The VLA Galactic Plane Survey

The VLA Galactic Plane Survey will observe the region within 18° < l < 67°. The longitude range overlaps the CGPS and SGPS surveys by 2 degrees at each end, allowing a measure of the reproducibility between the surveys. The area of the Galaxy covered by the combined VGPS, CGPS and SGPS is shown on the IGPS web page The global HI survey will image a 272° contiguous interval of Galactic longitude from 253° to 165°.

Science Goals

The overall goal of this project is obtain a complete picture of the HI Galaxy at 1' resolution. The mapping and analysis of the HI spectra will lead to a description of the spatial, dynamical, and thermal structure of the neutral atomic gas. The survey will cover with equal sensitivity and resolution regions of intense activity as well as the inactive or less active regions which show the more quiescent state of the ISM. We need this ``big picture'' in order to understand the relative importance of the various disruptive processes, and how the medium adjusts itself in response to them. The resulting equilibrium or steady state conditions may be very different in the inner galaxy from what they are at the solar circle and beyond. The VGPS will be the first that can probe the atomic medium in the first Galactic quadrant with sufficient detail to determine how the thermal and kinematic structure of the ISM changes across the Milky Way disk. The results will bring our understanding of the distribution and motion of the neutral atomic phase of the interstellar medium up to the level of detail and sophistication which are being obtained for the molecular gas and the dust.

The specific science goals that drive the specifications of the survey are discussed in more detail below.

The Disk-Halo Interaction

It is the HI medium of the Galaxy which traces the history of mass and energy exchange between the disk and halo. HI also provides us with the dynamical information which is a prerequisite to deciphering the energetics of the flows and implicates the input mechanisms which drive them. New arcminute scale CGPS, SGPS and VGPS images are revealing many cases of features which have been shaped by the vertical ejection of material from the Galactic disk. Such features cannot be detected in observations of even the most nearby external galaxies, due to lack of spatial resolution. In fact, some of the most exciting results, to date, from these surveys relate to the disk-halo connection and involve discoveries of structures never before seen. Examples are the Galactic Chimney (Normandeau et al. 1996; Taylor et al. 1999 and the Galactic Mushroom (English et al. 2000). The HI Galactic Chimney, also traceable in H-alpha and CO, is a striking confirmation of certain aspects of the "chimney model" (Norman & Ikeuchi 1989) in which hot gas from supernovae and stellar winds is expelled into the halo through conduits which form above the star forming region. In the case of the Galactic Chimney, the outflow can be explained by stellar winds alone from the hot stars associated with the W4 HII complex below it. The 340 pc high Galactic Mushroom, on the other hand, resembles the mushroom cloud of an atomic bomb blast and can be described by similar physics. It may represent a phase in disk-halo blowout which is earlier than that of the evacuated Galactic Chimney.

New HI supershells are also being discovered, such as a ~600 pc diameter supershell in the outer galaxy, which shows tentative evidence for breakout on both sides of the disk. A continuous ridge of swept up HI in both velocity and position space has been detected around the periphery of this shell (McClure-Griffiths et al. 2000).

One can only expect further such discoveries and further evidence for disk-halo dynamics as the VGPS HI mosaics become available. Determining an inventory of such features is both non-trivial and of greater interest than mere book-keeping. Indeed, an estimate of the number of chimneys per unit disk area, the clustering of such chimneys, and the relationship between these features and underlying star forming regions are all important inputs into models describing the global interchange between disk and halo gas. They are also very important parameters in distinguishing between models, some of which do not rely on star formation properties alone to describe the disk-halo phenomenon. For example, the Parker instability (Kamaya et al. 1996) is still an important contender in terms of explaining some loop and shell-like features. There are even further-reaching implications. There have been suggestions, for example, that a net flux of material is leaving the Galaxy altogether and not descending onto the disk again via a fountain (Breitschwerdt et al. 1991, 1993; Zirakashvili et al. 1996; Ptuskin et al. 1997). With a complete Galactic Plane survey, it may be possible to estimate the global net mass flux in order to test this hypothesis. It will certainly not be possible without the VLA Galactic Plane Survey.

