The 21-cm line emission from neutral atomic hydrogen (HI) in the Milky Way disk is being mapped with 1' resolution and brightness sensitivity of a few K rms by three groups using telescopes at the ATNF (the Southern Galactic Plane Survey, SGPS, McClure-Griffiths et al. 2001), the DRAO (the Canadian Galactic Plane Survey, CGPS, Taylor et al. 1999), and the VLA (the VLA Galactic Plane Survey, VGPS, Stil et al. 2006). These projects are charting the disk between longitudes 255° and 357°, between 65° and 147° and between 18° and 67° respectively. Working together as an international community, we are now in a position to obtain a complete image of the HI emission from the plane of the Galaxy at 1' resolution. The longitude range 18° and 67° can only be explored with the VLA, because at these longitudes the Galactic plane passes through the celestial equator. The VLA survey will provide a link between the northern and southern surveys and cover the first quadrant of the Galaxy, where the effects of star formation and the interaction between the disk and halo are expected to be dominant shapers of the ISM (e.g. Heiles 1984). The VGPS will also overlap the FCRAO survey of CO emission in the molecular ring from 18° to 52°, providing, together with high resolution infrared images, complete imaging of the major components of the interstellar medium in this region down to scales of a few pc.

The interstellar medium is the matrix within which the evolution of galaxies is played out -- governed largely by the processes that influence the cycles of star formation. The conditions of the ISM, its spatial, dynamical, thermal and chemical structure, reflect the evolutionary processes at work within the Galaxy. Observations over the last few decades have provided us with tantalizing glimpses of the complexity of the ISM. Far from a homogeneous and tranquil environment, the ISM displays large density and temperature variations. Velocity fields within the medium are turbulent, often supersonic. At the small end of the range of spatial scales, this highly disturbed state is maintained by point-like energy input from stars during both the formation and death stages. On the other end of the range of scales, energy input may take the form of global, large-scale phenomena, such as viscous dissipation or magnetic stress from Galactic rotation, the motion of spiral arm density waves, or gravitational infall from the halo. Despite the apparent flux of energy on all scales, there exist pockets of relative quiescence where dense, cold gas can become self-gravitating and the process of star formation begins.

Our knowledge of this system is still very much in the formative stage. Fundamental questions that remain unanswered are numerous.

Definitive answers to these and other, as yet unformulated, questions require knowledge of the 3-dimensional structure and conditions of the ISM over the disk of the Galaxy and over the full range of spatial scales, from the parsec scale typical of stellar separations and clustering, to the kiloparsec scale of spiral arm patterns.

Because of our vantage point within it, the Milky Way is the only Galaxy for which we have the potential to observe the relevant interactions and structures in sufficient detail and over the required range of spatial scales. An understanding of the evolution of external galaxies out to cosmologically significant distances must rely on a detailed knowledge of these processes gleaned from our own Galaxy.

Among the tracers of the interstellar medium, the 21-cm HI line uniquely traces the diffuse medium. The HI is widespread, with filling factor of 25% to 50% throughout the Galactic disk, and exhibits structure on all observed spatial scales. In contrast, the molecular gas is confined to much smaller clouds which fill a tiny fraction of the volume of the disk. High resolution survey projects at millimetre (Heyer et al. 1998) and infrared wavelengths (Cao et al. 1997; Kerton and Martin 2000) are providing large-scale images of the molecules and dust in the Galaxy at arcminute scales. Due to the long wavelength of the HI emission line, HI surveys of the Galaxy have, until recently, lacked sufficient angular resolution to be very useful for ISM studies. Advances in interferometric techniques and computing power now permit the application of wide-field synthesis imaging to Galactic HI studies. The impact of high angular resolution is demonstrated in Figure 1, which shows one HI velocity channel of a region in the northern galactic plane from the Leiden/Dwingeloo survey of Burton & Hartmann (1994) (35' resolution) compared to the same region from the CGPS (1' resolution). In this segment of the Perseus spiral arm, 1' corresponds to 0.6 pc. At this resolution the power of HI gas as a tracer of ISM processes is strikingly apparent. Features such as the chimney above the star cluster OCl 352, cold, dark, filamentary, parsec-scale HI clouds, and bright, arc-like shock structures become visible.

Figure 1. A 17 ° segment of the Perseus spiral arm at VLSR =-42 km s-1 as observed by the Leiden/Dwingeloo single dish survey (top) and the CGPS (bottom).

The VLA survey, in combination with the CGPS and SGPS, will provide a 3D image of this detail and quality for over 90% of the stellar disk of the Galaxy. The global image of the Galaxy will be a unique resource for the astronomical community. The observing parameters for the VLA Galactic Plane Survey are summarized in Table 1. The VGPS sky coverage is shown in Figure 2.

Survey area

990 pointings, longitude 18° to 67°, latitude coverage between -1° to +1°, and -2° to +2° (Figure 2)
Integration per field

9 minutes, broken into 3-minute snapshots at various hour angles
Resolution 1 arcminute
Sensitivity 11 mJy/beam (2 K for 1' beam)
Velocity resolution 6.1 kHz = 1.3 km s-1, sampled at 3.05 kHz
Band width 1.866 MHz (two times 1.56 MHz overlapped by 1.259 MHz)

Figure 2: Outline of the VGPS survey area and the 990 pointing centers shown on top 21-cm line emission from the Leiden/Dwingeloo survey.