A Monster Galaxy Hiding in Plain Sight

Not too far away in cosmic terms—about 250 million light-years—lies a monster. The thing is, we’ve known about it for a long time, but we didn’t know what it truly was because it was hiding its true nature from us.

It’s a galaxy, named UCG 1382. It was discovered decades ago, and in pictures taken using visible light telescopes it looks like a fairly typical elliptical galaxy. It’s relatively isolated, with just a handful of much smaller galaxies nearby, and doesn’t seem to be doing much. Astronomers didn’t ignore it, exactly, but there wasn’t much there to excite them, either.

That is, until a team of astronomers happened to look at ultraviolet images of it taken by the GALEX space telescope. What they saw was very surprising: a set of spiral arms extending well beyond the visible part of the galaxy! Right away that’s pretty dang weird; spiral arms are usually very bright in optical light because that’s where stars are born, including massive, luminous ones. But this also means the galaxy was larger than first thought. Much, much larger: The diameter across the arms is 500,000 light-years, five times the size of our home galaxy, the Milky Way!

Then it got even weirder: They investigated the galaxy with the Very Large Array, a set of radio telescopes in New Mexico. What they saw was nothing less than shocking. The radio telescopes detected hydrogen gas going out even further from the center, which means this galaxy is nearly 720,000 light-years across!

It’s not just huge. It’s a monster.

These new observations show that, far from being some uninteresting galaxy, UGC 1382 is actually one of the biggest known galaxies in the Universe.

It’s in a class known as Giant Low Surface Brightness, or GLSB, galaxies. Only about a dozen are known, including the previously thought biggest, Malin 1, which is about the same size as UGN 1382. The thing is, UGC 1382 is much closer than Malin 1 (250 million versus 1.2 billion light-years), making it far easier to study.

And when the astronomers studied it, it kept getting weirder. For one thing, it doesn’t have nearly as many stars as you’d think for such a huge beast. Its total mass of stars is about 80 billion times the mass of the Sun, which is much less than the Milky Way! Even adding in all the gas in the galaxy, its total mass is about 100 billion solar masses, 1/10^th of the Milky Way’s mass. That explains why it’s so faint; there isn’t a lot of there there, and it’s spread out over a lot of real estate.

It also has a huge halo of dark matter, the unseen and still mysterious substance that outnumbers normal matter (the stuff we’re made of, like protons, electrons, and neutrons) by about 5 to 1 in the Universe. The galaxy has about 2 trillion times the Sun’s mass of dark matter, which means UGC 1382 has a dark matter to normal matter ratio of 20:1. That’s really high.

And there’s one more weirdness: The outer parts of the galaxy (the gigantic disk with the arms in it) appears to be older than the inner part. That’s the opposite of “normal” spiral galaxies; in general the inner part is made of very old stars, because star formation there shut down early in the life of the galaxy, while it continues on in the spiral arms. Apparently UGC 1382 didn’t get that memo.

So how does a GLSB galaxy like UGC 1382 form? Galaxies like ours got big by eating other galaxies. Smaller ones get caught by bigger galaxies’ gravity, get torn apart, and fall into them. Sometimes you get collisions between bigger galaxies, too; we’ll be colliding with the massive Andromeda galaxy in a few billion years, forming one huger galaxy with twice the mass.

But that isn’t the case with UGC 1382. A collision like that would disrupt the spiral disk. But that’s the oldest part of the galaxy, so it’s unlikely it would still be around after the giant train wreck of a galactic collision.

What the astronomers posit is that it grew slowly by snacking gently on much smaller gas-rich dwarf galaxies. The events wouldn’t be so violent, so it would be able to maintain its disk (the massive dark matter halo is also helpful in maintaining the disk structure; it acts as a stabilizing influence on it). If the dwarf galaxies were old, that would explain the inside-out age problem, too. Those smaller galaxies would likely be pulled apart and distributed in the outskirts of UGC 1382, making it look older than the downtown region.

Also, remember that UGC 1382 is isolated, with no big galaxies nearby, so it’s unlikely to have big galaxies to collide with in the first place. In fact, all GLSB galaxies appear to be relatively isolated, so this is apt to be a key characteristic for them.

This is all a bit staggering. These monster galaxies weren’t even discovered until relatively recently, despite their ridiculous size. The problem is that the biggest parts of them, their disks, are very faint, making them hard to see. UGC 1382 shows us that many more could be hiding in plain sight, right in front of us.

It’s ironic that the biggest known single objects in the Universe are so hard to find, or at least so hard to identify. The cosmos is usually pretty good about letting us understand it, showing us a dramatic array of objects that all obey the laws of physics, allowing us to tease out information.

But no one ever said it would make it easy on us.