HI Absorption Toward HII Regions at Small Galactic Longitudes

TL;DR

This study analyzes HI absorption toward HII regions within 10 degrees of the Galactic center, revealing structures like the 3kpc Arms and H I Tilted Disk, and constraining distances using absorption spectra and recombination line velocities.

Contribution

It provides a comprehensive mapping of HI absorption features in the inner Galaxy and introduces a method to estimate HII region distances using combined spectral data.

Findings

Detection of HI absorption associated with key Galactic structures.

Constraints on HII region distances through spectral analysis.

Mapping of the inner Galaxy's HI absorption features.

Abstract

We make a comprehensive study of HI absorption toward HII regions located within Galactic longitudes less than 10 degrees. Structures in the extreme inner Galaxy are traced using the longitude-velocity space distribution of this absorption. We find significant HI absorption associated with the Near and Far 3kpc Arms, the Connecting Arm, Banias Clump 1 and the H I Tilted Disk. We also constrain the line of sight distances to HII regions, by using HI absorption spectra together with the HII region velocities measured by radio recombination lines.