Megan's Blog

In the news this month: giant black hole in a dwarf galaxy

Normal massive galaxies contain supermassive black holes in their central bulges. Observations have shown evidence for such objects in pretty much all massive Milky Way-like galaxies, as well as large elliptical galaxies. But not all galaxies are as large as the Milky Way. Many dwarf galaxies are known in the nearby universe, usually irregular in shape and often forming stars much faster than galaxies like our own. Now, a team of astronomers, led by Amy Reines at the University of Virginia, have found evidence for a supermassive black hole in the centre of one of these dwarf irregular galaxies.

The galaxy, known as Henize 2-10, is a small galaxy located some 30 million light years away in the southern constellation of Pyxis. It is classified as a blue compact dwarf and is highly irregular in shape. Despite being similar in mass to the Large Magellanic Cloud, Henize 2-10 is forming stars some ten times faster.

The team observed the galaxy with a number of telescopes operating in different parts of the electromagnetic spectrum as part of a survey of several galaxies. When they looked at data taken with the Very Large Array, a collection of radio telescopes situated in New Mexico in the USA, they found a small object at the centre of the galaxy which was very bright at radio wavelengths. When they looked with the Hubble Space Telescope they found that the object was not a large cluster of stars, several of which exist in other parts of Henize 2-10, but data from the archive of the Chandra X-ray telescope showed that there was a source of X-rays at the same position. The amount of energy being emitted by this object in different parts of the spectrum is consistent with it being a supermassive black hole with a mass estimated to be roughly two million times that of the Sun.

This is an exciting but unexpected result, since few dwarf galaxies are known to contain supermassive black holes, and those that do are forming stars far more slowly than Henize 2-10, and there is evidence that the black hole is growing by actively consuming material from the surrounding galaxy. This discovery has implications for our understanding of the growth of galaxies in the early universe, since the properties of Henize 2-10 (it's active black hole and simultaneous rapid star formation) resemble those of low-mass high-redshift galaxies seen in the distant universe when the early stages of galaxy assembly and evolution were happening on a large scale.

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This blog post is a news story from the Jodcast, aired in the March 2011 edition.

Reines, A., Sivakoff, G., Johnson, K., & Brogan, C. (2011). An actively accreting massive black hole in the dwarf starburst galaxy Henize 2-10 Nature, 470 (7332), 66-68 DOI: 10.1038/nature09724