Where is the Gaia data?

The European Space Agency’s Gaia Mission has provided distance estimates, magnitude, colour and other data for almost 2 billion stars, making it possible to map out the local galaxy in detail for the first time.

But how much of the galaxy? This is not an easy question to answer because Gaia data reliability depends upon distance, obscuring dust clouds, overcrowding in the galactic plane and so on.

A project called Gaia Unlimited has taken on this question using objects such as red clump stars, masers and Cepheid variables whose distances can be estimated independently of Gaia.

The map image leading off this article shows a colour range highlighting Gaia completeness based on a comparison with a red clump star catalog. The region near the Sun reliably plots the locations of almost all stars and the completeness gradually fades away, with completeness lasting longer in the outer galaxy than the dusty crowded inner galaxy.

Gaia should make it possible to map the near end of the galactic bar in considerable detail

Two conclusions we can draw from this is that first, Gaia’s 80% completeness region extends to the near end of the galactic bar so it should be possible to map that out in considerable detail and second, Gaia is reasonably complete in the direction of the outer galaxy.

Gaia has not detected any significant structures beyond the Perseus region, raising doubts about the existence of an Outer arm.