Bad Astronomy

Beautiful but mysterious galaxies litter the sky

How many galaxies can you handle today?

I hope it’s a lot. Because I have three to show you, or maybe 5, depending on how you count them.

First off, a gorgeous trio of interacting galaxies courtesy the Hubble Space Telescope. This group was the winner of the contest to have people vote on where to point Hubble:

Click this and the other pictures to get to embiggenated ones.

This group of three galaxies together are called Arp 274; Halton Arp cataloged quite a few oddly shaped galaxies, suspecting that they were more interesting than you might suppose at first glance. He was right; almost all the entries in his catalogue are galaxies interacting with one another, their mutual gravity distorting their shapes, their massive collisions triggering vast bursts of star birth across hundreds of thousands of light years of space.

It’s not clear what’s going on in this trio, located 400 million light years away. The two galaxies on the right and left are clearly undergoing massive star bursts, which can be seen as the large blotches of blue (when lots of stars are born, some are very massive and very luminous ones; these shine blue… and while they’re vastly outnumbered by lower mass redder stars, the dinkier stars shine more feebly and are harder to see).

The middle galaxy, however, appears relatively undisturbed. Oddly, it also seems to be at a different distance from the other two, as measured using its redshift (how fast it’s apparently moving away from us). Is it possible that the middle galaxy is merely superposed on the other two, and it’s just that pair that’s interacted sometime in the past? Or does the middle galaxy have an anomalous velocity for some unknown reason? Poking around the journal papers, I was unable to find an answer. That in itself is interesting!

The next two galaxies are from the European Southern Observatory. The first of these two is also a spiral, but a bit of a mess: NGC 7793, 12.5 million light years distant.

This photo, taken with the monster 8-meter Very Large Telescope, shows off the chaotic nature of this spiral. Most big spirals have well-formed spiral arms, but this one is a disaster. Why? Interestingly, it has a lot of gas in it, one of the largest supplies of hydrogen known in any normal galaxy. It’s also forming lots of very massive stars, so many that the gas is largely ionized (that is, stripped of electrons by the ultraviolet light from the stars) throughout the whole galaxy. Does this affect how the spiral arms form? It’s hard to say, but when you have two weird things in one place, it’s tempting to correlate them. It’s also not a terribly big spiral, less than half the diameter of the Milky Way. Does that play in to this as well? Again, it’s hard to say.

The last galaxy is NGC 55, an irregular, elongated galaxy. It’s flattened and we see it roughly edge-on:

This image was taken by the 2.2-meter MPG/ESO telescope in La Silla, Chile. NGC 55 is pretty close by, only 7.5 million light year away, maybe three times as far as the Andromeda Galaxy. It’s a bit of a mess, but its proximity gives us a great look at it. In this image, individual stars can be seen! We can also pick out planetary nebulae; giant shells of expanding gas blown off by dying stars. The properties of these nebulae can be used to figure out how far away they are from us, which gives us the distance of the galaxy itself. It’s an irony of nature that it’s easier to figure out the distance to remote galaxies, which are swept away from us by the expansion of the Universe itself, than it is for nearby galaxies, which are too close to use the cosmic expansion for distance.

NGC 55 is a bit like the Large Magellanic Cloud, an irregularly-shaped galaxy that orbits the Milky Way; both are oddly shaped, both are somewhat flattened. But NGC 55 is hugely larger than the LMC, perhaps five times as big. We’re not sure why some galaxies are irregular, and why some of these weirdos do have some shaping involved, like why being flattened.

Galaxies are huge, beautiful, intensely studied, and yet they still hold many mysteries for us to investigate. Even ones that are nearby – if you call quintillions of kilometers “close”! – and big in our telescopes still hold their secrets. But it’s images like these that will help us understand them better, and slowly unravel the tapestry of how these gorgeous objects behave.