Why are there blue shifted galaxies?

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 expnasion, 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.

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 in orbit about each other. Most are also dwarf galaxies which you've probably never heard of, although the Andromeda Galaxy, M31, is in there. A list of these galaxies can be found at this link.

This will present the galaxies, their positions on the sky, and their velocities (all negative, meaning blue-shifted) in the column "z or km/s."

January 1999, Dave Kornreich (more by Dave Kornreich) (Like this Answer)

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