1. Clusters and Superclusters
Galaxies are not distributed randomly through space
Concentrated in groups called Clusters
Clusters range from about 1013 MSun to about 1015 MSun
Range in size from 2 Mpc to 10 Mpc across
Clusters are categorized based on how many bright galaxies they have
Bright is vague, but roughly brighter than 10% of the Milky Way
The two types of clusters are rich and poor
Rich – 50+ bright galaxies, (about 1013 MSun)
Poor - <50 bright galaxies, (about 1014 MSun or 1015 MSun)
Sometimes these categories are called “groups” and “clusters” insteasd
Group = Poor cluster
Cluster = Rich cluster

2. The Local Group – our Cluster
It has about 50 total galaxies
Three spiral galaxies – two of them large
Milky Way Galaxy – a SBb or SBc
Andromeda Galaxy (M31) – SBb
Triangulum galaxy (M33) – Sc or Sd
Several small satellites of these galaxies
Several miscellaneous galaxies
Total diameter about 2 Mpc

3. Our Companions Sagittarius Dwarf Elliptical Galaxy Leo I
Canis Major Dwarf
Larger Magellenic Cloud
Ursa Minor Dwarf
Leo II
Sculptor Dwarf
Sextans Dwarf
Smaller Magellenic Cloud
Draco Dwarf
Carina Dwarf

4. The Great Galaxy in Andromeda (M31)
Large Spiral Galaxy - or probably barred spiral (SBb)
About 50% brighter than our galaxy
About the same mass as our galaxy
780 kpc away
Two black holes in the center (!)
Moving towards us at 300 km/s
Will merge with us in 2.5 Gy?

5. Companions of Andromeda
NGC 147
M110
NGC 185
Cassieopeia Dwarf
Andromeda V
Andromeda III
Pegasus Dwarf
Andromeda IV

6. Other Members of Local Group
Phoenix Dwarf
IC 1613
Triangulum galaxy (M33)
Wolf-Lundmark-Melotte
Tucana Dwarf
Pisces Dwarf
IC 10
Leo A

7. The Virgo Cluster More than 1000 galaxies Dozens of bright galaxies
Two giant ellipticals
Many bright spirals
18  1 Mpc away
A rich cluster
Total mass about 1.2 × 1015 MSun

8. The Coma Cluster 1000+ galaxies visible Many bright galaxies
Many giant ellipticals, especially in the center
Bright spirals towards the edges
About 100 Mpc away
A very rich cluster
Total mass about 3 × 1015 MSun

9. HGC 87
Hydra Cluster
Perseus Cluster
Markarian’s Chain
Fornax Cluster

10. Cluster Evolution
Galaxy Clusters, like stellar clusters, will change over time
Heaviest galaxies fall towards the center
Tidal Friction will enhance this effect
Galaxies will merge
For rich clusters, over time:
System will “virialize” to form a spherical distribution (more regular)
Core will shrink, eventually galaxies overlap
Giant ellipticals concentrated in the center
Spirals more towards the edge.
Collisions will knock gas out of the galaxies themselves
Typically, twice as much gas between the galaxies as in the galaxies

11. Ancient Galaxy Cluster
Old Clusters
Old Galaxy Cluster
Ancient Galaxy Cluster

12. Where is the mass in a galaxy cluster?
Most (all?) galaxies have much more mass than is in the stars and gas
Dark Matter is 90% of galaxies
How about clusters?
Distant Source
Need to find mass of cluster
Gravitational lensing!
Observer
Cluster
There’s much more mass than is visible
5% is in stars and other visible matter
10% is in hot gas between the galaxies
85% is in dark matter
Gravity bends light
You see two or more images of source
Can estimate mass of cluster!

16. Superclusters
Clusters are themselves grouped into larger structures called Superclusters
Size: Up to around 100 Mpc
Our cluster is called the “Virgo Supercluster”
Centered on the Virgo Cluster
Superclusters are much more poorly defined than clusters
They are probably not gravitationally bound!
They haven’t had time to virialize
They are always irregular, no particular shape to them
Gravity hasn’t had time to reshape them
Hence this represents primordial distribution of mass

17. Nearby Superclusters
Between the superclusters are “voids” almost devoid of galaxies
Little evidence of structure bigger than superclusters
No “hyperclusters”
Largest scale structure like soap bubbles
Mostly empty space
Superclusters are walls between the bubbles

18. On the largest scale:

19. Everyone should know where they live:
Classification: Our Address
Stellar system Solar System
Star Cluster (none)
Galaxy Milky Way
Galaxy Cluster The Local Group
Supercluster Virgo Supercluster
Universe Universe