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ns209hw4sol

# ns209hw4sol - Homework 4 due May 10 Thursday 1 Individual...

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Homework 4, due May 10, Thursday 1. Individual galaxies in a galaxy cluster have recession velocities between 6500 and 7500 km/s. The apparent diameter of the cluster is 1 degree. Estimate the mass of the galaxy cluster in solar masses. (Hint, see page 669 of your book. Assume a spherical distribution of galaxies. Imagine in what cases a galaxy would provide maximum and minimum speed? Remember the whole cluster is moving with Hubble flow, but individual galaxies may have smaller relative speeds with respect to the center of the galaxy) Solution. I assume the entire cluster is moving away from us with the average speed of galaxies, 7000 km/s. The maximum speed will be for a galaxy moving away from us in the cluster, and the minimum speed correspond to a galaxy moving towards us. In any case the speed of individual galaxies are around 500 km/s. v ind = 500 km/s distance = v cluster /H o = 7000 / 70 = 100 Mpc. the size (diameter) of the cluster ? 1'' = 10 8 AU in 100 Mpc, 1 degree = 3.6 10 11 AU = 5.4 x 10 19 km rad = 2.7 x 10 19 km = 2.7 x 10 22 m.

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