# Is there a theoretical maximum "size" for galaxies?

I am aware of upper limits to the mass of burning stars and neutron stars, beyond which they degenerate into neutron stars and black holes respectively. And also, if I understand it right, there is a maximum event horizon radius for black holes, related to the CMB, beyond which the black hole will evaporate down to an equilibrium "temperature" / "volume".

But is there any theoretical upper limit to the "size" of a single galaxy in any meaningful sense? It appears that all of the largest galaxies are of the elliptical type.

I suppose meaningful definitions of size would be effective radius, number of stars, total mass (including or excluding dark matter), and some measure of density using these other figures.

And if there are upper limits, what brings about these limits, and what happens to galaxies that exceed them through collision/absorption, collapse into the central black hole, etc?

I guess I'm wondering if there is something exotic at play, like if gravity can't hold a galaxy together past a certain size due to relativistic effects, or something like that.

• There is no maximum for black holes -- the bigger they are the slower they evaporate. Anything more massive than the Moon (and thus larger than about 0.1 mm) will gain mass from the CMB faster than it evaporates at this point in the history of the universe. There is however a rough minimum -- too small and they evaporate faster than you can give them mass, so they don't last long.
– user10851
Jul 15, 2016 at 1:24
• Also, Chandrasekhar is the limit for dead stars supported by electron degeneracy (i.e. white dwarfs). If the star is actively burning fuel it can get much, much larger than this limit (which is just 1.4 times the mass of the Sun). We observe stars up to around 100 times the mass of the Sun. Whether they can get more massive than this is an open question (the limits we understand well don't really kick in until $10^5$ times the Sun's mass or thereabouts).
– user10851
Jul 15, 2016 at 1:30

No. If the dark matter density is sufficiently high, a galaxy may remain gravitationally bound and thus there is no theoretical upper size limit for a galaxy.

However, if we talk amount luminous mass, constraints on an upper mass limit exist, and they are unsurprisingly dependent on a mass-luminosity relation. As an example, radiation pressure which pushes gas and dust outwards at a far enough distance from the galactic centre will counteract gravitational pull from the enclosed mass and let matter become gravitationally unbound.

The limiting factor really comes down to dark matter density as a function of distance from the galactic centre. If the density of dark matter does not increase quickly enough to sustain luminous baryonic matter, counteractive effects of luminous matter will see the galaxy fall apart. This is because, as far as we know, dark matter only acts gravitationally, so aides galaxies being held together.

This is especially important if rotational velocity is too high at the outskirts of a galaxy; with sufficient velocity, stars and matter can escape the outskirts.

Well, I'm no expert, but I think part of the problem is that the definition of a galaxy is somewhat shaky. This article (which references this paper) has the definition,

"A galaxy is a gravitationally bound collection of stars whose properties cannot be explained by a combination of baryons and Newton’s laws of gravity."

So I suppose that we can't define a size limit on galaxies yet because a) we haven't agreed on what a galaxy is, and b) we don't know enough about, say, dark matter and other stuff that binds together galaxies, to even guess at what might cause an upper limit.

• So I guess we know a galaxy when we see one? Aug 9, 2016 at 0:08