Distance to the Galactic centre 7$^\prime$ halo from HI absorption

TL;DR

This study uses archival HI absorption data to determine that the 7 arcminute halo around the Sgr A complex is located at the Galactic Center, revealing its position relative to molecular clouds through absorption features.

Contribution

First to clearly establish the 7 arcminute halo's location at the Galactic Center using HI absorption and molecular cloud data.

Findings

The halo is partly in front of and partly behind the +50 km/s molecular cloud.

The halo is located at the same distance as the +50 km/s cloud, i.e., at the Galactic Center.

HI absorption features reveal the halo's position relative to known molecular clouds.

Abstract

Using archival data, we have made an HI absorption study of the 7$^\prime$ halo surrounding the Sgr A complex, observed towards the Galactic centre (GC) region. We find strong HI absorption near velocities of $-$53 km s$^{-1}$, which is due to the 3-kpc arm, placing it beyond 5 kpc from us. We further examined the HI absorption properties towards 5 different parts of the 7$^\prime$ halo. Absorption by +50 km s$^{-1}$ GC cloud is seen towards only 3 parts of the halo, but not towards the other 2 regions. Strong emissions in CO and CS are, however, identified toward all the above 5 parts of the halo by the +50 km s$^{-1}$ GC molecular cloud. This does show that the 7$^\prime$ halo is partly behind, and partly in front of the +50 km s$^{-1}$ cloud. To our knowledge, this, for the first time clearly shows the 7$^\prime$ halo to be located at the same distance as the +50 km s$^{-1}$ molecular cloud, i.e., at the GC region.