More than 150 years after British astronomer John Herschel spied Centaurus A in the southern sky, the spectacular galaxy (right) is still captivating scientists. Radio telescope observations have shown it to be the strongest radio source in the Centaurus constellation and, at 11 million light-years away, it is the radio galaxy closest to us. Now new evidence indicates that Centaurus A has been keeping a dark secret. Reporting in the March 10 issue of the Astrophysical Journal, an international team of astronomers concludes that the center of the galaxy houses a supermassive black hole.

With the help of the ISAAC instrument at the European Southern Observatory's Very Large Telescope (VLT), Ethan Schreier of the Space Telescope Science Institute in Baltimore, Md., and his colleagues were able to look through the opaque dust lane that enshrouds the center of the galaxy. Their observations revealed a thin, rotating disk of gas. By analyzing the rotational speed of the nuclear disk, the team was able to determine that the mass of the material residing inside the disk amounts to about 200 million times the mass of our sun.

Such enormous mass, they realized, cannot be accounted for by normal stars, which would impart far greater luminosity than that observed. A more likely explanation for Centaurus A's dark, weighty center is that it is a supermassive black hole. The new findings confirm earlier suspicions based on unusually strong radio emissions emanating from the galactic center, and suggest that ISAAC, which makes measurements using infrared spectroscopy, has the potential to identify and weigh many more black holes in the future.