DWARF GALAXY: FORMATION, EVOLUTION, CHARACTERISTICS, EXAMPLES - ARTE - 2025

A dwarf galaxy is considered one whose number of stars is one-hundredth of those contained in large spiral galaxies, such as Andromeda and our Milky Way. Dwarf galaxies are the most common type of galaxy in the Local Group of galaxies and are presumed to be so in the rest of the universe as well.

Dwarf galaxies are regularly found orbiting much larger galaxies. Around the Milky Way alone, more than fifteen orbiting dwarf galaxies have been identified.

Figure 1. Ultra-compact dwarf galaxy M60-UCD1 orbiting the giant galaxy M60. M60 is 54 million light years away and the majestic spiral galaxy NGC 4647 is 63 million light years further to the background of M60. Source: NASA Hubble Space Telescope.

The smallest galaxy identified so far by astronomers contains about a thousand stars, is spheroidal in shape and is known as Segue 2, in the constellation Aries. In proportion to large galaxies, that would be like discovering an elephant the size of a mouse.

Formation and evolution

The most recent and controversial theories about the formation of dwarf galaxies, as well as other larger ones, indicate that they are the result of the gravitational attraction of interstellar matter towards regions with dark matter or with clouds formed by heavy elements.

However, relatively new findings made with the space telescope in the ultraviolet range, operated by NASA, have shown dwarf galaxies formed by gases of light elements coming from the Ring of Leo: an immense 650 thousand light-year cloud made of hydrogen and helium.

General characteristics

Dwarf galaxies are the most abundant in the universe, but difficult to detect due to their small size and low luminosity.

For dwarf galaxies the different types of shape observed in larger galaxies also apply, identified by Edwin Hubble on the "Hubble tuning fork". For example, the following types have been identified: spiral, irregular, spheroidal, and ellipsoidal.

Figure 2. The Hubble tuning fork with the different types of galaxies.

Additionally, compact and blue dwarf galaxies have been found, as well as ultra-compact ones.

Spiral dwarf galaxies are generally far from clusters of other galaxies, as otherwise gravitational interaction with nearby neighbors would produce alterations in their spiral disk.

This type of galaxy has low luminosity and its diameters are less than 16 thousand light years. They usually have a large amount of dark matter.

Although some experts believe they are very old, compact-blue dwarf galaxies are made up of hot, massive clusters of young stars that emit blue light and make the galaxy itself appear bluish.

A characteristic representative of this type of galaxy is PGC-51017 shown in Figure 3.

Figure 3. Compact blue-dwarf galaxy PGC-51017. Source: Wikimedia Commons

The stars that make up compact blue dwarf galaxies have different periods of formation and are in permanent evolution.

Dwarf galaxies, small and large black holes

One surprising finding was that of a dwarf galaxy with a small black hole at its center. It is NGC 4395 with a central hole of 10,000 solar masses. This is in contrast to black holes in the center of large galaxies, whose masses are between millions and billions of solar masses.

Figure 4. Dwarf galaxy NGC 4395 captured by the Shulman telescope in Arizona. Source: Shulman Telescope.

But at the other extreme are the ultra-compact dwarf galaxies that have a supermassive black hole at their center, with tens of millions of solar masses. Due to this, and despite being small galaxies, they have an enormous density of stars, as is the case of the M60-UCD1 galaxy shown in figure 1.

Examples of dwarf galaxies

Below we will give a variety of examples of well-identified dwarf galaxies of different shapes, sizes, and characteristics, to give the reader an overview of their diversity.

Spheroidal, ellipsoidal and spiral dwarfs

The low-brightness spheroidal dwarf galaxy PGC 19441, in the constellation of Carina, is a satellite galaxy of the Milky Way, forming part of the Local Group of galaxies. It has a diameter of 2,000 light-years and is at a distance of 330,000 light-years.

Another good example is the Sagittarius Ellipsoidal Dwarf Galaxy (M 54), 10,000 light-years in diameter and 50,000 light-years from the center of our galaxy, which it orbits. It should not be confused with another galaxy with a similar name: the much closer irregular galaxy of Sagittarius.

It is estimated that in about 100 million years it will approach the nucleus of the Milky Way again, finally becoming part of it.

Figure 5. Ellipsoidal dwarf galaxy M 54 in Sagittarius. Source: wikimedia commons

An example of a spiral dwarf galaxy is NGC 5474 in the constellation Ursa Major. It is the closest of the many satellite galaxies in the large Pinwheel Galaxy (M101). Among dwarf galaxies, spiral-shaped galaxies are the least frequent.

Figure 6. Spiral dwarf galaxy NGC 5474. Source: Wikimedia Commons

Irregular and ultra-compact dwarfs

The small Magellanic Cloud (NGC 292) is an irregularly shaped dwarf galaxy, about a hundred times smaller than the Milky Way, which is home to around 3 billion stars. It can be seen without the need for a telescope, in the southern constellation of the Toucan.

It is 200 thousand light-years away. It is believed that it was originally spiral in shape, but that it was distorted by the gravity of the Milky Way, without being itself a satellite galaxy of it.

An example of an ultra-compact dwarf galaxy is M60-UCD1, a dwarf galaxy that orbits the giant galaxy M60, 22,000 light-years from its center. At the center of the ultra-compact M60-UCD1 dwarf is a supermassive black hole of 21 million solar masses, according to calculations of the orbital speeds of the stars around it.

Figure 1 shows a spectacular photograph taken by the Hubble space telescope showing the ultra-compact dwarf galaxy M60-UCD1, orbiting the giant galaxy M60.

In that same figure is also the majestic spiral galaxy NGC 4647, which is 63 million years farther than the giant M60.

References

1. Jet propulsion laboratory. New recipe for dwarf galaxies. Recovered from: jpl.nasa.gov
2. Observatory. The M60 elliptical and the NGC 4647 spiral. Recovered from: observatorio.info
3. MNN. Just how big is the smallest galaxy in the universe ?. Recovered from: MNN.com.
4. Phys.org. Studying dwarf galaxies to get the big picture. Recovered from: phys.org.
5. Space. Small Magellanic Cloud: A Satellite Dwarf Galaxy Neighbor. Recovered from: space.com
6. SCI news. Astronomers discover supermassive black hole in dwarf galaxy. Recovered from: sci-news.com
7. Wikipedia. Segue 2. Recovered from: wikipedia.com