What is at the edge of the galaxy?

  • 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?

    @SirCumference I think that is pretty much the question asked.

    @Hohmannfan I'd say he's asking about what lies at the edge of the Milky Way. Or maybe his title isn't related to the actual question.

    Also, the Solar System is usually defined by the objects orbiting the Sun — that is, everything within the Sun's Hill sphere.

    @SirCumference As stated in the question body, I'm (tentatively) thinking of "the edge of the galaxy" as the interface between space dominated by forces in the galaxy and space beyond that / intergalactic space. It may be a fuzzy boundary; there may not a precise line. Or perhaps it is precise at a point in time but it fluctuates over time due to dynamic interactions between the 2 regions.

    @SirCumference This is by analogy to the "heliopause" which is considered for many purposes the edge of the solar system. For example NASA announced Voyager 1 had reached interstellar space when it had crossed the heliopause: Voyager 1 Reaches Interstellar Space

    @SirCumference I don't know whether there is an analogous phenomenon in the outer galaxy not. I'm open to other definitions of "the edge of the galaxy" if there are better ways of thinking about it.

    @lycan the heliopause is based on a field which has a precise size that we can measure linearly from the source. The galaxy as far as i know has no such thing. Also it depends on what you call "part of the x" A lot of people wouldn't consider the Oort Cloud part of the solar system for example whether you extends that far or not and you might have the same thing with the galaxy, but worse because you might not consider a star part of the galaxy even though it's inside what you consider the galaxy and galaxies are pretty much defined by stars...

  • Peter Erwin

    Peter Erwin Correct answer

    6 years ago

    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"?

    Great answer, thank you. Exactly what I was looking for. Have searched for this multiple times in the past but never been able to find a discussion of it.

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