Clusters - Galaxy Clusters, Large Scale Structure and...

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Galaxy Clusters, Large Scale  Structure and Hubble’s Law Chapter 18
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Half of all galaxies are in clusters (higher density; more Es and S0; mass > few times 10 14 -10 15 ) or groups (less dense; more Sp and Irr; less than 10 14 M sun ) a Clusters contain 100s to 1000s of gravitationally bound galaxies a Typically ~few Mpc across a Central Mpc contains 50 to 100 luminous galaxies (L > 2 x 10 10 L sun ) a Abell’s catalogs (1958; 1989) include 4073 rich clusters a Both luminous Es and dEs more concentrated in clusters than mid-size Es (?) Coma Cluster o Nearest rich clusters are Virgo and Fornax (containing 1000’s of galaxies; d=15-20 Mpc) a Richer cluster, Coma, at d=70 Mpc and 7 Mpc across a Clusters filled with hot gas (T=10 7 10 8 K X-ray bright – strips away cool gas of infalling galaxies Galaxy Clusters
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Groups of galaxies are smaller than clusters Contain less than ~100 galaxies Loosely (but still gravitationally) bound Contain more spirals and irregular galaxies than clusters “The Local Group”
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Compare relative sizes of groups and clusters
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Distribution of galaxies in a cluster falls as r 1/4 (like surface brightness of elliptical galaxies) May be dynamically relaxed systems Crossing time in a typical cluster (galaxy moving at 1000 km/s, cluster size 1 Mpc) 10 9 years Thus, clusters must be gravitationally bound systems and have possibly had enough time to “relax” M = 5 x 10 15 M sun If clusters are relaxed systems, we can use the virial theorem to estimate their masses M = (5/3)(<v 2 >R/G) eq. 13.47 Using radial velocity component only (Doppler shifts) M = 5<v r 2 >R/G eq. 13.52 For Coma cluster, v rms = 860 km/s and cluster size 6.1 Mpc, what is mass?
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L B ~ 8 x 10 12 L B,sun M/L B ~ 250 M sun /L B,sun Adding up mass in DM halos of spiral galaxies still not enough Look for mass in hot, intracluster gas - T=10 7 K Estimate gas mass from diffuse X-ray emission Significant mass in gas – can be up to 10 times stellar mass Dynamical (virial) measurements indicate this accounts for about 20% of the mass. .. Clusters have a Dark Matter problem too.
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This note was uploaded on 07/11/2011 for the course AST 3019 taught by Professor Sarajedini during the Spring '08 term at University of Florida.

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Clusters - Galaxy Clusters, Large Scale Structure and...

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