Distance to the Galactic centre 7 halo from HI absorption
Abstract
Using archival data, we have made an HI absorption study of the 7 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 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 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 halo to be located at the same distance as the +50 km s-1 molecular cloud, i.e., at the GC region.
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