MMSS11/Astronomy

The Michigan Math and Science Scholars 2012 ASTRONOMY: Climbing the Distance Ladder: How Astronomers Survey the Universe EXERCISE: The Local Group

Let's explore the structure of the Local Group. The Local Group is a sparse group of more than forty galaxies that significantly influence each other through their mutual gravitational pull, and which in turn is "falling" towards the vastly more massive Virgo cluster.

A very striking feature of the Local Group is how difficult it is to work out how many members it has, and to learn in detail about each one. Why?

We have constructed a 3D model of the Local Group using the best data available to date -- but keep in mind that the distances to many objects are very uncertain, and so the relations between objects depicted here are tentative. Start MATLAB and add the path to the data by typing at the command line:
addpath('YOUR-PATH')
Activate a script that will help you explore the data by typing
lg3dmap
Once all the data have been displayed, you might want to increase the window size by dragging the lower right corner. Use the mouse to rotate the image, by placing it at (0,0) and dragging towards the upper left. You should now be looking into a 3D volume with axes running from -5000 to 5000 along the lower left, and -2000 to 4000 along the lower right. The axes are marked in kilo-light years -- this volume is millions of light years on a side!

Each point is a galaxy. Are they randomly distributed, or do you see some structure? Click on the "Zoom In" button on the control bar and click repeatedly on a point near to the center of the lower left cluster. Be careful to keep the mouse close to the point on which you want to zoom as you click. After about six zoom steps you will be able to clearly see structure. Galaxies are color-coded as follows: spirals (blue), ellipticals (red), irregulars (green), dwarfs (cyan) and miscellaneous (magenta; there is only one!). All galaxies are shown as simple spheres (although the sizes are about right) except the two massive spirals in the Local Group -- The Milky Way and M 31. Your field should show the Milky Way. Click on the "Zoom Out" button, click to restore your original view, and now zoom in on the cluster to the upper right. Wait! What do you expect to see? So why are there two distinct regions in the Local Group?

Zoom back in on the Milky Way region, but now keep going. You will eventually see a dark point on the disk; zoom in on that. It's the Sun -- about 25,000 light years from the Galactic Center, and slightly out of our Galaxy's central plane. Slowly zoom back out. The galaxy on the far side of our disk is the Sagittarius dwarf. Why was that so difficult to discover and observe? Check out this galaxy at the SEDS Local Group page. By the time you have zoomed out to encompass about a dozen galaxies, you will see that most are dwarfs, but that two spirals (blue) lie close to us. These are the Large and Small Magellanic Clouds, often classified not as spirals but as irregulars. Check out these galaxies at the SEDS Local Group page.

You can hardly fail to notice a stream of gas -- represented by a scattering of blue points -- associated with the LMC and SMC. This is the Magellanic Stream. In fact, we have taken the liberty of putting it in a certain position -- we don't have a very good idea of where it actually is, because we don't know the distance to it.

Why?
It has been suggested that this is due to gas, ram-stripped from the LMC/SMC; can you infer from that term, what the physical scenerio is?
Do any of the features that you encountered in the interacting galaxy exercise seem plausible explanations of this feature? This is now the favored model, strongly supported by the recent discovery of the so-called 'Leading Arm'. We think there was a close encounter 500 million years ago that pulled gas into a bridge between the LMC and SMC; the stream originates partly in this bridge, partly in the SMC.
Why the SMC, not the LMC?
The stream ends far from the LMC and SMC. How might that position relate to where the close encounter occurred?
Will the gas in the stream necessarily continue to move forward at the orbital speed of the LMC and SMC? If not, will the position of the stream change?

Zooming and rotating as necessary, explore the Local Group. Make sure you understand the relationship between the Milky Way and M 31 and their close neighbors. This will require skill and practice! It's hopeless trying rotate once you have zoomed far in -- you will lose track of where you are. Zoom in just far enough to see some detail, rotate, zoom in to explore, and then zoom back out before rotating again. In particular, try viewing the environs of the Milky Way and M 31 both from "above" edge on. Refer to the SEDS Local Group page. How much can you learn about the different members of the Local Group?

In the distant future, motions will cause an interaction between M 31 and the Milky Way. M31 is moving towards the Milky Way at about 120 km/s. We expect a collision in about 7 billion years. Here's what might happen: Duck! A Galaxy Is Coming!

Find the distance to M31 -- do the above numbers make sense?
Why is it uncertain just how 'head-on' the collision will be?