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Clusters and Superclusters

Clusters and Superclusters. Clusters. Galaxies are not distributed randomly through space Concentrated in groups called Clusters Clusters range from about 10 13 M Sun to about 10 15 M Sun Range in size from 2 Mpc to 10 Mpc across

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Clusters and Superclusters

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  1. Clusters and Superclusters Clusters • Galaxies are not distributed randomly through space • Concentrated in groups called Clusters • Clusters range from about 1013MSun to about 1015MSun • 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 1013MSun) • Poor - <50 bright galaxies, (about 1014MSunor 1015MSun) • 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. Sagittarius Dwarf Elliptical Galaxy Our Companions 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. M32 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 × 1015MSun

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

  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. 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!

  13. 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

  14. 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

  15. On the largest scale:

  16. 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

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