The Survey of Water and Ammonia in the Galactic Center (SWAG): Molecular Cloud Evolution in the Central Molecular Zone

Abstract

The Survey of Water and Ammonia in the Galactic Center (SWAG) covers the Central Molecular Zone (CMZ) of the Milky Way at frequencies between 21.2 and 25.4 GHz obtained at the Australia Telescope Compact Array at ∼0.9 pc spatial and ∼2.0 km s −1 spectral resolution. In this paper, we present data on the inner ∼250 pc (1°.4) between SgrC and SgrB2. We focus on the hyperfine structure of the metastable ammonia inversion lines (J, K )=(1, 1)-(6, 6) to derive column density, kinematics, opacity, and kinetic gas temperature. In the CMZ molecular clouds, we find typical line widths of 8-16 km s −1 and extended regions of optically thick (τ>1) emission. Two components in kinetic temperature are detected at 25-50 K and 60-100 K, both being significantly hotter than the dust temperatures throughout the CMZ. We discuss the physical state of the CMZ gas as traced by ammonia in the context of the orbital model by Kruijssen et al. that interprets the observed distribution as a stream of molecular clouds following an open eccentric orbit. This allows us to statistically investigate the time dependencies of gas temperature, column density, and line width. We find heating rates between ∼50 and ∼100 K Myr −1 along the stream orbit. No strong signs of time dependence are found for column density or line width. These quantities are likely dominated by cloud-to-cloud variations. Our results qualitatively match the predictions of the current model of tidal triggering of cloud collapse, orbital kinematics, and the observation of an evolutionary sequence of increasing star formation activity with orbital phase.

The life cycle of the Central Molecular Zone – II. Distribution of atomic and molecular gas tracers

“…and shielded gas at nH ∼ 10 4 − 10 5 cm −3 . In particular, the temperatures predicted by the weak model, Tg ∼ 15 − 60 K (second panel), are in agreement with those characteristic of the cold molecular component (T ∼ 25 − 50 K, Krieger et al 2017), while the temperatures predicted by the strong model, Tg ∼ 40 − 180 K (fourth panel), are in agreement with the observed temperatures of the warm molecular component (T ∼ 60 − 150 K, Krieger et al 2017). As expected, the intermediate model (third panel) produces intermediate temperatures between the two previous models, Tg ∼ 25 − 100 K. This result clearly suggests that the radiation field in the present-day CMZ is not constant, but instead varies based in the local star formation conditions.…”

“…Kruijssen & Longmore 2013;Kruijssen et al 2014;Battersby et al 2017), warmer temperatures (TCMZ ∼ 25 − 200 K, e.g. Ao et al 2013;Ginsburg et al 2016;Krieger et al 2017) and higher level of turbulence (e.g. Shetty et al 2012;Federrath et al 2016;Henshaw et al 2016).…”

“…Dame et al 2001;Oka et al 2007), NH3 (e.g. Purcell et al 2012;Krieger et al 2017), HCN (Jones et al 2012), and dust tracers (e.g. Rodríguez-Fernández et al 2004;Molinari et al 2011) have shown the presence of kinematically coherent structures mostly located within an angular distance of 1 • from the Galactic center (corresponding to a physical distance of ∼ 150 pc).…”

The dynamical evolution of molecular clouds near the Galactic Centre – II. Spatial structure and kinematics of simulated clouds

“…Oka et al 2001;Rathborne et al 2014b), temperatures (e.g. Huettemeister et al 1993;Ao et al 2013;Mills & Morris 2013;Ginsburg et al 2016;Krieger et al 2017), and velocity dispersions (e.g. Shetty et al 2012;Henshaw et al 2016a;Kauffmann et al 2017a) of CMZ clouds are similar to those in high-redshift galaxies at the time of the peak cosmic star formation rate (Kruijssen & Longmore 2013).…”

“…Gas above a density of 2.5 × 10 6 cm −3 is converted into sink particles, implying that the effective equation of state is isothermal, with an adopted temperature of T = 65 K to represent the high temperature of CMZ gas (e.g. Ao et al 2013;Ginsburg et al 2016;Krieger et al 2017). At the adopted numerical resolution (see below), these sink particles do not represent individual stars, but stellar subclusters.…”