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 \kms $< {\rm v} < -140$ \kms\ 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 \hcop, HCN, and HNC toward the extragalactic continuum source B1741-312 at l=-2.14\degr, b=-1.00\degr\ 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(\HH) from scaling of the \hcop\ 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(\HH). Results. At galactocentric radii \Rgal\ $>$ 1.5 kpc, conventional measures such as the standard CO-\HH\ conversion factor and locally observed N(\hcop)/N(\HH) ratio separately imply that H I and \HH\ 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.