Abstract
Abstract We present the most sensitive and detailed view of the neutral hydrogen ( ${\rm H\small I}$ ) 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 ${\rm H\small I}$ 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 ( $1.6\,\mathrm{mJy\ beam}^{-1}$ ) $\mathrm{per}\ 0.98\,\mathrm{km\ s}^{-1}$ spectral channel with an angular resolution of $30^{\prime\prime}$ ( ${\sim}10\,\mathrm{pc}$ ). 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 ${\sim}25\,\mathrm{deg}^2$ 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 ${\rm H\small I}$ test observations.
Highlights
The evolution of the interstellar medium (ISM), which is driven by processes such as gas infall, star formation and subsequent stellar feedback, is fundamentally linked to the formation and overall evolution‘ of galaxies
We instead developed a custom imaging pipelined that utilises a combination of Common Astronomy Software Application (CASA; McMullin et al 2007) tasks to pre-process the measurement sets, WSCleane to grid the multiple Australian Square Kilometre Array Pathfinder (ASKAP) pointings onto a single grid for joint deconvolution using corresponding primary beam models for each ASKAP primary beam, and miriad (Sault, Teuben, & Wright 1995) tasks to combine ASKAP and Parkes data to correct for short-spacings
We find that an single dish factor (SDF) of 1.0 produces virtually an exact agreement with the ASKAP-only profile that extends from the maximum recoverable scale of ∼33 down to the size of the 30
Summary
The evolution of the interstellar medium (ISM), which is driven by processes such as gas infall, star formation and subsequent stellar feedback, is fundamentally linked to the formation and overall evolution‘ of galaxies. This paper is structured as follows: Section 2 describes the philosophy and parameters of the GASKAP-HI pilot survey; Section 3 explains the calibration procedure, provides a thorough description of our custom imaging pipeline, including the combination with Parkes data to correct for the missing short-spacings, and discusses our quality assessment methods; Section 4 outlines the noise properties of our final image cubes and derives the global transfer function to fully characterise the represented spatial frequencies; Section 5 highlights the advantages of the increased sensitivity and angular/spectral resolution through global channel maps of the SMC and presents an analysis of a known outflowing. HVC; in Section 6, we summarise these results from the SMC and anticipate future results from our other pilot fields and the eventual full GASKAP-HI survey
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