HI absorption in the direction of the galactic centre

Abstract

A gaussian analysis of the H_I absorption spectrum of Sgr A obtained with the Parkes Interferometer has yielded the following results. The optical depth at zero velocity has contributions from both a narrow deep feature, and an unexpected, very wide, low optical-depth feature. The peak optical depths for these two components are 4.3 and 0.3 respectively. The width of the former feature, identified with conventional cold concentrations leads to a new determination for the external motions of interstellar clouds of σ = 4.8 km s^-1. The new wide feature represents an amount of neutral hydrogen comparable to that in standard cold concentrations and contains most of the energy in mass motions of the interstellar gas. Neglecting its contribution the amount of hydrogen seen in absorption towards Sgr A is in close agreement with that expected from studies in other directions in the galactic plane. Both the above components have mean velocities differing from zero by less than 0.25 km s^-1, thus confirming the standard value of the solar motion and contradicting galactic models which postulate a radial motion of the local standard of rest.