Gas in the Galaxy. The rotation curve
We have already seen what the line-of-sight
velocities should be for the stars in the disk of our Galaxy if they were
moving on perfectly circular orbits:
Note:
If the MW rotated like a rigid body, the angular
speed would be constant, and the line of
sight velocity would always be zero.
Typically, the angular speed drops with radius (recall for example,
in the Kepler problem, V = sqrt(GM/R), so that 
). This implies that
- 0 < l < 90 and R < Ro (e.g. nearby)
then Vlos > 0
- 0 < l < 90 and R > Ro (external to the solar
circle) then Vlos < 0
- 90 < l < 180 (R > Ro always)
then Vlos < 0
- 180 < l < 270 (R > Ro always) then
Vlos > 0
- 270 < l < 360 and R < Ro then Vlos <
0,
- 270 < l < 360 and R > Ro, then Vlos >
0
This very characteristic pattern of radial velocities
can be observed in the motion of gas in the disk of our Galaxy.
See Figure 1 (Fig.2.18 from Sparke & Gallagher)
This image shows the intensity of the HI line emission from gas in the disk
of the Galaxy: as expected there is no gas with positive velocities in the
2nd quadrant or with negative velocities in the 3rd quadrant.
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