I have been pondering a question that arose while I was reading a research paper that mentions galactic disk stars have been found up to distances as far as 25 kpc from the galactic center and possibly even up to distances as far as 31 kpc https://www.aanda.org/articles/aa/abs/2018/04/aa32880-18/aa32880-18.html (López-Corredoira et al, 2018).

Since this is much further than the commonly stated Milky Way radius (15-20 kpc) I was wondering if there is a particular distance (or distance range) observed where the galactic disk ends/reaches its edge and no disk stars are being observed and measured at? Or is it possible that the exponential disk continues (albeit at low densities) to the edge of the galactic stellar halo or further (> 200 kpc)? From our limited perspective within the Milky Way how do scientists know our spiral disk is similar to that of other galaxies in size, is it possible our galactic disk could be much wider than that of other observed galaxies?

Sorry if this comes across as an ignorant question, I am new to this field and just curious


1 Answer 1


So, I'm not an astrophysicist, much less a galactic dynamics astrophysicist. But I'm going to take a shot at this. How do you look at a star and classify it as "belonging" to the galactic disk? The paper you reference (Lopez-Corredoira, et al) is basically grappling with that question. The answer more or less has to do with looking at the distribution of velocity components to distinguish disk stars (a population with small velocity components perpendicular to the plane of the galaxy) from other stars, such as halo stars, which is a population with larger velocity components perpendicular to the galactic plane.

Once we recognize those classifications based on velocity components the rest of the answer comes down to "do some complicated statistics", and that's more or less what the paper you refer to is doing. The answer of where the disk "ends" (which will always be a fuzzy boundary) depends on how you do those statistics, and so it will be a contentious issue among the people who are studying this. Ultimately, if we want a single answer to the question we need to settle on choices in that statistical analysis which produce results that we agree are sensible in some way. What we feel is "sensible" probably depends on what other questions about galactic dynamics we are trying to answer.


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