Loner galaxies like the Milky Way are a bit unusual. We live out in the suburbs of space, but most galaxies prefer the hustle and bustle of the urban environment: the galaxy cluster. Clusters can have dozens of galaxies, but some are huge and have thousands of island universes swarming around inside them. We’re loosely affiliated with a cluster about 60 million light years away, the Virgo Cluster, but it’s only a middlin’-sized town.
The Coma Cluster is a metropolis.
Coma is about 300 million light years away or so, and has well over 1000 galaxies. It’s far enough away that you need a decent telescope to see even the brightest members, but when you point something like Hubble at it, well, you get the picture above, which was just released (the European version can be found here). That picture is only a section of the cluster, but you can still see hundreds of galaxies in it.
It’s worth taking a look at a few places up close. Check out this beauty:
That’s obviously a spiral galaxy, or something close to it. But it lacks the gorgeous spiral arms like we’re used to. Spiral arms are like traffic jams in a galaxy’s disk. When clouds slam into the jam, they collapse and form stars; the high-mass stars are very bright and light up the gas around them. This makes the spiral arms very obvious. But spirals with bright, clear arms are scarce in clusters. Why?
The galaxies in a cluster move around sort of like bees around a hive, each circling the center on their own orbit. The orbital speeds can be pretty fast due to the high mass and therefore strong gravity of the cluster. There is gas in between the galaxies, and they ram through this gas while they orbit the cluster. Any gas inside the galaxy is stripped away by this gas outside the galaxy, like the way you can blow a smell out of a car by opening the windows and letting outside air blow in.
So this galaxy is not really a spiral, it’s just a disk galaxy, or what astronomers call a lenticular (lens-shaped) galaxy. You see them all the time in clusters because their gas is stripped away, making the spiral arms harder to see. If you look closely you can just barely make out a spiral pattern in it, but that’s about it. The ring pattern halfway out from the center is odd, and to be honest I’m not sure I understand it. Some spirals do have rings (like M95) but their original is unclear. This one is pretty distinct, and it doesn’t look like it’s affiliated with any other structure in this image (the object at about 2 o’clock on the ring is probably a background galaxy). Interesting!
And you should expect interesting galaxies when you look in a dense cluster. To prove this point, check out this menagerie:
What a mess! You can see lenticulars, ellipticals, and a host of other weird shapes. I suspect a lot of the smaller galaxies (but not all) are background galaxies; much farther away than 300 million light years. But a lot of the galaxies you see here live in the Coma cluster. That flat, edge-on one at the lower right is particularly cool. It looks like it may have suffered some sort of collision a few hundred million years ago; the outer parts are puffed up, like a pizza crust. That sometimes happens when a big galaxy eats a smaller one. I’m guessing: I’m not sure, but it seems like a good bet.
Images like this are fun! It can be a lot of fun to grab a big version (6000×4260, 33Mb), or, if you can handle it, the monster full-size image (10816 x 7679, and 125 Mb! Holy Haleakala!) and just tool around. You can find all sorts of interesting things if you look carefully. How many stars do you see? How many small red dots, which may be galaxies billions of light years distant? How many are distorted from collisions? How many are bright blue, indicating they are bursting with star formation? Do you spy any gravitational lenses, arcs of light which are the distorted views of more distant galaxies? I found at least one in this image. I bet there are more!
Images like this are a treasure trove. Enjoy digging.