QUESTION #384

# Are there any galaxies that have a blue-shift?

Asked by: Mike Cahill

### Answer

Sure there are. In particular, the Andromeda galaxy exhibits a small blueshift.

To give a bit of background, if you observe the light coming from distant galaxies, for the most part the light seems to be redshifted. In other words, we know what wavelengths of light to expect when we look at a galaxy, and the wavelengths we actually see end up being considerably longer.

As you probably know, we interpret the redshifts of galaxies to mean that the universe is expanding. So if you could staple the galaxies to the 'fabric' of space, all of them would appear to be moving away from us -- the farther away they are, the faster. This is Hubble's Law.

A useful analogy here is to take an empty balloon, draw dots all over it to represent galaxies, and pretend that we live on one of the dots. As you blow up the balloon, ALL of the dots move apart from each other. And the ones that are farthest away from us move the fastest.

The difference between this analogy and the actual universe is that although the galaxies are being pulled away from each other by the universe's expansion, they are not stapled down. Replacing the dots on the balloon by a bunch of ants give a feel for this idea. Astronomers refer to the velocity a stapled down galaxy would have as its 'Hubble recessional velocity.' Any deviation from this speed is its 'peculiar velocity.'

So, in a nutshell, if a galaxy's peculiar velocity is toward us and larger than its Hubble recessional velocity, then its light will appear blueshifted. This is possible for galaxies that are nearby like Andromeda, but as galaxies get farther away, their Hubble velocities dwarf any peculiar velocities they might have. As such, it's better to study far away galaxies when you're interested in how the universe is expanding.
Answered by: Leven Wadley, M.A., Physics Grad Student, Columbia University

Almost all the galaxies are red shifted; they are moving away from us, due to the Hubble expansion of the Universe. There are a handful of the nearby galaxies that are blue shifted. In addition to the apparent motion due to Universal expansion, individual galaxies also have their own intrinsic or peculiar motions; i.e. each galaxy is in motion irrespective of the universe's expansion and has its own unique velocity.

The velocities are in the order of hundreds of kilometers per second and 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 about 100 known galaxies with blueshifts out of the billions of galaxies in the observable universe. Most of these galaxies are in our own local group, and are all in orbit about each other. Most are dwarf galaxies among them include the Andromeda Galaxy, M31, etc. Click here for a list.

The negative velocities in the z column are the blue shifted galaxies (moving towards us).
Answered by: David Latchman, B.S., University of the West Indies, Trinidad