SPACE.com -- Supermassive Black Hole Found Inside Strange Radio Galaxy

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	Supermassive Black Hole Found Inside Strange Radio Galaxy By SPACE.com Staff posted: 10:05 am ET 09 March 2001

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A team of scientists recently stumbled upon a supermassive black hole when they calculated the nuclear mass of a much-studied nearby galaxy and arrived at the astonishing figure of 200 million times the mass of our own Sun.

Since this is far too much mass for normal stars with the luminosity of the object being studied in Centaurus A, the astronomers concluded the existence of the supermassive black hole lurking at its center.

Using the ISAAC instrument at the European Southern Observatory's Very Large Telescope (VLT), the team probed the thin disk of gas that surrounds the very center of this galaxy.

The unusual appearance Centaurus A (NGC 5128) was noticed by the famous British astronomer John Herschel in 1847 who catalogued the southern skies and made a comprehensive list of "nebulae."

Herschel could not know, however, that this beautiful and spectacular appearance is due to an opaque dust lane that covers the central part of the galaxy. This dust is likely the remains of a cosmic merger between a giant elliptical galaxy and a smaller spiral galaxy full of dust.

Radio neighbor

Centaurus A, 11 million light-years away, is even more spectacular when observed with radio telescopes. It is in fact one of the brightest radio sources in the sky (its name indicates that it is the strongest radio source in the southern constellation Centaurus).

The radio emission from the very compact center of Centaurus A, the nearest radio galaxy, exhibits strong activity. It has for some time been suspected that this powerful energy release is due to accretion of material onto a massive black hole.

Peering through the dust

The details of the center have remained largely unknown, due to the dense dust lane that completely obscures the central part of the galaxy in optical light. Infrared observations of the dust emission were carried out with the ISOCAM camera onboard the European Space Agency's (ESA) Infrared Space Observatory.

They revealed a structure extending across 16,500 light-years centered on the compact radio source, and very similar to that of a small barred galaxy. This bar may serve to funnel gas towards the active nucleus of the galaxy.

The spectroscopic observations required a high sensitivity in the infrared and excellent seeing conditions. This combination was achieved using ISAAC at VLT. Peering through the thick walls of dust enshrouding the nuclear region of Centaurus A, the astronomers succeeded in acquiring several high-quality spectra of the thin central disk.

The spectra did show the characteristic shape of a rotating disk. High-speed motions of the gas in this disk were detected, which are the hallmark of a black hole. An analysis of the rotational speed of the disk led to determination of the total mass of the material inside the disk. This showed that about 200 million solar masses of material resides inside the nuclear disk.

Infrared spectroscopy first

This discovery confirms a previous suspicion that the active nucleus of Centaurus A is powered by a supermassive black hole. It is the first time infrared spectroscopy has been used to weigh a black hole. Many other galaxies have dust-enshrouded nuclei, and the excellent capabilities of ISAAC now hold a great potential to discover and weigh many more black holes.

The finding is set for publication in the March 10 issue of Astrophysical Journal.