No direct link between black holes and dark matter
Scientists show that black hole growth is mostly connected to the formation of the galaxy bulge and not to dark matter.
January 20, 2011
Massive black holes have been found at the centers of almost all galaxies, where the largest galaxies —the ones also embedded in the largest halos of dark matter — harbor the most massive black holes. This led to the speculation that there is a direct link between dark matter and black holes — that exotic physics controls the growth of a black hole. Scientists at the Max Planck Institute of Extraterrestrial Physics in Garching, Germany, the University Observatory Munich, and the University of Texas at Austin have conducted an extensive study of galaxies to prove that black hole mass is not directly related to the mass of the dark matter halo, but rather seems to be determined by the formation of the galaxy bulge.
M101 (NGC 5457) is a galaxy with a massive dark halo but no bulge and no detected black hole. Observations show that this giant galaxy cannot contain a black hole that is even as small as the relatively small black hole in our Milky Way Galaxy. wikisky.org
Galaxies, such as our own Milky Way, consist of billions of stars, as well as great amounts of gas and dust. Most of this can be observed at different wavelengths, from radio and infrared for cooler objects to optical and X-rays for parts that have been heated to high temperatures. However, there are also two important components that do not emit any light and can only be inferred from their gravitational pull.
All galaxies are embedded in halos of so-called dark matter, which extends beyond the visible edge of the galaxy and dominates its total mass. This component cannot be observed directly, but can be measured through its effect on the motion of stars, gas, and dust. The nature of this dark matter is still unknown, but scientists believe that it is made up of exotic particles unlike the normal baryonic matter, which Earth, the Sun, stars, and we are made of.
The other invisible component in a galaxy is the supermassive black hole at its center. Our own Milky Way harbors a black hole, which is some 4 million times heavier than our Sun. Such gravity monsters, or even larger ones, have been found in all luminous galaxies with central bulges where a direct search is feasible; most, and possibly all bulgy galaxies, are believed to contain a central black hole. However, this component cannot be observed directly, and the mass of the black hole can only be inferred from the motion of stars around it.
In 2002, scientists speculated that there might exist a tight correlation between the mass of the black hole and the outer rotation velocities of galaxy disks, which is dominated by the dark matter halo, suggesting that the unknown physics of exotic dark matter somehow controls the growth of black holes. On the other hand, it had already been shown a few years earlier that black hole mass is well correlated with bulge mass or luminosity. Since larger galaxies in general also contain larger bulges, it remained unclear which of the correlations is the primary one driving the growth of black holes.
To test this idea, astronomers John Kormendy from the University of Texas and Ralf Bender from the Max Planck Institute for Extraterrestrial Physics and the University Observatory Munich carried out spectral observations of many disk, bulge, and pseudobulge galaxies. The increased accuracy of the resulting galaxy dynamics’ parameters led them to the conclusion that there is almost no correlation between dark matter and black holes.
By studying galaxies embedded in massive dark halos with high rotation velocities with small or no bulges, Kormendy and Bender tried to answer this question. They found that galaxies without a bulge — even if they are embedded in massive dark matter halos – can at best contain low mass black holes. Thus, they could show that black hole growth is mostly connected to bulge formation and not to dark matter.
"It is hard to conceive how the low-density, widely distributed non-baryonic dark matter could influence the growth of a black hole in a very tiny volume deep inside a galaxy," said Bender.
"It seems much more plausible that black holes grow from the gas in their vicinity, primarily when the galaxies were forming,” said Kormendy. In the accepted scenario of structure formation, galaxy mergers occur frequently, which scramble disks, allow gas to fall into the center, and thus trigger starbursts and feed black holes. The observations carried out by Kormendy and Bender indicate that this must be the dominant process of black hole formation and growth.
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