An image captured by the Hubble space telescope shows the dwarf galaxy ESO 495-21, really small having an estimated total mass of around 10 billion solar masses, about 3% of the Milky Way. The astronomers’ interest in ESO 495-21 is due to the fact that, despite its small size, it’s of the starburst type, which means that it has a fast rate of star formation, and has at its center a supermassive black hole with a mass estimated at at least one million solar masses. It’s a case that could be similar to the first galaxies of the universe and supports the hypothesis that the dwarf galaxy formed around a black hole that already existed before.
About 30 million light years from Earth, the dwarf galaxy ESO 495-21 is also known by various other designations depending on the catalog but in the research conducted in recent years it’s generally mentioned as Henize 2-10. Over the course of this decade, observations made it possible to discover truly curious features for a galaxy with a diameter of only 3,000 light years: not only within it the rate of star formation is about 1,000 times faster than in the Milky Way but at its center there’s a supermassive black hole with a really huge mass compared to the total of ESO 495-21.
Sagittarius A*, the supermassive black hole at the center of the Milky Way, has a mass estimated at just over four million solar masses, the one at the center of ESO 495-21 has a mass that was estimated around one million solar masses but according to an estimate reported in an article published in the journal “The Astrophysical Journal Letters” in October 2016 its mass could reach three million solar masses. In essence, the total mass of ESO 495-21 is about 3% of the Milky Way’s but the mass of its supermassive black hole could be almost 75% of of Sagittarius A*’s.
The characteristics of the dwarf galaxy ESO 495-21 are curious but could be similar to those of the first galaxies of the universe. Their formation and that of the supermassive black holes at their center are still under discussion because it’s not clear whether galaxies or black holes were born before or they grew together. The presence of a supermassive black hole at the center of a dwarf galaxy like ESO 495-21 suggests that it formed later from gas and dust attracted by the black hole.
The dwarf galaxy ESO 495-21 is at least for now a really unusual case, so much so that initially it was mistaken for a planetary nebula and the name Henize 2-10 – abbreviated as Hen 2-10 or He 2-10 – derives from a catalog of those objects. To understand if it can really represent the situation of the first galaxies of the universe more studies are needed and perhaps more sightings of similar objects.