Dwarf galaxies are regularly found orbiting much larger galaxies. In the Milky Way alone, more than fifteen orbiting dwarf galaxies have been identified
The smallest galaxy identified so far by astronomers contains about a thousand stars, is spherical in shape and is known as Segue 2 in the constellation Aries. In proportion to large galaxies, it would be like discovering an elephant the size of a mouse.
training and evolution
The most recent and controversial theories about the formation of dwarf galaxies, as well as other major ones, indicate that they are the result of the gravitational attraction of interstellar matter towards regions with dark matter or clouds formed by heavy elements.
However, relatively new discoveries made with the NASA-operated ultraviolet space telescope have shown dwarf galaxies formed by light element gases from the Leo ring: a huge 650,000 light-year cloud made of hydrogen and helium
Dwarf galaxies are the most abundant in the universe, but difficult to detect due to their small size and low brightness.
For dwarf galaxies, the different types of shapes observed in major galaxies identified by Edwin Hubble in the “Hubble Fingerboard” also apply. For example, the following types have been identified: spiral, irregular, spheroidal and ellipsoidal.
In addition, compact and blue dwarf galaxies were found, as well as ultra-compact galaxies.
Dwarf spiral galaxies are often far away from clusters of other galaxies, otherwise the gravitational interaction with close neighbors would cause changes in their spiral disk.
This type of galaxy has low luminosity and its diameters are below 16 thousand light years. They usually have a lot of dark matter.
While some experts believe they are very old, compact blue dwarf galaxies are composed of young, massive, hot star clusters that emit blue light and make the galaxy itself appear bluish.
A representative feature of this type of galaxy is the PGC-51017
The stars that make up the blue-compact dwarf galaxies have different formation periods and are constantly evolving.
Dwarf galaxies, small and large black holes
One startling discovery 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 contrasts with the black holes at the center of large galaxies, whose masses are between millions and billions of solar masses.
But on the other side are the ultra-compact dwarf galaxies that have a supermassive black hole at their center, with tens of millions of solar masses. For this reason, and despite being small galaxies, they have an enormous density of stars, as is the case of galaxy M60-UCD1
Examples of dwarf galaxies
Below, we’ll give a variety of examples of well-identified dwarf galaxies of different shapes, sizes, and characteristics to provide the reader with an overview of their diversity.
Spheroidal, Ellipsoidal and Spiral Dwarfs
The low-brightness dwarf spherical galaxy PGC 19441 in the constellation Carina is a satellite galaxy in the Milky Way that is 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 dwarf ellipsoid galaxy Sagittarius (M 54) 10,000 light-years across and 50,000 light-years from the center of our orbiting galaxy. Not to be confused with another galaxy with a similar name: the much closer irregular Sagittarius galaxy.
It is estimated that within 100 million years it will again approach the Milky Way’s core, finally becoming part of it.
An example of a dwarf spiral galaxy is NGC 5474 in the constellation Ursa Major. It is the closest of many satellite galaxies that the large Windlass galaxy has (M101). Among dwarf galaxies, spirals are the least frequent.
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 about 3 billion stars. It can be seen without the need for a telescope in the southern constellation of Toucan.
It’s 200,000 light years away. It is believed that it was originally a spiral, but that it was distorted by the gravity of the Milky Way, without actually being a satellite galaxy.
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 velocities of the stars around it.
In the same picture is also the majestic spiral galaxy NGC 4647, which is 63 million years older than the giant M60.