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Galaxy NGC4414 from HST Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30

Lecture 28: Groups & Clusters of Galaxies

Readings: Ch 26, section 26-6

Key Ideas

Galaxies tend to gather into Groups and Clusters

The Milky Way is part of the Local Group

Hierarchy of Structure:
Groups: 3 to 30 bright galaxies
Clusters: 30 to 300+ bright galaxies
Superclusters: Clusters of Clusters
Voids, Filaments, & Walls

Groups & Clusters of Galaxies

Most galaxies are found in groups and clusters

Basic Properties:

About 3000 clusters have been cataloged to date.


The Local Group

Group of 39 galaxies including the Milky Way and Andromeda:

Virgo Cluster

Nearest sizable cluster to the Local Group

Relatively loose cluster, centered on two bright Ellipticals: M87 & M84

Properties:

Image of the Virgo Cluster (83k) Credit: NOAO/AURA/NSF


Rich Clusters

Contain 1000's of galaxies: Image of the rich Coma Cluster (189k) Credit: NOAO/AURA/NSF


Superclusters

Clusters of Clusters

Properties:

Superclusters are the largest coherent structures seen in the Universe.

Local Supercluster

Roughly centered on the Virgo Cluster

Properties:

The Local Group is on the outskirts of the Local Supercluster, and falling into the Virgo Cluster at about 250 km/sec.


Voids, Filaments & Walls

The Universe looks foamy on large scales

Filaments:

Voids: Empty bubbles

Map of the Local Universe

The "Great Wall"

Found in a large-scale galaxy survey.

Sheet of superclusters:

Mass is ~2x1016 Msun

One of the largest structures known.


Implications

The existence of "Large Scale Structure" tells us something about how galaxies formed.

(Representative) Unanswered Questions:


Supplement: The Sloan Flyout Movie

In class, I showed a wonderful "fly-out" movie that used the Sloan Digital Sky Survey data. The movie was prepared by the Cosmus Project at the University of Chicago. The specific animation is available on this webpage:
Downloads from the SDSS/WMAP Universe
And look for the link under "Movies" labeled "Sloan Galaxies only". This is the 30Mb "straight flyout" movie I showed in class.

There are lots of other cool movies on the Cosmus site, but beware: the files are *big*.


Return to [ Unit 4 Index | Astronomy 162 Main Page ]
Updated: 2006 February 19
Copyright Richard W. Pogge, All Rights Reserved.