Another Hubble stunner

Bad Astronomy
The entire universe in blog form
Feb. 6 2007 3:14 PM

Another Hubble stunner

Astronomers at Space Telescope Science Institute just released a new image, and it's a beauty.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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What you're looking at is galaxy cluster Abell S0740. If galaxies are island universes, then clusters are archipelagos. Some are small, like this one, but others have hundreds or thousands of galaxies in them. In many of them, there is a central elliptical galaxy which dominates the cluster, and this cluster is no exception. The central galaxy grows through cannibalism, eating other galaxies and assimilating them. Sometimes the evidence for this is that the central galaxy has multiple cores -- think of them as the undigested remains of its conquests -- and sometimes the galaxy gets "puffed up" as the energy of the consumption is transferred to the stars in the galaxy. The central galaxy in this image has the look of the latter to me, but I'm not an expert. By the way, most of the little pinpricks of light you see surrounding the central galaxy are globular clusters. Cool.

The other galaxies in this cluster are a mess. This is expected. Imagine taking a hive of bees and compressing it. As the bees get jammed together, they get disturbed, right? Well, the same thing, kinda, is happening here. When you jam a few dozen galaxies into a small volume, their gravity starts to make a hash of things. They pass close by each other, disturbing their shapes. You get outright collisions as well.

That spiral above doesn't look too happy to me. I think a few passing galaxies have distorted its shape. You can see blue regions in it as well, which are locations of star formation. This can get triggered when passing galaxies gravitationally poke gas clouds in a neighboring galaxy, slamming them together, which in turn makes them collapse and form stars.

Now look at this one:

This is what's called a polar ring galaxy. If a small galaxy ploughs right through the center of a bigger galaxy, the bigger one can form a massive ring of stars surrounding a cylindrical mass in the center, like an axle on a wheel. We're seeing this one nearly edge-on, and it looks a lot like a normal spiral galaxy seen edge-on. But if you look carefully, along the disk you can see a dark gap, which indicates this is really not a disk seen edge on, but a ring.

Then there's this thing:

I have never seen a galaxy like this in my life. The press release says it's another ring galaxy seen edge on, but man. That's really weird.

I think what's most interesting about this cluster is that according to the release, the central galaxy has a mass of about 100 billion times the Sun. But that's about the same mass as our own Milky Way Galaxy! So in fact the galaxies in this cluster aren't all that big. And I'm wondering just how many of the galaxies you see in the image really belong to the cluster; some of those much smaller spirals may be part of a different cluster farther away. Or I might be totally wrong, and they might just be dinky galaxies. It looks like I'll have to find a paper on this cluster and do some reading. I love it when images from Hubble make me think, and I love it more when they make me rethink things I've already thought about!

Oh-- don't forget, I have more info on galaxies in my first ever episode of "Q & BA". And keep sending me questions!

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