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 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.