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SYFY WIRE Bad Astronomy

A galaxy with beautiful plumage

By Phil Plait

[Image credit: CFHT/Coelum]

Galaxies are amazing things. Huge, sprawling, and beautiful, they are vast collections of stars, gas, dust, and dark matter held together by their own gravity. They take on wonderful shapes, like spirals, ellipticals, rings, and more.

Some of them even have tails.

The galaxy above is NGC 4569, a spiral about 60 million light-years away. It’s a member of the huge Virgo Cluster, the nearest city of galaxies to us. Virgo has about a thousand galaxies in it, so it’s pretty beefy. Those galaxies all orbit the center of mass of the cluster, which means they sometimes pass very close to each other. When galaxies pass close to each other, the gravitational effects can distort their shapes and even strip them of the gas between their stars.

That gas becomes part of the intracluster medium, the thin soup of material between the galaxies themselves. This sets up another way for galaxies to lose gas: As they orbit the cluster, they pass through this gas. The pressure from that can push the gas out of the galaxy as well, in a process called ram pressure stripping.

That’s what’s happening to NGC 4569! The image is a pseudocolor* combination of observations from the Canada France Hawaii Telescope using three different filters: green (displayed as blue), red (displayed as green), and a filter called H-alpha that lets through a very narrow slice of color where warm hydrogen atoms emit strongly (displayed as red, and in this case it really is red). Stars and such show up as milky white, but the gas stripped from the galaxy appears as a fierce red.

It’s been known for some time that NGC 4569 is gas-poor in the galaxy itself, but now we know where it’s gone: into the cluster, to become part of the intracluster medium.

We’ve known about ram pressure stripping for decades. It’s quite common in clusters like Virgo and can create spectacular sights.

A new study just published found something very interesting, though: Gas can be stripped from galaxies even in much smaller collections than big clusters. Groups as small as a couple of dozen galaxies can manage it!

That surprised me when I read it. Gas would not be held terribly well in a group so small; the combined gravity of just a handful of galaxies wouldn’t be strong enough to allow them to keep the gas. So, what gives?

The researchers found that the likely culprit is dark matter. This stuff is still somewhat mysterious; we know it exists, but we don’t know exactly what it is. It’s most likely some odd flavor of subatomic particle we haven’t found, yet. The irony is that it outmasses “regular” matter (protons, electrons and so on that make up our everyday lives) by a factor of about five! It exists in haloes around galaxies, and in general makes up the majority of the mass of a galaxy.

...And, apparently, also a cluster, or even a smallish group. By carefully examining over 10,000 galaxies in different groups (ranging in distance of 250 to 700 million light years away from us), the scientists found that gas stripping of individual galaxies can happen even in small groups with just a few galaxies, and it appears to depend on how much dark matter is in the halo (it kicks in when there’s about a trillion times the Sun’s mass in the halo; that’s just a bit more than the mass of our Milky Way including its dark matter). The more there is, the faster it happens.

The question is how. It could be that groups with more massive dark matter haloes have more normal matter gas in them, so ram pressure stripping can happen. It’s also possible that the gravity from the galaxies and the dark matter can pull the gas out via tides. A third way is, as mentioned above, interactions between galaxies. This new research leans toward ram pressure stripping, but they can’t be sure.

This sort of result fascinates me. I would’ve thought gas stripping would only be in the realm of big clusters, but here we find that the Universe is more subtle than we first supposed, and allows smaller entities to have dominion over their members, too.

Oh, that cosmos of ours. It has a set of rules —physics— that it follows, and by displaying that subsequent behavior we can deduce those rules as well. We are the way the Universe knows itself, and I swear, it delights in doing so.

* Our eyes see color due to blue, green, and red receptors. “Natural” color images can be made by taking three separate images using filters and combining them, letting the red image be displayed as red and so on. You can mix and match colors, though; that won’t be how our eyes would see the object, but it sometimes has other advantages in helping us see structure. We refer to these kinds of images in lots of ways, but I rather like “pseudocolor”, because it lets you know the colors have been fiddled with.