Tiny galaxy has huge black hole at its center, gives clues to galactic evolution

Nestled within this field of bright foreground stars lies ESO 495-21, a tiny galaxy with a big heart. ESO 495-21 is just 3000 light-years across, a fraction of the size of the Milky Way, but that is not stopping the galaxy from furiously forming huge numbers of stars. There are also indicators for a supermassive black hole in its center — an unusual component for a galaxy of its size. ESA/Hubble, NASA

This image gathered by the Hubble Space Telescope’s Advanced Camera for Surveys and the Wide Field Planetary Camera 2 shows the tiny galaxy ESO 495-21, located 30 million light-years away in the constellation of Pyxis (the Compass).

The galaxy is only 3000 light-years across, which is just a fraction of the size of the Milky Way at 100,000 light-years across. But it could hold the clue to unraveling a longstanding question about the evolution of galaxies over time, and what causes a galaxy to grow.

Despite its small size, ESO 495-21 is thought to host a feature usually found in much larger galaxies — a supermassive black hole. These huge black holes sit right in the middle of galaxies and gobble up dust and gas which comes close to them, like the one at the center of our Milky Way. As a general rule of thumb, the bigger the galaxy, the bigger its supermassive black hole. But this little galaxy bucks that trend, seeming to have a black hole at its heart which is over a million times as massive as our Sun.

The possible presence of this black hole could help astronomers understand the way in which galaxies evolve. Currently, they are not sure about how the supermassive black holes develop — it could be that the black holes appear first and then galaxies form around them, or it could be that the presence of many stars forming a galaxy pushes mass together and creates the black hole.

As ESO 495-21 is small and has a high rate of star formation, it is thought to be similar to the galaxies which existed in the earliest stages of the universe. In that case, the fact it seems to have a large black hole suggests that the supermassive black holes develop first, and the galaxies around them form later.

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