Elliptical Galaxyとは何か？
An elliptical galaxy is a smooth, featureless spheroid of stars with no disc, no arms and almost no cold gas. They range from dwarfs smaller than a globular cluster to the largest galaxies in the universe, and by and large nothing new has been born in them for billions of years.
The stars inside are not orbiting in an organised way. Where a spiral disc is a coherent rotating system, an elliptical is dynamically hot: its stars are on randomly inclined orbits, and the shape of the galaxy reflects how those orbits are distributed rather than how fast it spins. The model builds it that way — the same orbital rule as the spiral, with the plane of each orbit chosen at random instead of shared.
The colour is a death certificate. Star formation stopped long ago, which means every short-lived blue star has already exploded and only the long-lived red and yellow ones are left. Astronomers call these galaxies red and dead, and the interesting question is what killed them. The leading answer is that they are the wreckage of collisions: when two spirals merge, the collision scrambles the ordered discs into a spheroid and drives most of the gas either into a violent burst of star formation or out of the galaxy altogether. What remains cannot make more stars.
At the top end this becomes cannibalism. The giant ellipticals at the centres of galaxy clusters — M87 is the nearby example — sit at the bottom of the cluster's gravitational well and swallow galaxies that fall in. They are the most massive galaxies known, they host the most massive black holes known, and they are surrounded by swarms of thousands of globular clusters, many of them stripped from the galaxies they consumed.