Hertzsprung-Russell diagram with stars from the Hipparcos and Gliese catalogs (from Wikimedia Commons).
Converting observations into models is always hard in science. Converting parallaxes from the TGAS dataset in Gaia DR1 into a map of the solar neighbourhood has its own unique challenges.
In previous blog posts I've described missing data in the TGAS data set caused by incomplete sky scans, lack of calibration for bright stars, etc. This missing data will get filled in (and dramatically expanded) by future Gaia releases. In this blog post I'm looking into more fundamental issues that would exist even if the TGAS dataset was complete and are relevant to future Gaia releases as well.
We cannot just take the entire TGAS parallax dataset and use it to compute density isosurfaces to produce a map using the marching cubes algorithm mentioned in my last blog post. Attempting to do that does not produce a map, but rather shows us what Gaia can see. It is important to get a better idea what Gaia can see, however, as we can use this information in choosing an appropriate TGAS subset that can be used for mapping.
Here is what happens if we select all the one million TGAS stars with err/plx <= 0.2. These isosurfaces are constructed from all low error TGAS stars, starting with the 5% isosurface dissolving until it reaches the 95% isosurface.
Here is another way of visualizing these isosurfaces by stacking them on top of each other:
We get this effect because Gaia can see more stars that are closer to the satellite and fewer stars further away. In principle, if stars were distributed equally in space, we would expect to see a series of spherical isosurfaces that get denser the closer they are to the Sun. In the real TGAS data we don't quite get that - the denser isosurfaces are flattened ellipsoids and the less dense isosurfaces are flattened disks with large craters in the upper third quadrant and lower first quadrant.
It is easy to see the reason for the flattening if we look at the z-coordinates of the TGAS stars:
Most of the stars in the Milky Way are concentrated in a thin disk, and the majority of the TGAS stars can be found within 300 parsecs of the galactic plane. The source of the other characteristics of the full low error TGAS isosurfaces (why an ellipsoid rather than a sphere for the denser isosurfaces? Why the large craters in the first and third quadrants for the less dense isosurfaces?) is not clear.
More to come