M33 in HI at different sensitivity levels

With acceptance of the publication now imminent, I suppose one little gif can't hurt...

This is M33 (Triangulum) as seen in the optical Sloan Digital Sky Survey and with atomic hydrogen data from AGES (the Arecibo Galaxy Environment Survey) at different sensitivity levels. It stars with a purely optical image. Next the hydrogen is added at a very low sensitivity so that only the brightest gas (blue) is shown, which follows the stellar disc pretty nicely. Then we ramp up the sensitivity a bit and the fainter hydrogen is seen to extend quite a bit further, especially in the upper-right where a "warp" in the disc is visible. Finally we go to the best sensitivity level we can (any more and you'd just see noise), showing how the full extent of the hydrogen is much greater than what was previously known.

The sensitivity levels are chosen entirely arbitrarily just to show how processing the data in different ways can reveal very different features from the same data set.

And yes, it looks very nice indeed in 3D, but you're going to have to wait for that one.