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Astronomy 162:
Introduction to Stars, Galaxies, & the Universe
Prof. Richard Pogge, MTWThF 9:30
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Lecture 28: Groups & Clusters of Galaxies
Readings: Ch 26, section 26-6
- 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:
- Groups: 3 to 30 bright galaxies
- Clusters: 30 to 300+ bright galaxies
- Sizes: 1-10 Mpc across
- Often contain many more dwarfs than bright galaxies.
- Total masses of 1012 to >1015 Msun
About 3000 clusters have been cataloged to date.
The Local Group
Group of 39 galaxies including the Milky Way and Andromeda:
- Size: ~1 Mpc
- 5 bright galaxies (M31, MW, M33, LMC and IC10)
- 3 Spirals (MW, M31, & M33)
- 22 Ellipticals (4 small Es & 18 dEs)
- 14 Irregulars of various sizes
- Total Mass ~5x1012 Msun
Virgo Cluster
Nearest sizable cluster to the Local Group
Relatively loose cluster, centered on two bright Ellipticals:
M87 & M84
Properties:
- Distance: ~18 Mpc
- Size: ~ 2 Mpc
- 2500 galaxies (mostly dwarfs)
- Mass: ~1014 Msun
Image of the Virgo Cluster (83k)
Credit: NOAO/AURA/NSF
Rich Clusters
Contain 1000's of galaxies:
- Extend for 5-10 Mpc
- Masses up to ~1015 Msun
- One or more giant Elliptical Galaxies at center.
- Ellipticals found near the center.
- Spirals found at the outskirts.
- 10-20% of their mass is in the form of very hot (107-8K)
intracluster gas seen only at X-ray wavelengths.
Image of the rich Coma Cluster (189k)
Credit: NOAO/AURA/NSF
Superclusters
Clusters of Clusters
Properties:
- Sizes up to 50 Mpc
- Masses of 1015 to 1016 Msun
- 90-95% empty space (voids)
- Often long and filamentary in shape
Superclusters are the largest coherent structures seen in the Universe.
Local Supercluster
Roughly centered on the Virgo Cluster
Properties:
- Size: ~20 Mpc
- Mass: ~1015 Msun
- only ~5% of the volume occupied by galaxies
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:
- Vast chains of superclusters
- Occupy ~10% of the Universe
Voids: Empty bubbles
- 25-50 Mpc in diameter
- 5-10x fewer galaxies than in superclusters
Map of the Local Universe
The "Great Wall"
Found in a large-scale galaxy survey.
Sheet of superclusters:
- 150 Mpc long
- 60 Mpc "high"
- 5 Mpc thick
Mass is ~2x1016 Msun
One of the largest structures known.
Implications
The existence of "Large Scale Structure" tells us something
about how galaxies formed.
- Large structures sculpted by gravity
- Concentrations of matter where galaxies form
(Representative) Unanswered Questions:
- Why do galaxies form only in particular places?
- How "empty" are the voids?
- Which formed first, galaxies or clusters?
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*.
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Updated: 2006 February 19
Copyright © Richard W. Pogge, All Rights Reserved.