Elliptical galaxy

Lore & Background

Elliptical galaxies range in size from dwarf ellipticals with tens of millions of stars to supergiants of over one hundred trillion stars. They are composed mostly of older, low-mass stars, with a sparse interstellar medium, and tend to be surrounded by large numbers of globular clusters. Star formation activity is typically minimal, though brief periods may occur when merging with other galaxies. The stars inside elliptical galaxies are on average much older than stars found in spiral galaxies. Edwin Hubble originally hypothesized that elliptical galaxies evolved into spiral galaxies, which was later discovered to be false, although the accretion of gas and smaller galaxies may build a disk around a pre-existing ellipsoidal structure. The formation of elliptical galaxies is an active area of research, with early models proposing rapid star formation at high redshifts and later models emphasizing mergers of disk galaxies. Gas-rich (wet) mergers may produce lower mass, fast rotating ellipticals, while gas-poor (dry) mergers may help build higher mass, slow rotating systems. Every massive elliptical galaxy contains a supermassive black hole at its center, with the mass of the black hole tightly correlated with the mass of the galaxy through relations such as the M–sigma relation. Elliptical galaxies are preferentially found in galaxy clusters and compact groups. Unlike flat spiral galaxies, elliptical galaxies are more three-dimensional, without much structure, and their stars are in somewhat random orbits around the center.

Reader's Guide

Elliptical galaxies are significant in astronomy as one of the three main galaxy classes defined by Edwin Hubble, forming a key part of the Hubble sequence. They represent the 'early-type' galaxy population, alongside lenticular and ES galaxies. Their smooth, ellipsoidal shapes and lack of prominent features contrast sharply with spiral galaxies, and they are dominated by old stellar populations with minimal ongoing star formation. This makes them important for studying galaxy evolution, particularly through merger processes, as both wet and dry mergers are thought to contribute to their formation. The presence of supermassive black holes at their centers, with masses tightly correlated to galaxy properties, provides crucial insights into galaxy-black hole coevolution. Elliptical galaxies are preferentially found in dense environments like galaxy clusters, where they can grow to enormous sizes—supergiant ellipticals are among the largest galaxies known. Their study has also revealed that many early-type galaxies retain residual gas and low-level star formation, challenging earlier assumptions. The Hubble classification system for ellipticals, based on axis ratios, remains a standard tool, though it is now understood that some apparent ellipticals are actually misclassified lenticular galaxies with disks.

Did You Know?

• Elliptical galaxies are believed to make up approximately 10–15% of galaxies in the Virgo Supercluster.
• Every massive elliptical galaxy contains a supermassive black hole at its center.
• Edwin Hubble originally hypothesized that elliptical galaxies evolved into spiral galaxies, which was later discovered to be false.