``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.
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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
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.
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Return to the CGPS Galactic H I Midplane Project page.