Artist's impression of accreting black hole in a dwarf galaxy
This artist's illustration shows a black hole within a dwarf galaxy.
Dwarf galaxies usually contains a few billion stars – very small when compared to the several hundred billion stars within the Milky Way. Unlike our home galaxy, these dwarf galaxies do not have a bulge of stars at their centre, and only a small number have been found to contain massive black holes.
Massive black holes lie between stellar-mass black holes, which have small masses of around 5 to 10 times the mass of the Sun, and the supermassive black holes that lurk at the centres of massive galaxies, which have masses of at least 1 million times the mass of the Sun.
In recent years, growing numbers of massive black holes have been identified within dwarf and bulgeless galaxies. An international team of astronomers have recently found one within one of the smallest galaxies yet, a dwarf named J1329+3234.
Although this galaxy shows no signs for a black hole in visible light, the team found infrared signatures of a feeding black hole within this galaxy in 2013. They followed up with ESA's X-ray satellite XMM-Newton, leading to a surprise find: a massive black hole at least 3000 times as massive as the Sun. If the black hole in J1329+3234 is similar to known low-mass supermassive black holes, then it would be much more massive, with a mass of around 150 000 times the mass of the Sun.
Black holes are thought to grow larger via successive galaxy mergers, which would also build up large galaxies with dominating central bulges. Finding a massive black hole within such a tiny and bulgeless galaxy instead supports the theory that black holes may have grown very efficiently within the gaseous haloes of forming galaxies, originating in massive, collapsing clouds of primordial gas. J1329+3234 also highlights the importance of multi-wavelength studies; although in optical light the galaxy looked unremarkable, the view in infrared and X-ray light revealed a different picture.