For the rest of this, I made a box around the largest hole in h516 (z=0) that included the whole disk. That's all I've analyzed so far. It would be interesting to see if we could see the hole using the entire galaxy, but that's an exercise for the rest of this week.
Two main ways to find these guys is to look at moment maps for obvious holes:
This is a total-intensity map of the HI in h516. Notice the edges of the box. Also notice the large hole. That's good. It's about 1.5 kpc in diameter.
The second way to find them is to use a position-velocity diagram. This effectively collapses your data cube along one of the position axes, leaving you with a map of position (usually along the major axis of the galaxy) versus velocity. If you see gas in the same position moving at two different velocities, that's indicative of a hole. In my limited experience, observers will take regions around suspected holes and collapse those. This is the main reason I made a tipsy box just around the hole. Anyway:
The axes are velocity versus position. The "split" in the center of the image is the main part of the hole. To find the velocity difference across two sides of the shell, I (by eye) looked at the average delta v for at a single (representative) position. It's about 15 km/s, which means that the shell expansion velocity is about half that: 7-8 km/s. This, however, gives a different size for the hole. To do that, you would (I believe!) take the total distance across the hole, regardless of velocity. (Basically, that's the projection of the hole on the x-axis only.) This gives a diameter of ~0.75 kpc. Hmmm.
To reconcile that with the above moment map, I made some new moment maps showing much deeper contour levels. When I plotted lower levels of HI emission, more of the hole filled in, and the diameter of the hole became ~0.75 kpc. So that's my current explanation for the difference, but I don't currently have an image of the new moment map.
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