A new scale for black holes?
A team of astronomers says it has found a new and remarkably simple way to measure the mass of a black hole: examine the shape of its home galaxy. If the technique proves accurate, scientists may have a fast method for weighing supermassive black holes in the cores of distant galaxies.

Many galaxies, including the Milky Way, harbor central black holes that weigh millions or even billions of times as much as our sun. Traditionally, the only way to accurately determine the mass of these hungry monsters has been to gauge the orbital velocities of stars nearby. However, these measurements are difficult to obtain and can only be gathered for galaxies in our cosmic neighborhood. An easier approach was developed 2 years ago, after astronomers found a strong correlation between a black hole's mass and the velocity dispersion of its host galaxy. Velocity dispersion is a statistical measure of the range of velocities of all stars in a galaxy, easily obtainable with a spectrograph.

Now, a team led by Alister Graham of the Astrophysics Institute of the Canary Islands has taken the velocity dispersion approach one step further. After examining 21 galaxies, they discovered a correlation between black hole mass and the distribution of starlight in its galaxy. Galaxies with more massive black holes turn out to have a higher concentration of stars in their central bulges, and consequently, the starlight is brighter in that region. An image of the galaxy reveals this "bulge concentration." A paper describing the results will be published in an upcoming issue of Astrophysical Journal Letters.

The new method will be invaluable for statistical studies of large numbers of galaxies, says Laura Ferrarese of Rutgers University in Piscataway, New Jersey, co-discoverer of the velocity dispersion correlation. But, she says, it turns out that not all galaxies for which black hole masses are known conform to the new correlation, and it's impossible to predict which will and which will not. "If you really want to have a precise mass measurement of the black hole in a specific galaxy, you'd better use the slightly more cumbersome velocity dispersion method," Ferrarese says.

� Govert Schilling

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ScienceNOW, 18 december 2001

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