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 km s-1<v<−140 km s-1 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 HCO+, 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 21 cm 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(H I) and N(H2). Results. At galactocentric radii Rgal> 1.5 kpc, conventional measures such as the standard CO-H2 conversion factor and locally observed N(HCO+)/N(H2) ratio separately imply that H I 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 Rgal< 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.