Astronomers have found the edge of the Milky Way at last, maybe

by | Mar 26, 2020 | Galaxies, Milky Way | 0 comments

Astronomers have found the edge of the Milky Way at last(?)
In this image from the Fermi space telescope, the Milky Way’s stellar disk, which runs horizontally along the middle, glows in gamma rays. A vast halo of dark matter engulfs the disk and emits no light at all, which makes measuring the galaxy’s total size a challenge. CREDIT: Fermi Lat Collaboration/DOE/NASA

Our next story of the day is one I debated talking about, because theoretically you shouldn’t talk if you’re not sure you have anything nice to say, and here, I’m not sure. A new paper posted on the astronomical part of the preprint server is getting a lot of coverage for claiming our galaxy’s edge has finally been found, and our galaxy is significantly bigger than anyone ever thought. The problem with this paper, right off the bat, is there are a gazillion ways to define “edge”. Even with humans this can be tricky. Most people, if you ask them how tall they are, will give you the distance from the soles of their feet to the tops of their head, as measured while standing flat footed, but you could also measure to the top of their hair, giving some punk rockers one hell of a height advantage. You could also go by the maximum extension, and measure from the tip of a ballerina on pointed toes to the tip of her outstretched arm.  Those latter measurements aren’t standard, but you can make those measurements and proclaim humans to be of unusual size.

In the same manner, we can measure galaxies in all sorts of different ways and get all sorts of different numbers, and that’s what’s happening here. Normally, we base our size measurements on looking at a galaxy through a specific filter, so we’re seeing light from the same kinds of objects, and we define the size based on where the brightness is some set fraction of the bright inner region of the galaxy. It’s a whole lot of math, but it keeps everything more or less consistent. What the folks in this paper have done is to instead try and sort using computer models and observations of nearby dwarf galaxies, and tried to sort how in-falling material might decide “I shall fall into the Milky Way” versus falling into something else like Andromeda.

There are soo many problems with saying “this is the edge” the first being that over the 13 billion year history of our universe, stuff has moved, and since galaxies grow through accumulation of colliding galaxies, these kinds of models can’t know how to accurately take into account stuff that we can’t see because we already ate it.

Anyway – folks have asked me about this story, and I’m just going to say this is a neat idea, but the physics of our reality doesn’t make this paper a solid way to say how big our galaxy may or may not be.


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