Kinetic temperatures toward X1/X2 orbit interceptions regions and giant molecular loops in the Galactic center region^⋆

¹ Instituto de Radioastronomía Milimétrica (IRAM), Av. Divina Pastora 7, Local 20, 18012 Granada, Spain
e-mail: riquelme@mpifr-bonn.mpg.de
² Centro de Astrobiología (CSIC/INTA), Ctra. de Torrejón a Ajalvir km 4, 28850 Torrejón de Ardoz, Madrid, Spain
³ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
⁵ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile

Received: 17 October 2011
Accepted: 6 September 2012

Abstract

Context. It is well known that the kinetic temperatures, T_kin, of the molecular clouds in the Galactic center region are higher than in typical disk clouds. However, the T_kin of the molecular complexes found at higher latitudes towards the giant molecular loops in the central region of the Galaxy is so far unknown. The gas of these high-latitude molecular clouds (hereafter referred to as “halo clouds”) is located in a region where the gas in the disk may interact with the gas in the halo in the Galactic center region.

Aims. To derive T_kin in the molecular clouds at high latitude and understand the physical process responsible for the heating of the molecular gas both in the central molecular zone (the concentration of molecular gas in the inner  ~500 pc) and in the giant molecular loops.

Methods. We measured the metastable inversion transitions of NH₃ from (J,K) = (1,1) to (6,6) toward six positions selected throughout the Galactic central disk and halo. We used rotational diagrams and large velocity gradient (LVG) modeling to estimate the kinetic temperatures toward all the sources. We also observed other molecules like SiO, HNCO, CS, C³⁴S, C¹⁸O, and ¹³CO, to derive the densities and to trace different physical processes (shocks, photodissociation, dense gas) expected to dominate the heating of the molecular gas.

Results. We derive for the first time T_kin of the high-latitude clouds interacting with the disk in the Galactic center region. We find high rotational temperatures in all the observed positions. We derive two kinetic temperature components (~150 K and  ~40 K) for the positions in the central molecular zone, and only the warm kinetic temperature component for the clouds toward the giant molecular loops. The fractional abundances derived from the different molecules suggest that shocks provide the main heating mechanism throughout the Galactic center, also at high latitudes.