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SYFY WIRE Bad Astronomy

I’m a Loner, Dottie. A Dwarf Galaxy.

By Phil Plait

Poor UGC 4879. Doomed to wander the Universe forever, alone.

Or is it? Dun dun dunnnn.

UGC 4879 is a dwarf galaxy, a small collection of stars, gas, and dust located about 4.5 million light-years away from us. The Milky Way, our galaxy, is part of a small clutch of galaxies called the Local Group, which is a few million light-years across. UGC 4879 is probably located just outside that group, far enough away to be on its own.

In fact, the nearest galaxy to UGC 4879 is another dinky thing called Leo A, and even that’s more than 2 million light-years distant from it. Those other galaxies you see in the image are much much farther away; hundreds of millions or even billion of light-years past it. It really is pretty isolated.

But that’s good! For astronomers, anyway. We love nearby galaxies because if they’re close we see them well. We can see individual stars in them, measure their gas clouds, and do all sorts of studies on them that help us understand galaxies as a whole.

The problem is, most of the nearby galaxies are near other galaxies, and over the past few billion years have interacted with them. Close passes gravitationally distort the galaxies, sending gas clouds careening into each other and triggering bursts of star formation. Also, small galaxies passing through the thin gas in the Local Group get their internal gas stripped from them (my favorite analogy to this: It’s like opening a car window when a passenger inside befouls the air in one way or another, and the wind as you drive blows the gas out). The point is, galaxies being near other galaxies changes them.

But UGC 4879 is a loner. As far as we can tell, it has never interacted with any of the galaxies in the Local Group, and so in many ways it’s a pristine example of what happens when you let a galaxy form and just kinda … sit there.

What you’d expect is that it probably formed a bunch of stars right after the galaxy itself formed, and either hasn’t done much since, or just steadily produced stars at a low rate.

But that’s not what UGC 4879 did.

Using the Hubble Space Telescope to observe it (as shown in the image at the top of this post), astronomers looked at the colors of the stars, which can give you an idea of their age. Looking at the star formation history, they found that indeed a bunch of stars were formed around the same time as the galaxy itself, and then nothing happened for about 9 billion years (most of the age of the Universe). The stars left over from that initial burst of fecundity are now red, which you can see in the Hubble image.

But then something odd happened. Between 500 million and a billion years ago, the galaxy underwent a brief surge of star birth (in the Hubble image, those stars appear blue). But why? No nearby galaxies came close enough to trigger star formation, so that’s out. What could have caused this?

Perhaps you noticed, looking at the image, that the upper part of the galaxy looks flat, especially compared with the lower part. It almost looks like it ran into something and got squished.

Maybe it did. We know there are nearly invisible but huge clouds of very old gas out between the galaxies, called primordial gas clouds. They’re almost entirely made of hydrogen and helium. If UGC 3879 bumped into one, the cloud would have mixed with native gas in the galaxy and formed stars. If you look carefully, you may see that the blue, younger stars are not centered in the galaxy; they seem to be huddled closer to the flat edge, just like you’d expect if they formed there. Hmmmmm.

This isn’t conclusive. But it’s interesting. If true, then UGC 4879 isn’t as lonely as we thought, or at least it had a brief tryst an eon or so ago with a single gas cloud. Then they were fruitful, and multiplied.

That’s a nice story, isn’t it? And there’s still some gas in the galaxy. Not much, but who knows. Maybe some day it’ll meet the right gas cloud, they’ll merge, and once again its stellar family will swell.

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