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

作者：Gerin M^*; Liszt H

来源：Astronomy & Astrophysics, 2017, 600: A48.

DOI：10.1051/0004-6361/201730400

• Galaxy: bulge
• ISM: atoms
• ISM: molecules

Aims. We aim to understand the unexpected presence of mm-wave molecular absorption at 200 km s(-1) < nu < 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 degrees, b = -1.00 degrees 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.

• 出版日期2017-4