Living on the edge of the Milky Way's central molecular zone

G1.3 is the more likely candidate for gas accretion into the CMZ^★

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: labusch@mpifr-bonn.mpg.de
² Institute for Astrophysical Research, 725 Commonwealth Ave, Boston University Boston, MA 02215, USA
³ Haystack Observatory, Massachusetts Institute of Technology, 99 Millstone Road, Westford, MA 01886, USA

Received: 2 September 2022
Accepted: 21 October 2022

Abstract

Context. The 1°.3 (G1.3) and 1°.6 (G1.6) cloud complexes in the central molecular zone (CMZ) of our Galaxy have been proposed to possibly reside at the intersection region of the X1 and X2 orbits for several reasons. This includes the detection of co-spatial low- and high-velocity clouds, high velocity dispersion, high fractional molecular abundances of shock-tracing molecules, and kinetic temperatures that are higher than for usual CMZ clouds.

Aims. By investigating the morphology and deriving physical properties as well as chemical composition, we want to find the origin of the turbulent gas and, in particular, whether evidence of an interaction between clouds can be identified.

Methods. We mapped both cloud complexes in molecular lines in the frequency range from 85 to 117 GHz with the IRAM 30 m telescope. The APEX 12m telescope was used to observe higher frequency transitions between 210 and 475 GHz from selected molecules that are emitted from higher energy levels. We performed non-local thermodynamic equilibrium (non-LTE) modelling of the emission of an ensemble of CH₃CN lines to derive kinetic temperatures and H₂ volume densities. These were used as starting points for non-LTE modelling of other molecules, for which column densities and abundances were determined and compared with values found for other sources in the CMZ.

Results. The kinematic structure of G1.3 reveals an ‘emission bridge’ at intermediate velocities (~150 km s⁻¹) connecting low-velocity (~100 km s⁻¹) and high-velocity (~180 km s⁻¹) gas and an overall fluffy shell-like structure. These may represent observational evidence of cloud-cloud interactions. Low- and high-velocity gas components in G1.6 do not show this type of evidence of an interaction, suggesting that they are spatially separated. We selected three positions in each cloud complex for further analysis. Each position reveals several gas components at various peak velocities and of various line widths. We derived kinetic temperatures of 60–100 K and H₂ volume densities of 10⁴–10⁵ cm⁻³ in both complexes. Molecular abundances relative to H₂ suggest a similar chemistry of the two clouds, which is moreover similar to that of other GC clouds and, especially, agrees well with that of G+0.693 and G−0.11.

Conclusions. We conclude that G1.3 may indeed exhibit signs of cloud-cloud interactions. In particular, we propose an interaction of gas that is accreted from the near-side dust lane to the CMZ, with gas pre-existing at this location. Low- and high-velocity components in G1.6 are rather coincidentally observed along the same line of sight. They may be associated with either overshot decelerated gas from the far-side dust line or actual CMZ gas and high-velocity gas moving on a dust lane. These scenarios would be in agreement with numerical simulations.