17 February 2011

Supermassive black holes not so big after all

By
Cosmos Online
Supermassive black holes are between two and 10 times less massive than previously thought, according to new calculations published by German astrophysicists.
black hole

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.”

A clue to galaxy formation

According to Emmanuele Berti from the University of Mississippi it is important to have accurate estimates of black hole masses: “It is widely believed that the black hole mass is intimately related to other properties of their galactic environment.”

He continued, “If we can measure this at different times during cosmic history, we may learn something about how the Universe became what we see today.”

Since a black hole only has two properties: mass and angular momentum, an accurate estimate of the mass is essential if astrophysicists are to have any hope of understanding what is going on now and what went on during galaxy formation.

Gas can potentially corrupt results

Scientists are confident that they know the mass of the supermassive black hole at the centre of the Milky Way, as Berti explained, “Observing the orbits of stars at the centre of our own galaxy yielded the most precise supermassive black hole mass measurement to date.”

However, David Ballantyne of Georgia Institute of Technology in the U.S. urged caution when using other methods.

“For active galactic nuclei it is tricky to estimate the black hole mass because they are relatively rare (and farther away), and they emit a lot of light which blocks our view of the nucleus,” he said.

Hence, said Ballantyne, scientists are forced to use gas tracers, but gas can be compressed, heated and/or shocked, which can corrupt its velocity signature. “The accuracy of these methods is not fully known.”

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