Why are there blue shifted galaxies? (Intermediate)

I have heard that there are galaxies which are not red shifted relative to earth, but are blue shifted. In the mapping projects of the heavens are there any ways to predict what percentage of galaxies are blue shifted, and what percentage are red shifted?

Almost all galaxies are redshifted because of the Hubble expansion of the universe. Only a handful of the most nearby galaxies are blue-shifted. You see, in addition to the apparent motion imparted to galaxies due to universal expansion, individual galaxies also have their own intrinsic, what we call "peculiar" motions. This is not because they are peculiar, as in strange, but rather because each galaxy is in motion irrespective of the universe's expansion, and each galaxy has its own unique velocity due to the gravitational attraction of other galaxies in its vicinity.

Generally, that velocity is some hundreds of kilometers per second. In regions close enough to our own galaxy where the Hubble expansion results in less outward expansion than this, the galaxies' peculiar velocities (if they are large enough and sufficiently towards us) can overcome that expansion, resulting in a blue-shift.

There are in all about 100 known galaxies with blueshifts out of the billions of galaxies in the observable universe. Most of these blue-shifted galaxies are in our own local group, and are all bound to each other. Most are also dwarf galaxies which you've probably never heard of, although the Andromeda Galaxy, M31, is in there.

This page was last updated on June 27, 2015

About the Author

Dave Kornreich

Dave was the founder of Ask an Astronomer. He got his PhD from Cornell in 2001 and is now an assistant professor in the Department of Physics and Physical Science at Humboldt State University in California. There he runs his own version of Ask the Astronomer. He also helps us out with the odd cosmology question.

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