Question

What are physical reasons for why most spirals are blue (or blueshifted) and most ellipticals are red (or redshifted)?

Answer 1

The distribution over the sky of the 65 galaxies with negative radial velocities is characterized by several features that may be important for understanding the kinematics and evolution of the cluster. (a) The blueshifted galaxies are distributed over the sky much more compactly than the rest of the population of Virgo. All the galaxies with VLG < 0 lie, without exception, inside the virial radius ΘV IR = 6◦ . 0 of the cluster. (b) The centroid of the present sample does not coincide with the dynamic center of Virgo (M 87), and is shifted significantly to the NW by 1◦ . 10 ± 0 ◦ . 35. (c) The galaxies of early and late types have significantly different positions relative to the center of the cluster: E and SO galaxies lie predominantly to the east, while objects with a young population (dIr, BCD, S) lie mainly to the west and south. (d) The dwarf galaxies with VLG < 0 manifest a small scale clumping effect. Thus, the galaxies VCC 181, AGC 224385, and IC 3094 form a triplet in projected onto the sky, while VCC 322 and VCC 334 form a pair with a small difference in radial velocities. Other examples of multiple systems are VCC 810/815/846, VCC 892/928, and UGC 7795/VCC 1750. The characteristic scale of the clumping for these objects is ∼ 10’ or 50 kpc, while the median difference in their velocities is about 70 km s−1 . These groups contain galaxies with young, as well as old, populations. If it is real, the dynamic isolation of these pairs and triplets within the cluster is highly unexpected.

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.