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Size Of Black Hole Dependent On Amount Of Dark Matter In A Galaxy, Says Study

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All massive galaxies have black holes at their cores. The bigger the galaxy, the bigger the black hole in the center.

A new study involving elliptical galaxies, which was published in The Astrophysical Journal, sheds light on the connection between galaxies and black holes, noting the role that dark matter plays in the relationship.

Carried out by Akos Bogdan from the Harvard-Smithsonian Center for Astrophysics and Andy Goulding from Princeton University, the study was designed to tackle a controversy in the field. It had been previously observed that the total mass of stars in an elliptical galaxy and the mass of its black hole are related.

However, recent research is suggesting that how big a black hole is has something to do with the dark matter halo a galaxy has. It wasn't clear which kind of relationship had more bearing, and this is what the study sought out to clear up.

Using data from the ROSAT X-ray satellite's all-sky survey and the Sloan Digital Sky Survey, the researchers examined over 3,000 elliptical galaxies to find the link between black holes and dark matter halos. They turned to star motions to trace the weight of black holes in galaxies while x-ray measurements of hot gas around galaxies helped calculate how heavy dark matter halos were. The more hot gases there are, the bigger the expanse of dark matter: galaxies would need it to hold hot gas.

According to their findings, masses of black holes and dark matter halos had a stronger relationship compared to what stars and black holes had.

This relationship is likely to be connected to how elliptical galaxies form and grow. An elliptical galaxy forms when a number of smaller galaxies are merged, with all their dark matter and stars mixing together. Since dark matter is heavy (it outweighs normal matter by 6 to 1), everything else follows the mold it dictates, guiding how an elliptical galaxy will turn out.

"In effect, the act of merging creates a gravitational blueprint that the galaxy, the stars, and the black hole will follow in order to build themselves," explained Bogdan.

Scientists only know dark matter exists based on its gravitational effect on everything else around it. It is capable of holding galaxies and clusters of galaxies together, as demonstrated by its ability to control how an elliptical galaxy would be formed. Each galaxy with a halo has dark matter as heavy as a trillion suns and as expansive as hundreds of thousands of light years.

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