``A High-Resolution Longitude-Velocity Study of the Galactic Midplane''

S. J. Gibson, J. M. Stil (University of Calgary)

2002, Bull. A.A.S., 34, 1187, #50.02

Presented on Tuesday, January 7, 2003

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We construct a detailed longitude-velocity diagram through the actual midplane of the Galactic H I disk. The Galactic midplane definition is based on single-dish sky surveys, and the detailed l-v slice (Fig. 1) is extracted from the Canadian Galactic Plane Survey (CGPS; Taylor et al. 2003).


We define the Galactic midplane as a smooth surface passing through the brightest H I emission in the disk. We use the Dwingeloo and Parkes H I sky surveys (Hartmann & Burton 1997; Strong et al. 1982) to map the latitude of the peak brightness as a function of longitude and velocity, excluding areas with poor S/N. Only emission within 10 degrees of the Galactic plane is considered. The signature of the warp in (l,b) channel maps resembles a simple cosine function in longitude (Fig. 2). We approximate the midplane as a set of cosine functions fitted to each velocity and smooth the derived (l,v) surface by 10 km/s in velocity to ensure continuity. The fits assume the (l,v)  ->  (-l,-v) symmetry suggested by the data (Fig. 2), and they exclude the longitude-velocity loci of inner Galaxy material as well as LSR velocities within 40 km/s of zero; we assume the midplane lies at b = 0 for |v | <= 40 km/s. Fig. 3 shows the resulting midplane map. This was used to construct the detailed CGPS midplane slice in Fig. 1.


The characterization of the Galactic midplane allows us to investigate departures from the smooth large-scale structure of the Galactic disk potential and the warp. The CGPS longitude-velocity diagram shows a wealth of structure on scales of 1 to 10 degrees (175 to 1750 parsecs at a distance of 10 kpc). This enables us to study the coherence of spiral arms and inter-arm regions on these scales, which provides a Galactic context for continuum absorption and H I self-absorption. How empty are inter-arm regions really? Small-scale features intruding into forbidden positive velocities in the second Galactic quadrant allow us to study the tail of the velocity distribution of local clouds. For more details, please see www.ras.ucalgary.ca/~gibson/himp.


Larger versions of each image are available via links.

Figure 1. CGPS H I emission in the Galactic midplane surface (see text). The emission is divided broadly into local arm gas near 0 km/s, Perseus arm gas near -40 km/s, and outer Galaxy gas at more extreme velocities. The arcminute resolution of the CGPS reveals considerable fine-scale detail not visible in prior surveys. Superposed on the general arm structure are many small velocity corrugations tracing gas kinematics in the disk. Note: other slices offset from this one were not shown in the AAS poster for lack of space, but they are available here.

1272x417 GIF [273 kB] | 6358x2086 GIF [1.8 MB] | 1-page PS [3.4 MB gzipped]

Figure 2. Variation of the H I midplane with longitude and velocity. The red curve shows the cosine fit at each velocity pair. Outer Galaxy gas with both positive and negative velocities is shown in each panel.

|v | = +/- 61 km/s: 846x319 GIF [93 kB] | PS [57 kB]
|v | = +/- 78 km/s: 846x319 GIF [82 kB] | PS [53 kB]
|v | = +/- 95 km/s: 846x319 GIF [59 kB] | PS [48 kB]
|v | = +/-110 km/s: 846x319 GIF [53 kB] | PS [43 kB]

Figure 3. Midplane surface defined by cosine fits (e.g. Fig. 2), from which the CGPS slice was constructed (gray box; Fig. 1). The black curves indicate constant Galactocentric radii of R = 9, 10, ..., 25 kpc, assuming a flat rotation curve with R0 = 8.5 kpc and V0 = 220 km/s.

842x850 GIF [54 kB] | PS [95 kB gzipped]


Return to the CGPS Galactic H I Midplane Project page.