The Central 300 pc of the Galaxy Probed by Infrared Spectra of H₃ ⁺ and CO. III. Locations of Sgr B2 and Star Iota

Abstract

Using a simple relation between the radial expansion velocity of diffuse gas in the Central Molecular Zone (CMZ) of the Galaxy and its distance from Sgr A* we estimate the physical depths within the CMZ of Star ι (2MASS J17470898-2829561) and two Sgr B2 far-infrared continuum sources with respect to the location of Sgr A^*. To do this we use velocity profiles of infrared absorption lines of ${{\rm{H}}}_{3}^{+}$ and of far-infrared absorption lines of H₂O⁺, OH⁺, and ¹³CH⁺. The distances to Star ι and to the Sgr B2 sources are found to be ~90 pc greater than the distance to Sgr A*. Our conclusion that Sgr B2 lies toward the rear of the CMZ is contrary to most previous models in which it has been placed shallower than Sgr A*.