A panoramic view of the Milky Way HI gas

Abstract

Neutral atomic hydrogen (H I ) traces the interstellar medium (ISM) over a broad range of physical conditions. Its 21-cm emission line is a key probe of the structure and dynamics of galaxies. This line comprises very different temperature and density regimes on all scales, from tens of astronomical units to kiloparsecs. H I is the key element to study the evolution of the ISM in detail. To understand the physics of the ISM and to analyze the interplay between different phases it is mandatory to cover observationally a broad range of scales. But large scale imaging of galaxies, resolving at the same time all these scales is difficult; spatial resolution, as well as sensitivity and the field of view are currently rather limited. The observational situation is much more favorable if we consider our own galaxy. Two major all sky 21-cm line surveys of the Milky Way will become available soon. The Galactic All Sky Survey (GASS) obtained with the Parkes 64-m telescope§ for the southern hemisphere with a resolution of 16 arcmin is close to completion. The northern extension, the Effelsberg Bonn HI Survey (EBHIS)¶with 9 arcmin resolution, will be available in 2010/2011; we refer to the talks by Kerp and Winkel. Here we discuss briefly the GASS and demonstrate the unprecedented quality of this survey. The Galactic single dish 21-cm line surveys prepare the ground for future high resolution imaging of the Galactic H I distribution. Using the available short spacing informaton, the Australian Square Kilometre Array Pathfinder (ASKAP) will be capable to generate a truly panoramic view of the Milky Way HI gas distribution with arcsecond resolution for all declinations < 30. Data from the Widefield ASKAP L-band Legacy All-Sky Blind Survey (WALLABY, see talk by Staveley-Smith) can be used to generate high resolution all sky maps. In comparison to the currently available interferometric International Galactic Plane Surveys (IGPS) the sensitivity will improve by a factor of 10. Most important is the all sky coverage which will overcome the rather limited spatial coverage of a few degrees around the Galactic plane for the IGPS.