ALMA hints at the existence of an unseen reservoir of diffuse molecular gas in the Galactic bulge

摘要

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 ( H_(2) ) 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 ( H_(2) ). Results. At galactocentric radii R _(gal) & 1.5 kpc, conventional measures such as the standard CO- H_(2) conversion factor and locally observed N (HCO ~(+) )/ N ( H_(2) ) ratio separately imply that H I and H_(2) 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) & 1.5 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.