H�I Emission and Absorption in the Southern Galactic Plane
Survey¹
N. M. McClure-Griffiths ,
John M. Dickey ,
B. M. Gaensler ,
A. J. Green ,
R. F. Haynes ,
M. H. Wieringa
, PASA, 18 (1), in press.

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Introduction

The Southern Galactic Plane Survey (SGPS) is an H�I spectral line and $\lambda$ 21-cm continuum survey of the fourth quadrant of the plane of the Milky Way (Dickey et�al. 1999). The survey covers the region

$253\hbox{$^\circ$}\leq l \leq 358\hbox{$^\circ$}$ ;

$\vert b\vert \leq 1\hbox{$^\circ$}$ . The dataset, which is a combination of data from the Australia Telescope Compact Array (ATCA) and the Parkes Radiotelescope, is sensitive to angular scales larger than

$\sim 1-2\hbox{$^\prime$}$ . The ATCA data is a mosaic of 2212 pointings. The Parkes data, which was obtained with the inner seven beams of the multibeam receiver, has extended coverage to

$\vert b\vert \leq 10\hbox{$^\circ$}$ . The final data product includes H�I data cubes with a spectral resolution of 0.8 km� ${\rm s}^{-1}$ and continuum maps in all four Stokes parameters. Observations for the SGPS are expected to be finished in late 2000 with a full data release in early 2001. Currently the Parkes observations are complete, as are observations of the SGPS Test Region (

$325\mbox{$.\!\!^\circ$}5 \leq l \leq 333\mbox{$.\!\!^\circ$}5$ ;

$-0\mbox{$.\!\!^\circ$}5 \leq b \leq 3\mbox{$.\!\!^\circ$}5$ ). This paper is based on a talk given at the ASA Annual Meeting in July 2000. Here we present a representative sample of the SGPS H�I data, including low resolution data from Parkes and full resolution data for the Test Region. The continuum emission has been subtracted from these H�I data. A full description of the observations and data analysis are discussed in detail elsewhere (McClure-Griffiths�et�al. 2000a, 2000b). In this introduction to the data we explore how the SGPS can contribute to our understanding of the interstellar medium (ISM) over a large range of size scales. The SGPS data span several orders of magnitude in angular size, allowing us to probe opposite ends of the H�I spatial power spectrum over a significant portion of the inner Galaxy. On the large scale we are studying H�I emission structures, particularly shells and supershells to understand the effect of these shells on the structure and dynamics of the ISM. The SGPS data may help us to understand how these deterministic structures are ``pumping'' the spatial power spectrum. On the small scale, we are using H�I self-absorption (HISA) to probe the coldest, smallest clouds in the ISM. We seek to understand the ISM on all scales in between, but the clues may be at the furthest extremes.

Next Section: H�I Shells
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Introduction