masses, and there are fewer of the most massive galaxies and many more of the dwarfgalaxies.The galaxies of the Local Group are distributed within a volume of about 1Megaparsec (Mpc) in radius. The volume of a sphere with a radius of 1 Mpc is about 4cubic Mpc. This means that there are about 10 galaxies per cubic Mpc in the Local Group.If you move outward from the Local Group, you encounter several other groups that aresuperficially similar to our own. M81 and M82 are two of the largest members of theirgroup about 4 Mpc away from us, which also includes a total of about two dozen knowngalaxies. Centaurus A is a member of the M83 group, which is about 5 Mpc away. TheLocal Group, the M81 group, and the M83 group are examples of what are generally called“loose groups,” which are just what they sound like: bound groups of galaxies that are notvery densely packed together.Astronomy for Teachers
2Unit 5: Galaxy InteractionsRemember from our lesson on stars in the Milky Way, that, on average, inside of agalaxy, the stars are separated by vast distances. However, on average, in the universe,galaxies are only separated by about 20 times the size of the diameter of the averagegalaxy. In all galaxy environments, we see examples of galaxies apparently in the processof interacting with each other. In fact, if you look back closely at all the images of galaxiesthat you have seen so far (the Hubble Deep Field, images of the Virgo Cluster, etc.), I amsure that you can pick out examples of galaxies that appear to be right on top of eachother.So, what happens when galaxies do encounter each other? There are a fewquantities that control the interaction:The mass ratio of the galaxies: Are their masses nearly equal, or is one muchmore massive than the other?The relative velocity of the galaxies: Are they zooming past each other, or arethey slowly passing by each other?How close is it to a direct hit?: Are the galaxies going to pass right througheach other, or just pass close to each other?In general, there are three different types of interactions that fall into the followingrough categories:Major mergersWhen two nearly equal mass galaxies with small relative velocities pass very closeto each other, or even right through each other, a major merger occurs. If the galaxies aremoving too fast relative to each other, a major merger is unlikely to occur, because thecollision will not slow the galaxies down enough. For this reason, major mergers do notoften occur in the cores of galaxy clusters. Instead, we find that they occur in loose groups(e.g., the Milky Way and Andromeda appear to be on a path to collide several billion yearsfrom now), compact groups, and in the lower density outskirts of galaxy clusters.If you look at images of the most well-known example, NGC 4038 and NGC 4039—the Antennae Galaxies—you will get a sense for what happens in one of these collisions.
Upload your study docs or become a
Course Hero member to access this document
Upload your study docs or become a
Course Hero member to access this document
End of preview. Want to read all 16 pages?
Upload your study docs or become a
Course Hero member to access this document
Term
Summer
Professor
Charl Waminal
Tags
