Galactic Taffy Pulling

Galactic Tidal Interactions Lab

In this lab, you will have a chance to play with some galactic tidal interactions. Given the number of galaxies in the universe, and the fact that many come in clusters, we have to expect that we will sometimes see the galaxies interacting closely.

Indeed, this is exactly what we do see. Despite what may be your first guess about this, interacting galaxies, even when the actually "collide", don't actually make a lot of physical contact. Thus, the stars seldom bump into each other (the is a lot of empty space in galaxies). What does happen, however, is that he galaxies get distorted. These distortions are tidal effects, which you have studied.

First off, let's see some galaxies that have interacted. Some of the loveliest galaxies in the universe were shaped mainly in this manner. Lucky for us, Hubble has taken some really slick shots of some very pretty galaxies, so we can see lots of these.

In the Beginning

To start with, we'll take a look at some of the first tidally disrupted galaxies known. As such, we have to start with The Antennae (while you are here, read the captions and such). These were the first tidally disrupted galaxies known (or, rather, the first we knew were messed up somehow). While we didn't know what was happening until relatively recently, we could tell something was up, here. Take a look and you'll see why.

By the same token, The Mice are also a good, early example. Again, we didn't really understand what was up here right off, but clearly something is happening.

For more peculiar galaxies, you can go to An Arp catalogue and go click-crazy!

Back to Our Story

Astronomers were seeing these kinds of things in galaxies, and knew something was up. At first, there were few enough examples that the astronomers were able to write them off (read: sweep 'em under the rug) as "peculiar" galaxies. But more and more started popping up the more closely they looked around. Since they almost always seemed to occur in situations where galaxies were paired up, it seemed logical that some kind of interaction was occuring. Tidal forces were an obvious suspect, but there were problems.

One such problem is that we saw more tails than bridges (tails spew from the far sides of the interacting pairs, while bridges connect them). The forces should be greater in the middle, so why don't we see more structure there (you may want to record your ideas here)?

Time for Some Answers!

In this case, the breakthrough was not analytical, it was in computing. Modeling these kinds of interactions without computers would be nearly impossible, of course. But with the advent of computing, astrophysicists (and even some non-physics people!) were able to set up models of galaxies interacting. Prominent among these were the Toomre brothers, who have written an excellent article in Sci. Am.

I have prepared a Mathematica (isn't it cool when that doesn't italicize?) Notebook for you to try this yourself. Read over my comments and see if you can understand the set-up. It should be fairly familiar to you.

By keeping the number of stars small, the code should run in a reasonable time. Try executing the code, and see what you can get. Try some different initial parameters to see what happens. Record your findings and observations.

If you are so inclined, I have a few notebooks in which I ran the simulation for days at a time to get more accurate, nicer results. Ask me to show them to you, if you want to see...

Analysis

Don't be worried if you didn't get these results. These took the Toomre brothers a lot of work to see. What the noticed, though, was that the bridges don't appear often because the tidal forces are so great in between the galaxies that the are torn apart so quickly that we seldom are looking at the right time. The tails, on the other hand, are less subject to the forces, and are able to last much longer.

Hopefully, you got some nice, spiral arm structures going. The Toomre brothers found that when the galaxies passed with partial contact and the interlopping galaxy was moving in the same direction as the target galaxy was rotating, they got nice spiral arms. This appears to be one source or spiral arm structure, although it does not seem to explain all such.

To see some nice spiral armed galaxies, try The Whirlpool Galaxy (my personal favorite). Notice the small companion? On over-exposed images, the tidal bridge shows up very clearly, by the way.

Going 3D

So far, we have pretty much worked in 2 dimensions. What would happen in the extreme other case, where the interloping nucleus passed through the target galaxy perpendicular to the plane of the target galaxy?

To investigate, open FaceOnTidal1.nb. This is my first approximation to the face on collision. I assumed that the stars from the first galaxy would stay in the plane of the galaxy (not true, of course, by I reserve the right to approximate as far as I see fit :). Toy with this for a bit and notice what kind of structures you see.

For a more realistic approach, try FaceOnTidal2.nb. This still neglects the interactions between the nuclei, unfortunately. But it is in 3D, which is always cool. Give it a go, and record your findings.

If you want to see a good example of this kind of interaction, hop to STScI, and see The Cartwheel Galaxy. Read the caption that goes with the picture, since it explains this much better than I can ever manage.

Wrapping it All Up

Well, I hope you've gotten a feel for the tidal interactions between galaxies. If you'd like to explore more, feel free. As I mentioned, I have a few notebooks that I ran for long periods of time (5 days; free computing!). Also, if you ask your lab asst. or Cindy, we'll be happy to talk more about them.

This web page was written by The Cheshire Cat, known on the Internet as The Cheshire Cat (occasionally John Weiss). It was last updated on 12 December 1997. Any typos, errors, or outright lies should bemailed to me. Thanks!