The diffuse molecular component in the nuclear bulge of the Milky Way^★

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: riquelme@mpifr-bonn.mpg.de
² Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
³ Astronomy Department King Abdulaziz University, PO Box 80203, 21589 Jeddah, Saudi Arabia
⁴ NSF Division of Astronomical Sciences. Suite 1053, 4201 Wilson Blvd., Arlington, VA 22230, USA

Received: 10 February 2017
Accepted: 4 September 2017

Abstract

Context. The bulk of the molecular gas in the central molecular zone (CMZ) of the Galactic center region shows warm kinetic temperatures, ranging from >20 K in the coldest and densest regions (n ~ 10^4-5 cm^-3) up to more than 100 K for densities of about n ~ 10³ cm^-3. Recently, a more diffuse, hotter (n ~ 100 cm^-3, T ~ 250 K) gas component was discovered through absorption observations of H₃⁺. This component may be widespread in the Galactic center, and low density gas detectable in absorption may be present even outside the CMZ along sightlines crossing the extended bulge of the Galaxy.

Aim. We aim to observe and characterize diffuse and low density gas using observations of 3-mm molecular transitions seen in absorption.

Methods. Using the Atacama Large (sub)Millimeter Array (ALMA) we observed the absorption against the quasar J1744-312, which is located toward the Galactic bulge region at (l, b) = (−2°.13, −1°.0), but outside the main molecular complexes.

Results. ALMA observations in absorption against the J1744-312 quasar reveal a rich and complex chemistry in low density molecular and presumably diffuse clouds. We detected three velocity components at ~0, −153, and −192 km s⁻¹. The component at ~0 km s⁻¹ could represent gas in the Galactic disk while the velocity components at −153, and −192 km s⁻¹ likely originate from the Galactic bulge. We detected 12 molecules in the survey, but only 7 in the Galactic bulge gas.