# Chapter 25: Galaxy Clusters and the Structure of the Universe

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1 Chapter 25: Galaxy Clusters and the Structure of the Universe Distribution of galaxies Evolution of galaxies Study of distant galaxies Distance derived from redshift Hubble s constant age of the Universe: 14 billion years Example: galaxy at distance 6 billion LY seeing it as it was when the Universe was 8 billion years old Exploring the Universe almost back to the time it began A Million Galaxies The known part of the Universe - to a distance of ~ 4000 Mpc

2 Clusters of galaxies Galaxy - huge group of stars, dust, gas, and other celestial bodies bound together by gravitational forces. Single galaxies (field galaxies): less than 8 % of total number of galaxies Groups of galaxies - the smallest aggregates of galaxies (N=10 to 100 members in a diameter of 2 Mpc) Cluster of galaxies - contain up to one thousand galaxies (N=10 2 to 10 3 ) Superclusters of galaxies - contain up to thousands of galaxies (N=10 3 to 10 4 ) Enormous clouds of extremely hot intergalactic gas and Dark matter Clusters of galaxies Classification of clusters Shape: regular or irregular Richness: based on number of members Elliptical dwarfs most numerous observational selection affects the classification at distances > Mpc the elliptical dwarfs are not visible statistical approaches to count them Average distance between clusters r ~(10 x Cluster s Diameter ) -- much larger spatial concentration in comparison to the concentration of stars in a given galaxy ( r between stars = 10 5 to 10 6 x D star ) Contemporary theory No principal difference between groups and clusters Hierarchical structure of the Universe the field galaxies form groups and the groups form clusters

3 Clusters typically have the following properties: Content 10 to 1000 galaxies hot X-ray emitting gas large amounts of dark matter The distribution of these three components is approximately the same from one cluster to another Total mass: to solar masses Diameter: 3 to 8 Mpc The spread of velocities for the individual galaxies is about km/s. The average distance between clusters is approximately Mpc Poor Clusters & Groups AWM 7 NGC 2300 N~100 visible objects MKW 4

4 Rich clusters N~ visible objects D~ 3-4 Mpc Total known number ~ 4000 Cluster dominant galaxies The brightest members 1 or 2 giant E or S0: L~ 3x10 11 L Sun M v = -24 mag mass ~ mass Sun Galaxies of type cd (cluster dominant) Standard candles distance indicator all tests of the expansion of the Universe are done using cd galaxies

5 Red color Smooth Profile High Surface Brightness Egg shaped Many globular clusters Little or no dust line Absorption lines only No rotation Found in clusters -22 < M < -18 Elliptical Galaxies Cluster dominant galaxies multinuclear structure formed by capturing other galaxies or by repeatedly merging with other cluster members These mergers occur at the centre of the cluster dynamical friction makes galaxies lose kinetic energy: The motion of a galaxy creates an enhanced "wake" of galaxies behind it The excess gravitational pull of this wake slows the motion of the galaxy

6 Dynamical friction The net effect of this force is to make a galaxy slowly spiral in toward the centre of the cluster Once there, it will merge with all the galaxies that have proceeded it. Evidence for this scenario comes from the large number of "multiple nuclei" seen in cd galaxies: These secondary condensations of light are thought to be the partially digested remains of merging latecomers. The Local Group Members and Size The Canis Major galaxy Dominant galaxies (M31, MW, M33) Dwarf elliptical galaxies 50 similar groups in a sphere with a radius 20 Mpc

7 The Local Group The Local Group NGC 6822

8 Coma Clusters of galaxies The closest clusters Vigro (20 Mpc) and Coma (90 Mpc) Virgo The Virgo Cluster Moving away from the Local group at 1000 km/s (should be 1300 km/s) The Local group belongs to the Virgo Supercluster.

9 Fig M 87 The Coma Cluster 9

10 Clusters of galaxies The Perseus Cluster of Galaxies The cluster CL

11 The formation and evolution of galaxies: galaxies in clusters vs. galaxies between clusters How do clusters of galaxies form? The pancake model: very large clumps of gas collected to form protoclusters Fragmentation to form individual galaxies Pancakelike shape

12 How do clusters of galaxies form? The hierarchical model: small clouds of gas formed clusters of stars, which later formed individual galaxies and clusters of galaxies How do clusters of galaxies form? The explosion model: gigantic explosions in the early Universe swept up the surrounding matter into dense shells. Individual galaxies formed in the shells; clusters of galaxies formed at the intersections of the expanding shells.

13 Pre-galactic Blobs Our picture of galaxy evolution Observations + Computer simulations

14 Our picture of galaxy evolution First billion years: very smooth distribution of matter directly after the Big Bang gravity of the more massive clumps of stars starts to attract more matter Our picture of galaxy evolution billion years larger : clumps grow from merging of smaller once

15 Our picture of galaxy evolution After 1-2 billion years: after growing to a fraction of the size of our own galaxy, the clumps are large enough for the Hubble Space Telescope to see them Our picture of galaxy evolution After 2-4 billion years: larger irregular looking objects form through collisions and mergers between these sub-galactic sized clumps

16 Our picture of galaxy evolution After 4-13 billion years: Galaxies as we see them today form and take their final shape. The elliptical and the spiral galaxies with old red stellar populations in their centers form first The spiral galaxies disks form later from infalling surrounding gas.

17 Evolution of Galaxies 1. All galaxies have formed at the same time; they are equally old ~ x 10 9 yrs 2. Different galactic types are at different evolutionary stages Elliptical Galaxies formed remarkably early in the universe all stars born long ago during giant single star-forming process, exhausting all interstellar star-forming material Spiral Galaxies took much longer to form ongoing star-forming processes 3. Reason for this difference different physical parameters Compactness Rotation Environment Hidden matter

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