2017A&A...600A..48G

Aims. We aim to understand the unexpected presence of mm-wave molecular absorption at -200km/s<v←140km/s in a direction that is well away from regions of the Galactic bulge where CO emission at such velocities is prominent.
Methods. We compared 89GHz Cycle 2 ALMA absorption spectra of HCO⁺, HCN, and HNC toward the extragalactic continuum source B1741-312 at l=-2.14°, b=-1.00° with existing CO, HI, and dust emission and absorption measurements. We placed the atomic and molecular gas in the bulge and disk using circular and non-circular galactic kinematics, deriving N(HI) from a combination of 21cm emission and absorption and we derive N(H₂) from scaling of the HCO⁺ absorption. We then inverted the variation of near-IR reddening E(J-K) with distance modulus and scale E(J-K) to a total gas column density N(H) that may be compared to N(HI) and N(H₂).
Results. At galactocentric radii R_gal>1.5kpc, conventional measures such as the standard CO-H₂ conversion factor and locally observed N(HCO⁺)/N(H₂) ratio separately imply that HI and H₂ contribute about equally to N(H), and the gas-derived N(H) values are in broad agreement with those derived from E(J-K). Within the Galactic bulge at R_gal<1.5kpc, HI contributes less than 10% of the material inferred from E(J-K), so that the molecular absorption detected here is needed to understand the extinction.