What is at the edge of the galaxy?

Question

The solar system is thought to have a heliopause which can be considered its edge for many purposes:

The heliopause is the theoretical boundary where the Sun's solar wind is stopped by the interstellar medium; where the solar wind's strength is no longer great enough to push back the stellar winds of the surrounding stars. This is the boundary where the interstellar medium and solar wind pressures balance.

Heliopause: the boundary between solar wind and interstellar wind where they are in equilibrium.

Is there a similar phenomenon at the interface between space dominated by the Milky Way and intergalactic space?

What is at the edge of the Milky Way galaxy?

Answer 1

The short answer is: probably nothing much, because galaxies are very fuzzy objects without "edges".

If you look at the stellar disk, it just fades out in density, with no evidence for a sharp cutoff (this is true of disk galaxies in general). There may or may not be a "break" at some radius, but this just marks the beginning of a steeper falloff in density, not an "edge" in any real sense. The gas disk extends even further out (that is, further out than we can easily trace the stars in the disk), and doesn't show much evidence for a sharp cutoff either.

There's also a round halo of stars, which is low density but extends to at least 400,000 light years; two stars were recently discovered which are plausibly part of the halo and located at distances of about 800,000 light years. (Note that there are other galaxies -- such as the Large and Small Magellanic Clouds -- which are closer to us and to the center of the Milky Way, so you have to accept that individual galaxies can be found "inside" other galaxies.)

And then there's the halo of dark matter...

I can think of two ways in which you might talk about the outer boundary of an isolated galaxy (which the Milky Way sort of is) from a theoretical perspective. The first is the idea of a halo "accretion shock", in which relatively cold gas falling onto the galaxy from intergalactic space runs into the galaxy's halo of hot gas ("hot" = millions of degrees), creating a shock where the infalling gas slows down and the local density of gas goes up. (This is a little bit analogous to the heliopause, though the direction of gas flow is the other way around: the heliopause is where the outflowing solar wind shocks into the more or less stationary interstellar gas.) The problem is that extensive, hot halos of gas are thought to form only around massive galaxies, and it's not clear the Milky Way is massive enough to qualify. It also won't work for galaxies in massive groups and clusters, where the individual galaxies aren't accreting gas any more (the cluster is what has the hot halo and the accretion shock).

The other possibility is a region where the infalling dark matter particles merge into the dark-matter halo (whose particles are orbiting within their common gravitational potential, and so are not as a whole falling towards the center any more). Simulations suggest that as one moves inward the local density of dark matter starts to increase rather rapidly at this point (and the average velocity of the dark matter particles changes), but it's still a fairly smooth process (unlike gas, the dark matter particles aren't "shocking" into each other). There's no way of determining where this is for the Milky Way, other than something really vague like "maybe 1 million light years"?

Answer 2

It is argued that the edge of the galaxy has the halo of dark matter at around 100,000 light years, but there are arguments that extend the galaxy from anywhere between it being 120,000 to 200,000. The major factor though for putting the edge at around 100,000 light years is that there is a major star density fall off for no known reason.

Similarly the depth of the galaxy is said to be 10,000 ly at the bulge and 1,000 ly for the rest of the galactic plane. There is similarly no reason for the fall off in star density in that direction known. And there are massive xray and gamma ray structure coming out of the galactic center that is 50,000 ly on either side which you could argue is the real edge of the galaxy along that axis.

So what lies at the edge of the galaxy depends on how you define the edge of the galaxy which could be:

• 100k x 100k x 10k
• 100k x 100k x 100k
• 200k x 200k x 10k
• 200k x 200k x 100k
• 120k x 120k x 10k
• 120k x 120k x 100k

Or you could use some other arbitrary number, but you'd need to make that argument.

I however think that a star density definition should be how we should define things "officially" because it is more or less how most people think of the galactic limits in the first place. Of course if you define it like that then you're dealing with issues like the New Outer Arm where the density falls off and then increases, the actual density isn't a perfect shape, and we might have to say that things we call 2 or 3 galaxies are actually 1 galaxy, such as the 2 dwarf galaxies orbiting the Milky Way or the where there exist 2 galaxies with a stream of stars between the two as they merge over millions/billions of years.

Answer 3

While the Solar wind is pretty much homogenous in all directions, the galaxy puffs out stellar winds from discrete and temporary star forming regions. Protostars and giant stars which go supernova young, punch "chimneys" through the interstellar medium in the galactic disk here and there. The Local Fluff is btw our region inside the Local Chimney, a relatively low density region formed by pressure from the young and exploding stars in the Scorpius-Centaurus Association 400 light years from here. And the supermassive black hole turns its jet on and off depending on when it accretes material. I don't think there's a pan-galactic Milky Way wind that looks anything like the Solar wind. But around an intensely starbursting quasar galaxy maybe?