselves in a normal dark matter dominated potential. This explanation is consistent with the cold dark matter paradigm.