# Quasar Number Densities and Lifetimes

• I
aurora7790
From deep galaxy counts, it is estimated that there are about 40 billion galaxies in the observable universe (not including probable multitudes of dwarf galaxies too faint to observe). Assuming that the mean age of these galaxies is 10 Gyr, and that each one goes through an AGN episode once, with a mean duration of 108 yr, estimate the total number of quasars that we can see at any one time.

This was an interesting question that my astronomy professor asked our class the other day, just wanted to see what others came up with!

## Answers and Replies

stefan r
Science Advisor
From deep galaxy counts, it is estimated that there are about 40 billion galaxies in the observable universe (not including probable multitudes of dwarf galaxies too faint to observe). Assuming that the mean age of these galaxies is 10 Gyr, and that each one goes through an AGN episode once, with a mean duration of 108 yr, estimate the total number of quasars that we can see at any one time.

This was an interesting question that my astronomy professor asked our class the other day, just wanted to see what others came up with!

(4 x 1010/1 x 1010) x 108 = 432

Did you mean 108 instead of 108? In that case 4 x 108

Is also unclear what "that we can see at any one time" means. Large parts of the sky are blocked by the milky way or other galaxies, stars, and clouds. It will also be an uneven distribution. Not a random distribution of quasars in any time window over 10 billion years.

Not all active galactic nuclei are quasars.