‘The Brick’ is not a brick: 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 of G0.253+0.016 (a.k.a. ‘the Brick’); one of the most massive and dense molecular clouds in the Galaxy to lack signatures of widespread star formation. As a potential host to a future generation of high-mass stars, understanding largely quiescent molecular clouds like G0.253+0.016 is of critical importance. In this paper, we reanalyse Atacama Large Millimeter Array cycle 0 HNCO J = 4(0, 4) − 3(0, 3) data at 3 mm, using two new pieces of software that we make available to the community. First, scousepy, a Python implementation of the spectral line fitting algorithm scouse. Secondly, acorns (Agglomerative Clustering for ORganising Nested Structures), a hierarchical n-dimensional clustering algorithm designed for use with discrete spectroscopic data. Together, these tools provide an unbiased measurement of the line-of-sight velocity dispersion in this cloud, $\sigma _{v_{\mathrm{ los}}, {\rm 1D}}=4.4\pm 2.1$ km s−1, which is somewhat larger than predicted by velocity dispersion-size relations for the central molecular zone (CMZ). The dispersion of centroid velocities in the plane of the sky are comparable, yielding $\sigma _{v_{\mathrm{ los}}, {\rm 1D}}/\sigma _{v_{\mathrm{ pos}}, {\rm 1D}}\sim 1.2\pm 0.3$. This isotropy may indicate that the line-of-sight extent of the cloud is approximately equivalent to that in the plane of the sky. Combining our kinematic decomposition with radiative transfer modelling, we conclude that G0.253+0.016 is not a single, coherent, and centrally condensed molecular cloud; ‘the Brick’ is not a brick. Instead, G0.253+0.016 is a dynamically complex and hierarchically structured molecular cloud whose morphology is consistent with the influence of the orbital dynamics and shear in the CMZ.