Another very interesting aspect of disk-halo dynamics is that there appear to be different properties between disk-halo features depending on location in the Galaxy. Most supershells, for example, occur in the outer Galaxy, whereas most ``worms" (i.e. chimneys, if open-topped) occur in the inner Galaxy (Heiles 1979,1984). In addition, outflows in the vicinity of the Galactic Center may be dictated by different kinds of sources and different physics than those elsewhere. Thus, it is particularly important to observe each region in detail. The VLA survey, covering both the Galactic Center and inner Galaxy, will provide crucial data for such a study.

Structure and Kinematics

A major objective of the IGPS is to get an inventory of the shells, bubbles, and chimneys throughout the Milky Way disk. Such structures represent the feedback mechanism by which the star formation process acts back on the larger ISM (e.g. Knee & Wallace 1999). Ironically, we have more complete knowledge of these features in nearby galaxies such as the LMC than we do in the Milky Way. Our purposes here are both to provide data for astrophysical modeling of these objects (Oey & Clarke 1997), and to get an idea of how their collective action determines the structure of the ISM. These objects can act as probes of the conditions in the surrounding ISM. For example, the kinematical analysis of the W4 chimney showed that the surrounding ISM had an unexpectedly low scale height (Basu et al. 1999). This result leads to interesting lines of thought about the relationship between the scale height of the ISM and massive star forming complexes. This result also has implications for the ability of these complexes to form conduits to the halo.

We wish to study the motions of the HI on all scales. The large scale kinematics are dominated by the rotation of the galaxy. Non-circular motions are present throughout the disk, but we do not have a very good idea of the amplitudes and spatial scales of these departures from cylindrical, circular rotation. This survey will allow measurement of the distribution function of non-circular motions and their spatial coherence by studying the terminal velocity drop-off and its variation with longitude on scales of a few arc-minutes up to several degrees. On smaller scales, we will study the kinematics of discrete structures such as expanding shells (e.g. Stil & Irwin 2001) and chimneys.

This VLA Galactic plane survey, when combined with single dish observations with the GBT, will provide complete spatial frequency sampling from the zero spacing to roughly three thousand wavelengths (one arcminute). This will allow us to study the spatial power spectra of the 21-cm emission integral (and thus of the column density of the HI) and of individual velocity channel-maps (thus of the space density of the HI). Assuming that the medium has in general come into a dynamical steady state, this spatial power spectrum can be compared with the turbulence spectrum of the ionized gas as derived from scintillation studies of pulsars, and of the spatial power spectrum of the far-IR emission (see Stanimirovic et al. 1999). Overall, we are interested in the transition from the discrete structures which we can map and analyse dynamically, to the stochastic density and velocity fields which represent the evolutionary end-product of populations of supernova remnants, stellar wind-blown bubbles, ionization fronts, and other interstellar shocks.

A powerful means to obtain the turbulent energy spectrum of interstellar gas is via the technique of Principal Component Analysis ("PCA"; Heyer & Brunt 1999, Brunt 1999); this is being applied to both the CO (FCRAO OGS, FCRAO/BU Galactic Ring) data and the HI (CGPS) data. The VGPS helps to complete the statistical picture of the dynamical state of the ISM by incorporating the crucial atomic component in the inner galaxy, including the region of the molecular Galactic Ring Survey. The spatial dynamic range afforded by the VGPS and the other surveys provides an exciting opportunity to study the transfer/redistribution of turbulent kinetic energy over a wide range of physical scales, and over a wide variety of environments. This type of study is exactly what is needed by the rapidly developing field of supercomputer simulations of interstellar turbulence. Magnetohydrodynamic simulations of the ISM are now sophisticated enough to include self-gravity and star formation (Vazquez-Semadeni et al. 1995), and use grid sizes up to 512³ (Stone et al. 1998), which often exceeds the spatial dynamic range of even exceptional previous ISM data sets. Only large scale survey-mode projects, such as the CGPS, SGPS and VGPS can provide the necessary data to regulate this exciting and powerful new astrophysical field of study.

