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This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited.--Full list of authors: Pingel, N. M.; Dempsey, J.; McClure-Griffiths, N. M.; Dickey, J. M.; Jameson, K. E.; Arce, H.; Anglada, G.; Bland-Hawthorn, J.; Breen, S. L.; Buckland-Willis, F.; Clark, S. E.; Dawson, J. R.; Denes, H.; Di Teodoro, E. M.; For, B-Q; Foster, Tyler J.; Gomez, J. F.; Imai, H.; Joncas, G.; Kim, C-G; Lee, M-Y; Lynn, C.; Leahy, D.; Ma, Y. K.; Marchal, A.; McConnell, D.; Miville-Deschenes, M-A; Moss, V. A.; Murray, C. E.; Nidever, D.; Peek, J.; Staveley-Smith, L.; Tepper-Garcia, T.; Tremblay, C. D.; Uscanga, L.; van Loon, J. Th; Vazquez-Semadeni, E.; Allison, J. R.; Anderson, C. S.; Ball, Lewis; Bell, M.; Bock, D. C-J; Bunton, J.; Cooray, F. R.; Cornwell, T.; Koribalski, B. S.; Gupta, N.; Hayman, D. B.; Harvey-Smith, L.; Lee-Waddell, K.; Ng, A.; Phillips, C. J.; Voronkov, M.; Westmeier, T.; Whiting, M. T.; Stanimirovic, S., We present the most sensitive and detailed view of the neutral hydrogen ( HI ) emission associated with the Small Magellanic Cloud (SMC), through the combination of data from the Australian Square Kilometre Array Pathfinder (ASKAP) and Parkes (Murriyang), as part of the Galactic Australian Square Kilometre Array Pathfinder (GASKAP) pilot survey. These GASKAP-HI pilot observations, for the first time, reveal HI in the SMC on similar physical scales as other important tracers of the interstellar medium, such as molecular gas and dust. The resultant image cube possesses an rms noise level of 1.1 K ( 1.6mJy beam−1 ) per 0.98km s−1 spectral channel with an angular resolution of 30′′ ( ∼10pc ). We discuss the calibration scheme and the custom imaging pipeline that utilises a joint deconvolution approach, efficiently distributed across a computing cluster, to accurately recover the emission extending across the entire ∼25deg2 field-of-view. We provide an overview of the data products and characterise several aspects including the noise properties as a function of angular resolution and the represented spatial scales by deriving the global transfer function over the full spectral range. A preliminary spatial power spectrum analysis on individual spectral channels reveals that the power law nature of the density distribution extends down to scales of 10 pc. We highlight the scientific potential of these data by comparing the properties of an outflowing high-velocity cloud with previous ASKAP+Parkes HI test observations. c The Author(s), 2022. Published by Cambridge University Press on behalf of the Astronomical Society of Australia., The Australian SKA Pathfinder is part of the Australia Telescope National Facility which is managed by CSIRO. Operation of ASKAP is funded by the Australian Government with support from the National Collaborative Research Infrastructure Strategy. ASKAP uses the resources of the Pawsey Supercomputing Centre. Establishment of ASKAP, the Murchison Radio-astronomy Observatory and the Pawsey Supercomputing Centre are initiatives of the Australian Government, with support from the Government of Western Australia and the Science and Industry Endowment Fund. We acknowledge the Wajarri Yamatji people as the traditional owners of the Observatory site. Pipeline development was tested on the OzSTAR supercomputer under the project code, oz145, which is available through Swinburne University’s Centre of Astrophysics and Supercomputing. This research was supported by the Australian Research Council (ARC) through grant DP190101571. N.M.-G. acknowledges the support of the ARC through Future Fellowship FT150100024. G. A. and J. F. G. acknowledge support from the State Agency for Research (10.13039/501100011033) of the Spanish MCIU, through grants AYA2017-84390-C2-1-R and PID2020-114461GB-I00 (co-funded by FEDER) and the ‘Center of Excellence Severo Ochoa’ award for the Instituto de Astrofsica de Andaluca (SEV-2017-0709). C. E. M. is supported by an NSF Astronomy and Astrophysics Postdoctoral Fellowship under award AST-1801471.

A crucial ingredient in establishing a theory of compressible turbulence is the characterization of the density fluctuation statistics, an information particularly relevant in Cosmology and interstellar gas dynamics. A first step towards this characterization is the determination of the probability density function (PDF) of the density field. Previous work [1, 2] has found log-normal distributions in isothermal simulations of 2D and 3D compressible turbulence, whereas power-law regions have been reported at high densities in simulations of interstellar gas subject to complex heating and cooling processes [3]. To understand the reason of this difference we have performed one-dimensional simulations of a randomly accelerated barotropic gas, for different values of the polytropic index γ [4]. The equations for the non-dimensional density ρ and velocity u read $$ \frac{{\partial \rho }}{{\partial t}} + \frac{{\partial (\rho u)}}{{\partial x}} = 0 $$ (1) and $$ \frac{{\partial u}}{{\partial t}} + u\frac{{\partial u}}{{\partial x}} = - \frac{1}{{\gamma {{{\text{M}}}^{2}}\rho }}\frac{{\partial {{\rho }^{\gamma }}}}{{\partial x}} + \nu \frac{{{{\partial }^{2}}u}}{{\partial {{x}^{2}}}} + {{a}_{r}} $$ (2) where M is the Mach number of the velocity unit. These runs start at rest, but are driven by a random acceleration a r at wave numbers 1–19, with a correlation time t corr = 3 × 10-3. The probability densities of log ρ are obtained by considering all grid points (typically 3072 or 6144 for the highest Mach numbers) and averaging over nearly 150 non-dimensional time units, sampling every t = 0.1. Thus, the histograms contain over 4.6 million points.