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

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.

Accepted for publication in ApJ