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:

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 M_sun

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 M_sun

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 M_sun

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 M_sun
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 (10^7-8K) 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 M_sun
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 M_sun
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 M_sun

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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