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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?

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    $\begingroup$ @SirCumference I think that is pretty much the question asked. $\endgroup$ – Hohmannfan Sep 5 '16 at 22:00
  • $\begingroup$ @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. $\endgroup$ – Sir Cumference Sep 5 '16 at 22:10
  • $\begingroup$ Also, the Solar System is usually defined by the objects orbiting the Sun — that is, everything within the Sun's Hill sphere. $\endgroup$ – Sir Cumference Sep 5 '16 at 22:11
  • $\begingroup$ @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. $\endgroup$ – Lycan Sep 5 '16 at 22:42
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    $\begingroup$ @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 $\endgroup$ – Lycan Sep 5 '16 at 22:47
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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"?

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  • $\begingroup$ 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. $\endgroup$ – Lycan Sep 6 '16 at 11:52
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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.

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  • $\begingroup$ Thanks for an interesting overview. What about the interstellar medium within a galaxy? Is there a point where it is overpowered by the intergalactic medium that could be considered an edge of the galaxy? That seems to be how astronomers think of the edge of the solar system: the point where the interplanetary medium is overpowered by the interstellar medium. $\endgroup$ – Lycan Sep 6 '16 at 0:53
  • $\begingroup$ @Lycan I dunno. If that's how they think of it then they define it so I guess that's right, but you're largely talking about plasma and radiation that would be more fluid, flows, ripples, twirls, etc and so you still have the smae problem, it's not a perfect shape and you'll get wonkey shapes that result in multi-galaxies becoming 1, or structures that "part of the galaxy" that stretch far out, making the galaxy longer, but noone thinks of them as defining to the size of the galaxy in shape. $\endgroup$ – Durakken Sep 6 '16 at 2:01
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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?

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  • $\begingroup$ Very interesting! Had no idea about the chimneys. $\endgroup$ – Lycan Sep 6 '16 at 11:51
  • $\begingroup$ @Lycan I'm not an expert, but I don't think you're question was really answered. Another way of asking this would be, "Does the Milky Way move through the local intergalactic medium or do they move together?" It's been published that the Milky Way moves toward the "Great Attractor" at about 600 km/s (the speed of the solar wind is about 400 km/s). The density of the intergalactic medium has been estimated at about 1H per cubic meter (1H per cubic cm for the ISM). If you say that the ISM meets the IGM at about 300 km/s then the equal pressure at the "galacticopause" would be about 10E-22 N/m2 $\endgroup$ – Jack R. Woods Sep 13 '16 at 3:09
  • $\begingroup$ @Lycan cont. By comparison, the solar wind pressure where there is an equilibrium with the ISM is on the order of 10E-13 N/m2. I arrived at this value since the solar wind pressure at Earth varies from about 1-6 x 10E-9 N/m2 and the heliopause is estimated to be between 80 and 120 AU. $\endgroup$ – Jack R. Woods Sep 13 '16 at 3:19
  • $\begingroup$ @JackR.Woods Definitely sounds like what I was visualizing. I don't really have the understanding to evaluate your numbers here. Just to throw something else in, I since found on the Interstellar medium page on Wikipedia this note: "[The ISM] fills interstellar space and blends smoothly into the surrounding intergalactic space." Possibly meaning it has no abrupt edge but blends out in a smooth gradient. Unfortunately it's unsourced. $\endgroup$ – Lycan Sep 13 '16 at 4:38
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    $\begingroup$ Maybe the local IGM is moving toward the "Great Attractor" at about the same speed as the galaxy. $\endgroup$ – Jack R. Woods Sep 13 '16 at 17:05

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