An astronomer observes two galaxies, A and B, which have redshifts of: Galaxy A

Question

An astronomer observes two galaxies, A and B, which have redshifts of: Galaxy A = 0.020 and Galaxy B = 0.010.
(a) Which galaxy is closest to us and what is its distance away from us in Mpc? You may assume that the Hubble constant is H0 = 75 km s–1 Mpc-1 and that the speed of light is
c = 3.0 × 105 km s–1.

(b) Assuming that Galaxy A is intrinsically of identical luminosity to Galaxy B, how does its apparent brightness on the night sky compare to that of Galaxy B?
How many times brighter or less bright is Galaxy A than Galaxy B.

Explanation / Answer

Redshift is defined as 1+ (velocity of the galaxy away from us/the Speed of light) so the velocities are Galaxy A .02 c = 6,000 km/sec Galaxy B .08 c = 24,000 km / sec so the larger the number the faster is it moving away from us. And from Edwin Hubble, we know that, in general, the faster a galaxy is moving away from us, the farther it is. Sorry, I do not remember my parsecs! I vaguely remember that the Hubble constant of 75 is something like 25 kilometers per second per million light years, so the velocity of Galaxy A: 6000 / 25 = 240 million light years distant And Galaxy B: 24,000 / 25 = 960 million light years distant. Being 1/4th the distance, one would expect galaxy A to be 4^2 times brighter.

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