Kinematics and properties of the central molecular zone as probed with [C ii]

¹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
e-mail: William.Langer@jpl.nasa.gov
² Department of Physics and Astronomy, UCLA, 403 Portola Plaza, Los Angeles, CA 90095-1547, USA

Received: 8 August 2016
Accepted: 12 December 2016

Abstract

Context. The Galactic central molecular zone (CMZ) is a region containing massive and dense molecular clouds, with dynamics driven by a variety of energy sources including a massive black hole. It is thus the nearest template for understanding physical processes in extragalactic nuclei. The CMZ’s neutral interstellar gas has been mapped spectrally in many neutral atomic and molecular gas tracers, but the ionized and CO-dark H₂ regions are less well traced spectroscopically.

Aims. We identify features of the UV irradiated neutral gas, photon dominated regions (PDRs) and CO-dark H₂, and highly ionized gas in the CMZ as traced by the fine structure line of C⁺ at 158 μm, [C ii], and characterize their properties.

Methods. We observed the [C ii] 158 μm fine structure line with high spectral resolution using Herschel HIFI with two perpendicular on-the-fly strip scans, along l = −0.̊8 to +0.̊8 and b = −0.̊8 to +0.̊8, both centered on (l, b) = (0°, 0°). We analyze the spatial-velocity distribution of the [C ii] data, compare them to those of [C i] and CO, and to dust continuum maps, in order to determine the properties and distribution of the ionized and neutral gas and its dynamics within the CMZ.

Results. The longitude- and latitude-velocity maps of [C ii] trace portions of the orbiting open gas streams of dense molecular clouds, the cloud G0.253+0.016, also known as the Brick, the arched filaments, the ionized gas within several pc of Sgr A and Sgr B2, and the warm dust bubble. We use the [C ii] data to determine the physical and dynamical properties of these CMZ features.

Conclusions. The bright far-IR 158 μm C⁺ line, [C ii], when observed with high spatial and spectral resolution, traces a wide range of emission features in the CMZ. The [C ii]  emission arises primarily from dense PDRs and highly ionized gas, and is an important tracer of the kinematics and physical conditions of this gas.