What Are Elliptical Galaxies?

Elliptical galaxies are united by the fact that they are, well, elliptical or ellipsoidal, along at least two dimensions, which would resemble a flat disk, or three dimensions, which could be roughly spherical in nature. Elliptical galaxies don't have to resemble an oval, they can in fact have a circular quality as well. (A circle is a special case of an ellipse, for those geometry-inclined folks out there.)

Within these forms, the density of stars is roughly constant, giving the appearance of an even brightness across the entire galaxy. The size of an elliptical galaxy can vary greatly, from a few hundreds of millions to upwards of a trillion stars, more variance than any other type of galaxy.

Elliptical galaxies are also host to very specific types of stars and star structures. The stars tend to be quite old, as well as being low-mass, and thus tend to have a yellow-reddish tinge to them. Between the stars the interstellar medium is sparse, with little dust or other gas, which means that there is little star formation in these galaxies, biasing the population of stars further towards an older one.

While an elliptical galaxy does indeed appear to have an even consistency of stars, there are also other structures within the galaxy. Globular clusters tend to be quite common, which may in fact be thought of as elliptical galaxies in miniature. Indeed, there's a bit of conflict amongst scientists about where one differentiates a large globular cluster from a dwarf elliptical galaxy!

Elliptical galaxies tend to be found nearer to the center of galaxy clusters than other galaxies. They comprise somewhere between 10-15% of the total galaxies in the (known) universe, and are thus not the dominant type. As would be expected of a galaxy with older stars, they are not found as often in images taken of the early universe.

From these pieces, one can begin to construct a hypothesis as to how elliptical galaxies form. Something that astronomers have noted is that features of elliptical galaxies are quite similar to that of the bulge within other forms of galaxies, such as the spiral or barred spiral forms. Astronomers have also noted that galaxies have a certain tendency to rip each other's spiral arms off when they pass near each other, or collide and merge into one galaxy if they pass directly through each other. So, it has been suggested that the aged elliptical galaxies we often see are in fact the core remnant of other galaxies after having been through some sort of gravitational turmoil with other galaxies, either merging into one larger elliptical galaxy or tearing enough of the outer structures off that only the bulge remains intact. Such collisions, ordinarily quite rare, would happen more often near the center of galaxy clusters, where elliptical galaxies are predominantly found. Indeed, it is thought that the future merger of our own Milky Way galaxy with the nearby Andromeda in three or four billion years will render us into one giant elliptical galaxy.

Paralleling theories regarding the evolution of the bulge in spiral galaxies, astronomers also think that the reason for the near-uniformity of the age of the stars in elliptical galaxies is because they all were created at approximately the same time in a brilliant initial burst of star formation. Also, as consistent with this theory of evolution is the existence of a supermassive black hole at their center, a remnant of when they were that bulge of a spiral galaxy.

For more on the evolution of galaxies, check out this article.