The hydrogen foothills of the Milky Way

And another one. More background for those who have no idea what's going on...

HI data cubes map the atomic gas content across the sky. As well as the "brightness" (actually density) of the gas, they also record what frequency it's emitted at. The frequency depends on how fast the gas is moving towards or away from us, which is very roughly correlated with its distance. The correlation gets better when you start to go to distant galaxies, but is pretty meaningless within our own galaxy - its own rotation dominates.

There are lots of ways to visualise the data depending on what you want to do with it. Normally you'd pick some fixed velocity and then make a 2D map of the density. Or you can render lots of different velocities at once and get a 3D image In this case I've used the density to set both the colour but also the height of each pixel, creating a landscape. Each pixel has the same frequency.

What this actually is is part of the gas in the Milky Way, with more dense regions shown as blue peaks and less dense ones shown as red troughs. You can also see some perfectly linear features - some very narrow, others broad. These happen because our survey isn't really designed for detecting signal across the whole field of view. While the telescope calibrates the data to some extent, we also rely on the signal only occupying a small fraction of the view at any given frequency. We use this to estimate the zero level of the data, and for pretty much every other data set this works extremely well. But in the Milky Way, where there's gas all over the place, it doesn't, hence the data levels are at best unreliable and at worst meaningless. The linear shape of these artifacts occur because we construct the data by making narrow scans across the sky, so if something changes from one scan to another (e.g. if there's a particular big, bright bit of gas in one scan but not so much in an adjacent one), they look noticeably different.

The reason I started trying this was because most of the sources are much less interesting : they're just unresolved points. Knowing the shape of the telescope beam (a Gaussian) and the maximum flux, it should be possible to remove them (hopefully revealing any more extended sources that might be present). I've done this before, but this time it wasn't working so well. The Gaussian model of the source looked like a good approximation of the real source, but it's hard to tell exactly how close it is by comparing colours. It's much easier to do this by comparing surface heights, which let you compare the data over the 2D area rather than pixel by pixel. But now I've got distracted by how pretty the data looks... well, it is a Saturday, after all. :P

Next up : animate the frequencies to generate a surface that changes both shape and colour...