An artist’s impression of a flat accretion disk rapidly spinning around a black hole.
Credit: NASA/Dana Berry, SkyWorks Digital
BRISTOL: Supermassive black holes are between 2 and 10 times less massive than previously thought, according to new calculations published by German astrophysicists.
At the centre of most galaxies, including our own, sit supermassive black holes, believed to be between 100,000 and several billion times more massive than the Sun. Previous estimates of black hole masses had contradicted theory, particularly for far away or young black holes. But new research shows that these estimates were wrong.
“It caused problems for the theory of galactic evolution that young galaxies should have these massive black holes,” said lead researcher Wolfram Kollatschny of the University of Göttingen in Germany. “Knowing the rotational velocity of surrounding material we could calculate the central black hole masses unambiguously.”
Probing the black holes
Supermassive black holes are thought to grow from massive star supernovae, sucking in so much surrounding gas that they eventually gravitate to the centre of their galaxy. They are surrounded by bright hot discs of material - called accretion discs - waiting to fall into the abyss.
Emission spectra – which identify the elements in matter - emanating from these discs contain important information about the black holes they surround. Scientists use one line in these spectra to estimate young and distant black hole masses and another for closer black holes.
What the latest research published in the journal Nature shows, however, is that one line “is always broader” than the other. “If we don’t correct for this effect we overestimate the masses of distant and young black holes,” said Kolatschny.
All previous calculations overestimated
Kollatschny and Matthias Zetzl, also from the University of Göttingen dissected spectra from 37 active galactic nuclei and found that the line widths of broad emission lines are caused by a combination of turbulence and rotational speed.
“We could separate their shares in individual emission lines,” explained Kollatschny. “Only the rotational velocity should be used to derive the central black hole masses.”
When they did this they found that previously calculated masses had all been overestimated. Furthermore, they found that “the ratio of the turbulence with respect to the rotational speed gives detailed information on the accretion disk geometrical structure.”
