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You might think this question would be easy to answer. If it’s big, it should be pretty straightforward to find, right? Yeah, well, the Universe isn’t always that simple.
First, what’s a galaxy? Basically, it’s a collection of stars, gas, and dust (as well as invisible dark matter) bound together by its own gravity. Some are elliptical (giant puff balls), some have disks and spiral arms, some are irregular (shapeless), and some peculiar (they have a shape, but it’s … weird). If you need a refresher, this episode of Crash Course Astronomy explains them:
Most galaxies have billions of stars. Our home galaxy, the Milky Way, has hundreds of billions strewn across a disk about 100,000 light-years in diameter. Some galaxies are much dinkier, and have only millions of stars; those are hard to find, even when they’re nearby, because they’re so faint.
I like to think of galaxies as the basic building blocks of the Universe. They’re like towns and cities strewn across the cosmos. Back when we were first figuring out their true nature, they were sometimes called “island universes.” Poetic, and not a bad description.
Galaxies can grow pretty big, usually by eating other galaxies. They can collide and merge to form a bigger, more massive galaxy. In a few billion years we’ll crash into the Andromeda galaxy, forming one around twice the size we are now.
So, how big can they get? What’s the biggest galaxy?
It turns out that this isn’t easy to answer for two reasons. One is that it depends on what you mean by “size,” and the other is that, paradoxically, the biggest galaxies may be very faint.
Let’s tackle the second one first. There exists a class of galaxy called Giant Low Surface Brightness galaxies. As the name implies, they aren’t terribly bright, even though they can be quite large. They’re rare, so they tend to be far away, and that means they’re hard to spot. One, called Malin 1, was only discovered in 1986, and was recently found to be far, far larger than previously thought: It’s a spiral galaxy a colossal 700,000 light-years across. At least. That’s five times the size of the Milky Way.
Another, UGC 1382, has a disk of stars about the same size as Malin 1’s but has gas measured out to a distance of 720,000 light-years! Malin 1’s disk is about that same size, within measurement error. Malin 1 also has a gas envelope that is 720,000 light-years across.
These GLSB galaxies are way bigger than normal galaxies, but they’re faint. There could be more of them, even bigger ones, but they’re really hard to find. Malin 1 looks like a relatively normal spiral until you take really deep images of it.
So there could be larger galaxies out there, and we don’t even see them!
And then there’s another complication, and that’s what you call a galaxy.
Let me introduce you to IC 1101. If UGC 1382 and Malin 1 are huge, IC 1101 is a behemoth. Its diameter has been measured at a staggering, overwhelming 2 million light-years. If one end were placed at the Milky Way, it would stretch two-thirds of the way to Andromeda!
But wait a sec, because it may not really be that big.
IC 1101 sits in the center of a large cluster of galaxies a billion light-years away called Abell 2029. Because of this it’s enjoyed the largesse of the cluster’s larder; it’s collided with a lot of other galaxies. This has made it grow large, but it’s also been puffed up; the way galaxies interact makes them swell in size for a while before settling down again.
Worse, IC 1101’s gravity has torn smaller galaxies apart as they merge, and its surrounded by all this debris. It’s hard to separate that from the glow of the cluster itself (it’s full of gas that adds to the light) so IC 1101 may be far smaller than generally claimed. It may still be bigger than Malin 1 and UGC 1382 though. We just don’t know.
And apropos of all is the final problem: What do you call the edge of the galaxy?
UGC 1382 has a disk with stars in it, and that fades away with distance from the center. But as I mentioned, it’s surrounded by a huge halo of gas. Do you count that? If you want to compare apples to apples, you need to be able to see if another galaxy has such a halo and that observation may be hard or even impossible.
So where does this leave us? What’s the biggest galaxy?
I think it’s a safe bet that IC 1101 is as far as we know at the moment, but with an asterisk due to not really being sure where it stops and the cluster environment begins. If it gets disqualified, then Malin 1 may edge out UGC 1382, but they’re so close it’s hard to be sure.
And of course, bigger ones may exist.
I’ve been pretty clear in the past that I’m not comfortable putting things into tightly regimented bins. Nature doesn’t, so why should we? There’s no real border between a planet and a brown dwarf, and even the line between brown dwarf and star is fuzzy. Would you say something the size of a beach ball orbiting the Sun between Mars and Jupiter is an asteroid? What about a baseball? A pea? A grain of dust?
Whenever you push boundaries, things get fuzzy. The same is true here. There may not really be a biggest galaxy. Instead, there may be the biggest galaxies, a porous container encapsulating specimens in which we needn’t be too concerned with individuals as far as records go.
Instead, we should study them to see how they tick, what made them so big, and how that might affect their own history and the evolution of other galaxies and the environment around them. That is a far more interesting task than picking out one and hanging a blue ribbon on it.