ALMA hints at the existence of an unseen reservoir of diffuse molecular gas in the Galactic bulge
Abstract
Aims. We aim to understand the unexpected presence of mm-wave molecular absorption at -200 < v < -140 \ in a direction that is well away from regions of the Galactic bulge where CO emission at such velocities is prominent. Methods. We compared 89 GHz Cycle 2 ALMA absorption spectra of , HCN, and HNC toward the extragalactic continuum source B1741-312 at l=-2.14, b=-1.00\ with existing CO, H I, 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(H I) from a combination of 21cm emission and absorption and we derive N() from scaling of the \ 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(H I) and N(). Results. At galactocentric radii \ > 1.5 kpc, conventional measures such as the standard CO-\ conversion factor and locally observed N()/N() ratio separately imply that H I and \ 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 < 1.5 kpc, H I 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.