About 23% of the Universe is made up of mysterious ‘dark matter’, invisible material only detected through its gravitational influence on its surroundings. Now two astronomers based at the National Autonomous University of Mexico (UNAM) have found a hint of the way it behaves near black holes. Their results appear in a letter in the journal Monthly Notices of the Royal Astronomical Society.
In the early Universe clumps of dark matter are thought to have attracted gas, which then coalesced into stars that eventually assembled the galaxies we see today. In their efforts to understand galaxy formation and evolution, astronomers have spent a good deal of time attempting to simulate the build-up of dark matter in these objects.
The UNAM astronomers, Dr. Xavier Hernandez and Dr. William Lee, calculated the way in which the black holes found at the centers of galaxies absorb dark matter. These black holes have anything between millions and billions of times the mass of the Sun and draw in material at a high rate.
The researchers modeled the way in which the dark matter is absorbed by black holes and found that the rate at which this happens is very sensitive to the amount of dark matter found in the black holes’ vicinity. If this concentration were larger than a critical density of 7 Suns of matter spread over each cubic light year of space, the black hole mass would increase so rapidly, hence engulfing such large amounts of dark matter, that soon the entire galaxy would be altered beyond recognition.
Dr. Hernandez explains, “Over the billions of years since galaxies formed, such runaway absorption of dark matter in black holes would have altered the population of galaxies away from what we actually observe.”
Their work therefore suggests that the density of dark matter in the centers of galaxies tends to a constant value. By comparing their observations to what current models of the evolution of the Universe predict, Hernandez and Lee conclude that it is probably necessary to change some of the assumptions that underpin these models — dark matter may not behave in the way scientists thought it did.
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Preprint of research paper: http://arxiv.org/abs/1002.0553
Image: http://www.astroscu.unam.mx/~wlee/FigurePR.pdf
Caption: Artist’s schematic impression of the distortion of space-time by a supermassive black hole at the center of a galaxy. The black hole will swallow dark matter at a rate which depends on its mass and on the amount of dark matter around it.
Credit: Felipe Esquivel Reed
The Royal Astronomical Society (RAS; www.ras.org.uk), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science. The RAS organizes scientific meetings, publishes international research and review journals, recognizes outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 3000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.
