Elliptical galaxy

An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies.

Most elliptical galaxies are made up of older, low-mass stars, with very little material between the stars, called a sparse interstellar medium. They often have many globular clusters around them. Because of this, very few new stars form in elliptical galaxies, though they can form some when they join with other galaxies.

Elliptical galaxies are not the most common type in the universe, making up about 10–15% of galaxies in the Virgo Supercluster. However, they are often found near the centers of big groups of galaxies called galaxy clusters. Their sizes vary widely, from small dwarf ellipticals with tens of millions of stars, to huge ones with over one hundred trillion stars that can dominate their galaxy clusters.

Examples

Here are some well-known elliptical galaxies:

• 3C 244.1
• M49 (NGC 4472)
• M59 (NGC 4621)
• M60 (NGC 4649)
• M87 (NGC 4486), whose huge black hole was the first black hole ever pictured by the Event Horizon Telescope.
• M89 (NGC 4552)
• M105 (NGC 3379)
• NGC 4697, part of the NGC 4697 Group
• ESO 383-76, one of the largest galaxies known
• IC 1101, the central galaxy of Abell 2029
• Hercules A, a giant elliptical galaxy
• Maffei 1, the nearest giant elliptical galaxy
• CGCG 049-033, famous for having the longest jet ever found in a galaxy
• Centaurus A (NGC 5128), an elliptical/lenticular radio galaxy with a unique shape and unusual dust bands
• NeVe 1, the source of the Ophiuchus Supercluster eruption, the most powerful space event we know of
• M86 an elliptical or lenticular galaxy located in the constellation Virgo

General characteristics

Elliptical galaxies look like round or oval balls of stars. They don't have much of the gas and dust needed to make new stars, so they have very few young stars. Instead, they are filled mostly with older stars, which makes them look red. Big elliptical galaxies often have many small, dense groups of stars called globular clusters.

These galaxies are usually found in places where many galaxies are close together. Unlike spiral galaxies, which are flat and have a clear structure, elliptical galaxies are three-dimensional and their stars move in more random paths around the center.

Sizes and shapes

Big galaxies called supergiant ellipticals, or type-cD galaxies, are the largest. Elliptical galaxies come in many different sizes and masses. Their diameters can range from 3,000 light years to more than 700,000 light years. Their masses can range from 10⁵ to nearly 10¹³ times the mass of the sun. This size range is much wider for elliptical galaxies than for other types. The smallest elliptical galaxies, called dwarf elliptical galaxies, might be about the size of a typical group of stars called a globular cluster, but they contain a lot of invisible material called dark matter. Most of these small galaxies are different from larger elliptical galaxies.

The way scientists classify elliptical galaxies looks at how stretched out their shapes are. They use the ratio of the longest part (a) to the shortest part (b) of the galaxy's light pattern, called isophotes. For a perfectly round galaxy where a equals b, the number is 0, and it is called an E0 galaxy. Scientists have found that some galaxies classified as E4 to E7 are actually lenticular galaxies seen from different angles. This shows that the shape we see depends on both the galaxy's true shape and our viewing angle. Some elliptical galaxies have disks, while others do not, and there are galaxies in between these two types.

Dwarf spheroidal galaxies are a special group that share more properties with irregular and late spiral-type galaxies.

At the largest end of elliptical galaxies, there are D-galaxies and cD-galaxies. These are similar to smaller giant ellipticals but are more spread out, with large areas that may belong to the galaxy cluster around them as much as to the central galaxy.

Formation and evolution

Scientists are still learning how elliptical galaxies form. Some ideas say they form quickly when many stars are made at once. Other ideas say they form when smaller galaxies crash into each other and combine.

Recent studies show that when galaxies with lots of gas crash together, they can make smaller, faster-spinning elliptical galaxies. When galaxies with little gas crash together, they might help build bigger, slower-spinning ones. Bigger elliptical galaxies in crowded areas of space are usually older and have more metals in their stars. Some elliptical galaxies also have dusty strips, which might be clues that they were formed from merging galaxies.

Star formation

In recent years, we have learned that about 25% of certain types of galaxies still have some gas and can form new stars, but only a little bit.

Researchers using the Herschel Space Observatory think that big black holes in the middle of these galaxies might stop the gas from getting cold enough to make many new stars.