Where does the Milky Way end?

Question

I was reading this article and it says the following:

Researchers measured the mass of the Milky Way and found that our galaxy is approximately half the weight of a neighbouring galaxy known as Andromeda which has a similar structure to our own.

So I was thinking, if we consider a galaxy neighbor to us, then our galaxy should be having an endpoint and the neighboring galaxy should be having a start point. So how do we know which is the starting and ending point of a galaxy, and how do we calculate it?

Answer 1

Here's a rough sketch of the Milky Way and the Andromeda galaxy,^* showing their approximate sizes and distance from each other to scale:

 

What the picture (hopefully) illustrates is the incredibly vast gap of empty space — around 2.5 million light years, to be exact — between the galaxies, each of which has a diameter of only(!) around 100 thousand light years or so.

While both galaxies are shaped pretty much like "fuzzy whirlpools" of dust and gas (and, apparently, dark matter), with no precisely defined sharp outer edge, they are still pretty unambiguously separated. While we may not be able to point to a specific line in space and say that "the Milky Way ends right here", it's still clear that the Milky Way is over here, while the Andromeda galaxy is over there, and there's a huge gap of pretty much absolutely nothing at all in between.

(Of course, there is some stuff even in intergalactic space, including some very diffuse gas, a little bit of dust and even the occasional stray star. Still, compared to the galaxies themselves — which, from a human viewpoint, are already pretty full of empty space — the intergalactic medium can be pretty well described as empty.)

_{*) The little specks in the picture around each major galaxy are meant to represent their smaller satellite galaxies, such as the lesser and greater Magellanic clouds. Their relative positions and distances, alas, are probably not very accurate.}

Answer 2

Update for clarity:

For the visible part of a standalone galaxy, the stars can all be measured to orbit that galaxy's core. So if you wanted to measure the furthest extent at this simplified level, it would be very easy. The problem is that there is a lot of mass which is not stars, and which is dark matter. Some of it is so far out, it may be impossible to calculate what it is orbiting.

If you read the paper, you will see that they chose a limit beyond which the effect of the negligible mass further out was not relevant to this calculation.

A galaxy can be thought of as a flattened sphere, as regards mass. Stars, at least for spiral galaxies, tend to lie in a plane, but there is mass orbiting the common centre in all planes. So there is no start point, there is just a need to decide how far out we want to say the galaxy extends.

For the Milky Way and Andromeda, a decision has been made that is the same for both (the exact decision doesn't appear to have been published for this paper, but as long as it is consistent, then the relative masses will be correct)

For galaxies that are colliding or near to each other it is much more difficult - do you calculate which galaxy a star belongs to by the direction of its motion? Stars may swap from one to the other. From Wikipedia's Milky Way page:

Surrounding the Galactic disk is a spherical Galactic Halo of stars and globular clusters that extends further outward, but is limited in size by the orbits of two Milky Way satellites, the Large and the Small Magellanic Clouds, whose closest approach to the Galactic center is about 180,000 ly (55 kpc).[51] At this distance or beyond, the orbits of most halo objects would be disrupted by the Magellanic Clouds. Hence, such objects would probably be ejected from the vicinity of the Milky Way.

tl;dr - it's a pretty arbitrary decision :-)

Answer 3

As already answered, the definition of the size of a galaxy must always to some extend be arbitrary. In astronomy, several definitions are used, according to the context in which it's used, e.g.:

• $R_{\mathrm{vir}}$ (the virial radius): Used when considering the galaxy's dynamics; defined by $GM_{\mathrm{vir}}/R_{\mathrm{vir}} \sim V^2$, where $G$ is the gravitational constant, $M_{\mathrm{vir}}$ is the mass inside the virial radius, and $V$ is the circular velocity (for disk galaxies) or velocity dispersion (for elliptical or irregular galaxies). For instance, if an initial density perturbation is smaller than its virial radius, it will collapse to form a galaxy. This radius is also the radius within particles will be gravitationally bound if their velocity does not exceed $V$.
• $R_{1/2}$ (the half-light radius): The radius within which half of the total observed light is emitted. This definition is useful when comparing the luminosity of galaxies.
• $R_{200}$: The radius within which the average density is equal to 200 times the average density of the Universe. Similarly, sometimes $R_{500}$ or $R_{1000}$ are used. These definitions make sense when discussing the mass of a galaxy, since beyond they sort of blend in to the intergalactic medium.

Although these definition give different sizes, they are all of the same order of magnitude.