In conjunction with the spatial power spectrum analysis (e.g. Crovisier & Dickey 1983, Green 1993) and utilizing a possible inversion to the three dimensional density field statistics (Lazarian 1995), the energy spectrum analysis via PCA will allow us to directly test turbulence theory beyond "Kolmogorov" turbulence and the passive scalar phenomenology. How are density and velocity structure coupled in a supersonic, compressible medium? Since, in the compressible case, the nature of the turbulent energy spectrum depends on the magnitude and nature of energy injection (e.g. Passot, Pouquet & Woodward 1988, Miesch & Bally 1994), it crucial to obtain measurements over a wide range of physical conditions, especially in the violent inner Galaxy. These studies together will provide a concise statistical description of the structure and dynamics of interstellar gas throughout the Galaxy. They will provide a means to relate the dynamical and structural state of the gas to the nature and rate of energy input. We will be able to locally compare the dynamics of atomic and molecular gas and place constraints on the type of events by which one phase could evolve into the other.

Cold Atomic Hydrogen

HI self-absorption (HISA) by cold atomic hydrogen has emerged from the CGPS and SGPS data as a striking feature of the 21-cm channel maps, with complex wispy and filamentary shapes ranging in size from one degree down to the arcminute limits of resolution. High angular and spectral resolution is critical to this study. The population revealed at 1' resolution is far richer and more diverse than expected from single dish surveys, whose large beams dilute most features into invisibility. Surprisingly, many of the strongest HISA features in the CGPS, which are found in the Perseus arm (Gibson et al. 2000), lack clear associations with CO or dust emission, in contrast to the traditional equilibrium picture of cold atomic hydrogen as a trace constituent of molecular clouds. Since these strong features occur predominantly at velocities expected for the spiral wave shock, they may represent HI cooling and condensing into molecular gas downstream of the shock in the first step toward star formation.

Figure 1. HISA features in the VGPS survey.

In the outer Galaxy, the velocity-distance relationship from galactic rotation is single valued. We thus rely on non-circular gas motions to reveal HISA clouds silhouetted against a background of HI emission. In the inner Galaxy, there are two points along the line of sight, located on either side of the tangent point, which have identical radial velocities, giving us a systematic probe of HISA clouds. We will be able to test whether HISA clouds are a ubiquitous ISM component or associated in velocity-position space with spiral arm shock patterns (as suggested from the Perseus arm study). The VGPS will capture HISA in the Sagittarius and Scutum-Crux arms, filling in the crucial longitude coverage gap between the SGPS and CGPS.

HI self-absorption may also solve the distance ambiguity problem for CO emission from the inner Galaxy. The absolute positioning of a given CO emission feature within the Galaxy is limited by the assumption of circular rotation and the near-far side distance ambiguity. The resolution of this ambiguity is critical to the construction of the gas distribution from the survey and to the determination of accurate gas and star formation properties such as masses and far infrared luminosities. Previous surveys have used scale height arguments to place an object on the near or far side of the tangent point. We are investigating a new means to rectify the ambiguity. The assignment of a molecular feature to the near side may be made by the presence of associated HI self absorption (see Figure 2). In this case, the data were obtained from Arecibo with an effective beam size of 4 arcminutes. By providing a high resolution HI data base to reduce the confusion, the VGPS will be a critical complement to the BU-FCRAO Galactic Ring Survey.

Figure 2. HI 21cm line (shaded histrogram) and ¹³CO spectra from the Galactic Ring Survey (line) but resampled to the HI resolution toward a selected direction. The presence of HI self-absorption at the ¹³CO velocity places a molecular cloud at the near kinematic distance.

In addition to HISA, HI absorption toward background sources and Galactic continuum emission offers a unique opportunity to investigate cold gas in the inner Galaxy. HI continuum absorption (HICA) complements HISA studies. The physical properties of the HI gas are more easily determined for HICA, while the extensive illuminating background of HISA allows a more complete view of the spatial distribution and structure of cold HI. Used together, they are a powerful probe of this phase of the ISM. HICA spectra toward the several hundred continuum sources brighter than a few hundred mJy will provide the first detailed survey of cold gas properties in the inner Galaxy. We will gain a clear picture of the nature of diffuse clouds across a significant fraction of the HI disk and in a variety of environments. This will measure, e.g., how the abundance of cool versus warm HI changes with galactocentric radius, and the variation of HI column density across and between spiral arms. In addition, HICA spectra will provide rough distance indicators to supernova remnants (Figure 3), HII regions, and other Galactic continuum sources.

Figure 3. HI continuum absorption of the Galactic supernova remnant W44 in the VGPS survey.