Elliptical Galaxies: Rounded Stellar Cities

Hubble ACS Image of Elliptical Galaxy M87
Hubble ACS Image of Elliptical Galaxy M87. NASA, ESA, Hubble Heritage Team (STScI/AURA): P Cote (Herzberg Institute) and E Baltz (Stanford)

Galaxies are huge stellar cities and the oldest structures in the universe. They contain stars, clouds of gas and dust, planets, and other objects, including black holes. Most galaxies in the universe are spiral galaxies, much like our own Milky Way. Others, such as the Large and Small Magellanic Clouds, are known as "irregular" galaxies, due to their unusual and rather amorphous-looking shapes. However, a significant percentage, perhaps 15% or so, of galaxies are what astronomers term as "ellipticals".

General Characteristics of Elliptical Galaxies

As the name suggests, elliptical galaxies range from spherically shaped collections of stars to more elongated shapes similar to the outline of a U.S. football. Some are only a fraction the size of the Milky Way while others are many times larger, and at least one elliptical called M87 has a visible jet of material streaming away from its core. Elliptical galaxies also appear to have a large amount of dark matter, something that distinguishes even the smallest dwarf ellipticals from simple star clusters. Globular star clusters, for example, are more tightly gravitationally bound than galaxies, and generally have fewer stars. Many globulars however, are as old as (or even older than) the galaxies where they orbit. They llikely formed around the same time as their galaxies. But, that doesn't mean they're elliptical galaxies. 

Star Types and Star Formation

Elliptical galaxies are noticeably absent of gas, which is the key component of star-forming regions.

Therefore the stars in these galaxies tend to be very old, and star formation regions are relatively rare in these objects. Furthermore, the old stars in ellipticals tend to be yellow and reddish; which according to our understanding of stellar evolution, means they are smaller, dimmer stars.

Why no new stars?

It's a good question. Several answers come to mind. When many large stars are formed, they die quickly and redistribute much of their mass during a supernova event, leaving the seeds for new stars to be formed. But since smaller mass stars take tens of billions of years to evolve into planetary nebulae, the rate at which gas and dust is redistributed in the galaxy is very low.

When the gas from a planetary nebula or a supernova explosion finally drifts into the intergalactic medium, there is usually not nearly enough to begin forming a new star. More material is needed. 

Formation of Elliptical Galaxies

Since star formation seems to have ceased in many ellipticals, astronomers suspect that a period of rapid formation must have happened early in the galaxy's history.One theory is that elliptical galaxies may primarily form through the collision and merger of two spiral galaxies. The current stars of those galaxies would become intermixed, while the gas and dust would collide.The result would be a sudden burst of star formation, using up much of the available gas and dust.

Simulations of these mergers also show that the resulting galaxy would have a formation much like that of elliptical galaxies.

This also explains why spiral galaxies seem to dominate, while ellipticals are more rare.

This would also explain why we don't see very many ellipticals when we survey the oldest galaxies we can detect. Most of these galaxies are, instead, quasars - a type of active galaxy.

Elliptical Galaxies and Supermassive Black Holes

Some physicists have theorized that at the center of every galaxy, almost regardless of type, lies a supermassive black hole. Our Milky Way certainly has one, and we've observed them in many others. While this is somewhat difficult to prove, even in galaxies where we don't directly "see" a black hole, that does not necessarily mean that one is not there. It's likely that at least all (non-dwarf) elliptical (and spiral) galaxies that we have observe contain these gravitational monsters.

Astronomers are also currently studying these galaxies to see what effect the existence of the black hole has on their past star-formation rates. 

Edited by Carolyn Collins Petersen