Radial HI Profiles at the Periphery of Galactic Disks: The Role of Ionizing Background Radiation

TL;DR

This study investigates whether external ionizing radiation alone explains the sharp HI density cutoff in spiral galaxy disks, finding that other factors likely contribute to the observed profiles.

Contribution

It challenges the assumption that background radiation solely causes HI cutoff, showing that other mechanisms are involved in most cases.

Findings

Ionization by background radiation is insufficient to explain HI cutoff in most galaxies.

In some galaxies, the HI profile steepening can be caused by ionization.

The cutoff occurs closer to galaxy centers than expected if caused solely by background radiation.

Abstract

Observations of neutral hydrogen in spiral galaxies reveal a sharp cutoff in the radial density profile at some distance from the center. Using 22 galaxies with known HI distributions as an example, we discuss the question of whether this effect can be associated exclusively with external ionizing radiation, as is commonly assumed. We show that before the surface density reaches $\sigma_{\textrm{HI}}\le 0.5 {\cal M}_\odot/{\textrm {pc}}^2$(the same for galaxies of different types), it is hard to expect the gas to be fully ionized by background radiation. For two of 13 galaxies with a sharp drop in the HI profile, the "steepening" can actually be caused by ionization. At the same time, for the remaining galaxies, the observed cutoff in the radial HI profile is closer to the center than if it was a consequence of ionization by background radiation and, therefore, it should be caused by other factors.