# Clusters - 1 Galaxy Clusters Large Scale Structure and...

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Unformatted text preview: 2/18/10 1 Galaxy Clusters, Large Scale Structure and Hubble’s Law Chapter 18 Half of all galaxies are in clusters (higher density; more Es and S0; mass > few <mes 10 14 ‐10 15 ) or groups (less dense; more Sp and Irr; less than 10 14 M sun ) Clusters contain 100s to 1000s of gravita<onally bound galaxies Typically ~few Mpc across Central Mpc contains 50 to 100 luminous galaxies (L > 2 x 10 10 L sun ) Abell’s catalogs (1958; 1989) include 4073 rich clusters Both luminous Es and dEs more concentrated in clusters than mid‐size Es (?) Coma Cluster Galaxy Clusters 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” 2/18/10 2 Compare relative sizes of groups and clusters 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? Luminous matter does not make up this mass 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 still more mass Clusters have a Dark Matter problem too 2/18/10...
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Clusters - 1 Galaxy Clusters Large Scale Structure and...

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