‘The Brick’ is not a<i>brick</i>: a comprehensive study of the structure and dynamics of the central molecular zone cloud G0.253+0.016

Abstract

In this paper we provide a comprehensive description of the internal dynamics\nof G0.253+0.016 (a.k.a. 'the Brick'); one of the most massive and dense\nmolecular clouds in the Galaxy to lack signatures of widespread star formation.\nAs a potential host to a future generation of high-mass stars, understanding\nlargely quiescent molecular clouds like G0.253+0.016 is of critical importance.\nIn this paper, we reanalyse Atacama Large Millimeter Array cycle 0 HNCO\n$J=4(0,4)-3(0,3)$ data at 3 mm, using two new pieces of software which we make\navailable to the community. First, scousepy, a Python implementation of the\nspectral line fitting algorithm scouse. Secondly, acorns (Agglomerative\nClustering for ORganising Nested Structures), a hierarchical n-dimensional\nclustering algorithm designed for use with discrete spectroscopic data.\nTogether, these tools provide an unbiased measurement of the line of sight\nvelocity dispersion in this cloud, $\\sigma_{v_{los}, {\\rm 1D}}=4.4\\pm2.1$\nkms$^{-1}$, which is somewhat larger than predicted by velocity dispersion-size\nrelations for the Central Molecular Zone (CMZ). The dispersion of centroid\nvelocities in the plane of the sky are comparable, yielding $\\sigma_{v_{los},\n{\\rm 1D}}/\\sigma_{v_{pos}, {\\rm 1D}}\\sim1.2\\pm0.3$. This isotropy may indicate\nthat the line-of-sight extent of the cloud is approximately equivalent to that\nin the plane of the sky. Combining our kinematic decomposition with radiative\ntransfer modelling we conclude that G0.253+0.016 is not a single, coherent, and\ncentrally-condensed molecular cloud; 'the Brick' is not a \\emph{brick}.\nInstead, G0.253+0.016 is a dynamically complex and hierarchically-structured\nmolecular cloud whose morphology is consistent with the influence of the\norbital dynamics and shear in the CMZ.\n