Molecular gas kinematics within the central 250 pc of the Milky Way

Abstract

Using spectral-line observations of HNCO, N 2 H + , and HNC, we investigate the kinematics of dense gas in the central ∼ 250 pc of the Galaxy. We present scouse (Semi-automated multi-COmponent Universal Spectral-line fitting Engine), a line-fitting algorithm designed to analyse large volumes of spectral-line data efficiently and systematically. Unlike techniques which do not account for complex line profiles, scouse accurately describes the {l, b, v LSR } distribution of Central Molecular Zone (CMZ) gas, which is asymmetric about Sgr A* in both position and velocity. Velocity dispersions range from 2.6 km s −1 < σ < 53.1 km s −1 . A median dispersion of 9.8 km s −1 , translates to a Mach number, M 3D 28. The gas is distributed throughout several "streams", with projected lengths ∼ 100 − 250 pc. We link the streams to individual clouds and sub-regions, including Sgr C, the 20 and 50 km s −1 clouds, the dust ridge, and Sgr B2. Shell-like emission features can be explained by the projection of independent molecular clouds in Sgr C and the newly identified conical profile of Sgr B2 in {l, b, v LSR } space. These features have previously invoked supernova-driven shells and cloud-cloud collisions as explanations. We instead caution against structure identification in velocity-integrated emission maps. Three geometries describing the 3-D structure of the CMZ are investigated: i) two spiral arms; ii) a closed elliptical orbit; iii) an open stream. While two spiral arms and an open stream qualitatively reproduce the gas distribution, the most recent parameterisation of the closed elliptical orbit does not. Finally, we discuss how proper motion measurements of masers can distinguish between these geometries, and suggest that this effort should be focused on the 20 km s −1 and 50 km s −1 clouds and Sgr C.

The Link Between Turbulence, Magnetic Fields, Filaments, and Star Formation in the Central Molecular Zone Cloud G0.253+0.016

“…Thus, the turbulent velocity dispersion is significantly smaller than the total velocity dispersion. Henshaw et al (2016) recently measured a 1D velocity dispersion of -11 km s 1 for G0.253+0.016, 25% higher than our estimate that includes the contribution of the large-scale gradient. This difference arises because Henshaw et al (2016) measured the LOS velocity dispersion, while we measure the dispersion in the plane of the sky.…”

“…We further correct for the large-scale gradient. However, the LOS velocity dispersion includes the contributions from the large-scale gradient and thus the dispersions and Mach numbers determined in Henshaw et al (2016) are not the purely turbulent dispersions and Mach numbers.…”

A theoretical explanation for the Central Molecular Zone asymmetry

“…Walsh et al 2011, Purcell et al 2012. They have a spatial resolution of ∼ 60 arcsec and a spectral resolution of 2 km s −1 , and have been fit using the Semi-automated multi-COmponent Universal Spectralline fitting Engine (SCOUSE, Henshaw et al 2016, https://github.com/ jdhenshaw/SCOUSE).…”

Nuclear spirals in the inner Milky Way

“…High resolution molecular line studies of the CMZ have revealed coherent features spanning the width of the (l, b, v) data cubes in several different species, including CO, CS, NH 3 and HNCO (Nagayama et al 2007;Oka et al 2007;Henshaw et al 2016b). The origin of these features has been of interest for several decades, and proposed explanations include two spiral arms (Sofue 1995;Sawada et al 2004), a twisted elliptical orbit (Molinari et al 2011), or an open stream (Kruijssen et al 2015).…”