January 10, 2011

A Supermassive Black Hole in a Dwarf Galaxy

A supermassive black hole exists in the center of our galaxy, with an incredible mass on the order of 4 million times that of the Sun. Our galaxy is not alone; most large galaxies also possess very large black holes as well. This leads to some interesting questions in galaxy formation and development: Which comes first, the black hole or the galaxy? Does a galaxy develop first, and then later, a massive black hole forms? Or does a black hole appear first, around which a galaxy much later forms? The discovery of a supermassive black hole could help answer that question.

A composite (X-ray, optical, and radio) image of the Henize 2-10 dwarf galaxy.
Image: X-Ray (NASA/CXC/Virginia/A. Reines et al);
Radio (NRAO/AUI/NSF); Optical (NASA/STScl)

Amy Reines, a graduate student, and her colleagues at the University of Virginia initially wanted to study the rapid star formation in the Henize 2-10 galaxy. The dwarf galaxy possesses many starburst regions and interesting newly formed super star clusters, where star formation has recently taken place. However, while looking back at the data, Reines and the researchers realized that there were signs of a black hole in the center of the galaxy. Radio radiation was originating at the spot, suggesting the presence of jets of radiation resulting from matter falling into the black hole. Further data from the Chandra X-ray Space Telescope revealed a high amount of X-ray radiation in the region, more evidence for the existence of a black hole.

Looking at black holes in the center of other galaxies has shown that there is a correlation with the mass of the central bulge in the galaxy and the mass of the black hole in the galaxy center. This has led some to speculate that a bulge in the galaxy was necessary for a black hole to form. The discovery of a black hole, with a mass about 2 million times that of the Sun, in Henize 2-10 challenges that notion. Henize 2-10 lacks a bulge, and being a dwarf galaxy, it has a low mass, only 10% of the Milky Way’s mass. It also has an irregular shape, with a rapid rate of star formation. These characteristics suggest that Henize 2-10 could be an early phase in the evolution of a galaxy and the answer to the above questions could be that black holes develop before galaxies form. With that though, we still have to keep in mind that the black hole in Henize 2-10 might be an outlier. Until we find more examples, the questions are still open.

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