Dwarf elliptical galaxy

Some space groups are very small.

These are called dwarf elliptical galaxies. They are smaller than most other galaxies. They often live near bigger galaxies.

Some say these small groups are building blocks. They may join together to make big ones. Other scientists think they used to be spiral shapes. Moving near big groups might change their shape.

These small groups can even have gas. This gas helps them grow. It is fun to learn about space!

Dwarf elliptical galaxies are small groups of stars. They are smaller than ordinary elliptical galaxies. You can often find them in galaxy clusters. They usually live near other galaxies as companions.

Scientists have two ideas about how they form. One idea is called hierarchical merging. This is a way where small things join together. Small bits of dark matter and gas might form first. These small bits merge to make bigger objects. These small galaxies may be the building blocks for large ones.

Another idea is called galaxy harassment. This happens when a small spiral galaxy moves near big galaxies. The gravity from the big galaxies can change its shape. This can turn a spiral shape into a rounder one. Some dwarf ellipticals even show weak spiral arms. This might be a sign of their old shape.

One dwarf galaxy is named CG 611. It lives all by itself. It has a disk of gas. This gas helps the galaxy grow. This shows that not all dwarf ellipticals need a cluster to change.

Dwarf elliptical galaxies are small groups of stars. They are smaller than ordinary elliptical galaxies. These galaxies are very common in groups and clusters. They often live as companions to larger galaxies.

Some people might confuse them with compact elliptical galaxies. One example of a compact type is M32. M32 is a satellite of the Andromeda Galaxy. However, dwarf ellipticals are a different class of object. They have a fainter brightness than ordinary elliptical galaxies.

Scientists have two main ideas about how these galaxies form. One idea is called hierarchical merging. This way of working follows the Lambda-CDM model. In this model, small objects form first. These objects are made of gas and dark matter. Gravity pulls these small objects together to merge. These mergers create more massive objects over time. Dwarf galaxies might be the building blocks for large spiral galaxies.

Another idea is called galaxy harassment. This happens when a small spiral galaxy moves near other galaxies. The gravity from those galaxies changes the shape of the small one. This process can remove much of the stellar disk. This can turn a spiral shape into a rounder one. Some dwarf ellipticals show weak spiral arms. These arms might be leftovers from an old spiral shape.

We learned more about these galaxies through history. Walter Baade confirmed two of them in 1944. These galaxies are named NGC 147 and NGC 185. They are members of our Local Group. He could see the individual stars because they are close. In the 1950s, scientists found more in other places. They found them in the Virgo and Fornax clusters.

One galaxy named CG 611 helps us understand these shapes. It lives all by itself and is very isolated. It has a disk of gas that rotates. This gas disk actually rotates the opposite way of the stars. This shows the galaxy is growing by taking in new material. If it fell into a cluster, it would change. Hot gas in a cluster could strip its gas away. This would make it look like other dwarf ellipticals.

Dwarf elliptical galaxies, or dEs, are small collections of stars shaped like ellipses. They are much smaller than ordinary elliptical galaxies. These galaxies are very common in galaxy clusters and galaxy groups. Most dEs act as companions to larger galaxies. It is important to distinguish them from compact elliptical galaxies. The galaxy M32 is a prototype for that rare class. M32 is a satellite of the Andromeda Galaxy. However, dEs are a distinct type of object.

Scientists study how the brightness of these galaxies changes from their center to their edges. This is called a surface brightness profile. Ordinary elliptical galaxies were once described using a model called de Vaucouleur's model. In contrast, dEs were often described by an exponentially declining surface brightness profile. Today, scientists use a more general tool called Sersic's model. This model fits both types of galaxies well. The Sersic index is a number that quantifies the shape of the profile. There is a continuity in this index based on galaxy luminosity. This suggests that dEs and ordinary ellipticals belong to a single sequence.

There are two main theories regarding the origins of dwarf elliptical galaxies. The first theory involves hierarchical merging. This idea fits within the Lambda-CDM cosmological model. In this model, small objects made of dark matter and gas formed first. Gravity caused these small objects to coalesce, or merge together. These mergers created more massive objects over time. This process suggests dwarf galaxies were the building blocks for large spiral galaxies. Eventually, those spirals might merge to form giant ellipticals.

The second theory is known as galaxy harassment. This theory suggests dEs are the remnants of low-mass spiral galaxies. In this scenario, a spiral galaxy enters a cluster of galaxies. Repeated gravitational interactions with other galaxies change its shape. This process can remove much of the galaxy's stellar disk. The galaxy becomes rounder as a result. Some dEs show weak spiral arms or stellar disks. These features might be modified versions of an original spiral disk.

Our understanding of these galaxies grew through important discoveries. In 1944, Walter Baade confirmed two dEs named NGC 147 and NGC 185. He proved they were members of the Local Group. He could do this by resolving them into individual stars. This was possible because they are relatively close to us. During the 1950s, researchers discovered more dEs in the Fornax and Virgo clusters. These discoveries helped map how common these galaxies are in the universe.

The isolated galaxy CG 611 provides a unique way to test these theories. CG 611 lives far away from other galaxies. It possesses attributes like coherent rotation and faint spiral arms. Previously, these traits were thought to prove the galaxy harassment theory. However, CG 611 is not in a cluster. It has a gas disk that counter-rotates to its stellar disk. This means the gas moves in the opposite direction of the stars. This shows the galaxy is growing through accretion events, or taking in new material.

We can also imagine what would happen to CG 611 in a cluster. If it fell into a dense galaxy cluster, it would face ram-pressure stripping. A cluster has a halo of hot X-ray gas. This gas would strip away the gas disk of CG 611. This would leave behind a gas-poor dwarf elliptical galaxy. This new galaxy would immediately look like the dEs found in clusters. This process would not require any reshaping of the stars. This finding challenges the idea that all dEs were once spiral galaxies.