Galactic tentacles of DOOM

Contributed by
Oct 7, 2008

Today is a good day for awesome images. Check out M86 and NGC 4438:

These two galaxies lie in the Virgo Cluster, a city of galaxies about 50 million light years away. Life is crowded in the cluster, and collisions between galaxies are common.

Now, that line sounds pretty mundane, but think about it: entire galaxies collide! They slam into each other at hundreds of kilometers per second! Billions of stars interact gravitationally! Gas clouds impact creating cosmic fireworks on an epic scale! AIIIIEEEE!!!!!!

Phew. OK. That's outta my system now.

So we see galaxy collisions all the time, but sometimes the evidence is weak. NGC 4438 is the galaxy on the left, and it's all twisty and distorted. M86 is a more normal looking elliptical. But looking at the gas content of M86 has indicated something is going on; it's heated up pretty well, and distorted. But it wasn't until now we could see why.

That image above is from a 4 meter telescope in Arizona. It has a camera that allows it to collect a lot of light over a big area of the sky. When a filter was used that isolates warm hydrogen gas, astronomers found these tendrils connecting the two galaxies. Those tentacles are the shrapnel of the impact, streamed out in the aftermath of the collision... and the galaxies are now 400,000 light years apart. That's four times the size of our Milky Way.

The sciencey part of this is that they looked for new stars being born in those filaments; that's common after collisions. However, there aren't any! The collision happened at such high speed that the gas got really hot, and couldn't condense to form stars. That has implications for the galaxies themselves. It's been something of a mystery as to why elliptical galaxies stopped forming stars early in their lives. It's thought the central supermassive black hole in the center of every galaxy plays a part; as the black hole feeds on matter it blows off a huge wind, blowing out the galaxy's gas and cutting off star formation.

But now we see that collisions may play a role as well, heating up the galaxy's gas and preventing it from making stars. It's hard to say how much each process contributes; early in a galaxy's life it hasn't had much time to collide with others, so maybe this becomes important later. And spirals have those black holes too, yet stars still form in them. Obviously, there's a lot of complicated stuff going on.

Either way, with these new detectors we can look deep into the goings-on of nearby galaxy clusters and learn more about them. And there's a lot to learn! I'll note that Virgo is the nearest big cluster, and M86 is visible easily in binoculars, yet here's something new and surprising to discover about it.

Oh, and that edge-on spiral in the lower right? That's NGC 4388, which was once thought to be associated somehow with M86. But the filaments seen emanating from it are really high velocity, so something else may be going on there as well.

In a couple of billion years, our Milky Way galaxy will collide with Andromeda in a pretty decently ginormous event. They'll pass through each other, but then fall back again. Over the course of about five billion more years, and a handful of passes, the two will merge into one massive galaxy. What will happen to us then? Will the Sun fall toward our own central black hole? Or will we get ejected from the galaxy, flung out into intergalactic space?

We don't know. But I know where you can find out more info... Chapter 8. Mwuhahahahahaha!

Image credit: Tomer Tal and Jeffrey Kenney/Yale University and NOAO/AURA/NSF

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