A new study of elliptical galaxies led by Dr Akos Bogdan of the Harvard-Smithsonian Center for Astrophysics has revealed a surprising link between galaxies’ dark matter halos and their central black holes.
Resembling looming rain clouds on a stormy day, dark lanes of dust crisscross the giant elliptical galaxy NGC 5128, also known as Centaurus A. The warped shape of NGC 5128’s disk of gas and dust is evidence for a past collision and merger with another galaxy. The resulting shockwaves cause hydrogen gas clouds to compress, triggering a firestorm of new star formation. At a distance of just over 11 million light-years, NGC 5128 contains the closest active galactic nucleus to Earth. The center is home for a supermassive black hole that ejects jets of high-speed gas into space, but neither the supermassive black hole or the jets are visible in this image. Image credit: NASA / ESA / Hubble Heritage / STScI / AURA / Hubble Collaboration / R. O’Connell, University of Virginia / WFC3 Scientific Oversight Committee.
“There seems to be a link between the amount of dark matter a galaxy holds and the size of its central black hole, even though the two operate on vastly different scales,” said Dr Bogdan.
Previous observations of elliptical galaxies had found a relationship between the mass of the central black hole and the total mass of stars.
However, more recent studies have suggested a tight correlation between the masses of the black hole and the galaxy’s dark matter halo. It wasn’t clear which relationship dominated.
In our Universe, dark matter outweighs normal matter – the everyday stuff we see all around us – by a factor of 6 to 1. Every galaxy is surrounded by a halo of dark matter that weighs as much as a trillion suns and extends for hundreds of thousands of light-years.
To investigate the link between dark matter halos and supermassive black holes, Dr Bogdan and his colleague Dr Andy Goulding of Princeton University studied more than 3,000 elliptical galaxies. They used star motions as a tracer to weigh the galaxies’ central black holes.
X-ray measurements of hot gas surrounding the galaxies helped weigh the dark matter halo, because the more dark matter a galaxy has, the more hot gas it can hold onto.
The scientists found a distinct relationship between the mass of the dark matter halo and the black hole mass – a relationship stronger than that between a black hole and the galaxy’s stars alone. This connection is likely to be related to how elliptical galaxies grow.
The results have been accepted for publication in the Astrophysical Journal (arXiv.org preprint).
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Akos Bogdan & Andy D. Goulding. 2015. Connecting Dark Matter Halos with the Galaxy Center and the Supermassive Black Hole. ApJ, accepted for publication; arXiv: 1502.05043
