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

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 NH 3 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, C 34 S, C 18 O, and 13 CO, 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.

Footpoints of the giant molecular loops in the Galactic center region

“…A.3). Thus, we decided to use 10 K which is consistent with our estimations in Riquelme et al (2013) for CS, and with the discussion in Section 5.1.…”

Dense gas in the Galactic central molecular zone is warm and heated by turbulence

“…6 of Hüttemeister et al (1993), who make the same plot for NH 3 (4, 4) and (5, 5). Riquelme et al (2013) also found no correlation using NH 3 over a similar range of velocity dispersions. While there is no clear correlation between velocity dispersion and temperature, most data points fall within a range that is consistent with turbulent heating if the appropriate size scale is selected; see Fig.…”

The Galactic Center Molecular Cloud Survey

“…6.2 of Lucas 2015) because the gravitational force F g ∝ m/r 2 increases with increasing galactocentric radius for the observed CMZ mass profile, where m ∝ r 2.2 (Launhardt et al 2002; see Kruijssen et al 2015 for the measurement of the power-law slope in the radial interval of about 10 to 100 pc that is of interest here). The clouds are further compressed by the high external pressure; gas temperatures of typically 50 to 100 K in CMZ clouds (Güsten et al 1981;Hüttemeister et al 1993;Ao et al 2013;Mills & Morris 2013;Ott et al 2014;Ginsburg et al 2016; also see Riquelme et al 2010aRiquelme et al , 2012 imply H 2 densities 10 4 cm −3 to balance 2 the aforementioned pressure of 10 6 to 7 K cm −3 . Finally, the strong and widespread SiO emission tracing shocks (Martín-Pintado et al 1997;Hüttemeister et al 1998;Riquelme et al 2010b), prevalence of molecules likely ejected from grain surfaces via shocks 1 The classical reviews by, for example, Güsten (1989) and Morris & Serabyn (1996) state that clouds are subject to disruptive shear.…”