The galaxy that ate Detroit

Bad Astronomy
By Phil Plait
Feb 5, 2008 7:46 PMNov 5, 2019 6:55 AM


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Actually, it's The Galaxy That Ate A Whole Lot Of Other Galaxies... And Got Gas. Check it, babies:

This new Hubble picture release (American version) of the elliptical galaxy NGC 1132 has some way cool stuff going on. Usually, when I see a new Hubble image, I like to look at it for a few minutes and see what I can see before reading the actual text. I do this to see what I can figure out on my own, but also so that I don't get prejudiced by the press release. The PRs are generally very good, but if I read it first I may bias myself against seeing something that I would otherwise miss. In this case, I was pretty close. My first thought was, big deal, an elliptical galaxy. They are very pretty and very interesting, but generally not worth a press release. But then I spotted something odd: the core is very bright. Hmmm, is it an active galaxy? Maybe the supermassive black hole in the core of NGC 1132 is furiously gobbling down matter, and spewing out vast amounts of energy in the process. Then I looked at the Chandra X-ray image (superposed on the visible light image taken by Hubble):

Hmmm, the core isn't that bright. Active galaxies, as a rule, have tons of X-rays pouring out of the center, and the core would be the brightest thing in the X-ray image. So NGC 1132 isn't active, or is at best mildly active. Then I noticed the weird distribution of X-rays. X-rays usually come from very hot gas in galaxies, and ellipticals tend not to have much gas in them. Also, most ellipticals, if they emit X-rays at all, tend to have a nice symmetric distribution of them. It would look like a smooth cloud of light surrounding the center. That's not the case here! The gas is lumpy, and there's a lot more "above" the core than "below". Hmmm... Aha! NGC 1132, I says to myself, says I, is a giant elliptical. It has been busily eating other galaxies, building up its mass. Now it's huge, and the asymmetric distribution of X-rays indicates how the hot gas inside has been roiled up by collision after collision of smaller galaxies. Bingo! From the press release:

In visible light NGC 1132 appears as a single, isolated, giant elliptical galaxy, but this is only the tip of the iceberg. Scientists have found that NGC 1132 resides in an enormous halo of dark matter, comparable to the amount of dark matter usually found in an entire group of tens or hundreds of galaxies. It also has a strong X-ray glow from an abundant amount of hot gas – an amount normally only found in galaxy groups. Its X-ray glow extends over a region of space ten times larger than the 120,000 light-year radius it has in visible light. An X-ray glow that is equal in size to that of an entire group of galaxies.

Kewllllll. Galaxies tend not to live alone: they usually travel in groups. The Milky Way, for example, lives in a smallish cluster called the Local Group, which has us, the Andromeda galaxy, the Triangulum galaxy, and a few dozen smaller galaxies (what my Mom would call pitzulahs, "little things"). When you travel in packs, collisions are likely, and over time a medium size galaxy can swallow lots of little ones to become a Monster Galaxy. NGC 1132 is such a monster. Like I said before, ellipticals tend not to have much hot gas, but NGC 1132 is lousy with it. It got all that matter by eating smaller gas-rich galaxies. As they spiral in to the galaxy's center, their gas is stripped away where it becomes part of the bigger galaxy. Furthermore, the gas gets stirred up, which heats it enormously. Eventually, the smaller galaxies fall to the core and are fully absorbed, building up the mass of the gourmand galaxy's heart (which is why it looks bright in the visible light Hubble image). NGC 1132 is also quite huge, bigger than the Milky Way (which has eaten its share of smaller galaxies over time too), and sits in a vast cloud of dark matter - normally invisible, but in this case revealing itself by its gravitational effect on the stars in the elliptical itself (actually, to be honest, I'm guessing how they found it: there are many ways to detect dark matter, but the venerable method is to measure the velocities of the stars in the galaxy, which are affected by dark matter; stars move faster when more dark matter is present in a halo around the galaxy). The amount of dark matter detected is way more than usually seen for a single galaxy; more evidence that this galaxy has been busily gobbling up companions -- and their dark matter, too. The final nail: the galaxy is surrounded by zillions of globular clusters. These are collections of stars, sometimes with a million or more stars in them, packed into a tight ball shape sometimes only a few light years across. Giant ellipticals tend to have lots of globulars, which survive the acts of galactic cannibalism, changing allegiance from their original galaxy to the bigger one (globulars are pretty but fickle). The Milky Way has over 100 globulars around it, another indication that we too are the big guy on the block, but at the cost of dozens of other galaxies' existence. It's amazing to me that just by critically examining one or two images of a galaxy we can see so much; so much of the galaxy's present, so much of its past. And we can be confident of our interpretation! It just takes practice... and of course centuries of slow but solid scientific progress to bring us to the point where we can understand the Universe we live in. And appreciate its beauty too. NGC 1132 is pretty.

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