Supermassive black hole found in nearby galaxy
Posted: March 9, 2001

The nearby galaxy Centaurus A harbors a supermassive black hole at its center.

Using the ISAAC instrument at the ESO Very Large Telescope (VLT), an international team of astronomers has peered right through the spectacular dust lane of the peculiar galaxy Centaurus A, located approximately 11 million light-years away.

They were able to probe the thin disk of gas that surrounds the very center of this galaxy. The new measurements show that the compact nucleus in the middle weighs more than 200 million solar masses!

This is too much just to be due to normal stars. The astronomers thus conclude the existence of a supermassive black hole lurking at the center of Centaurus A.

A small area in the direction of the heavily obscured center of the peculiar radio galaxy Centaurus A, as seen in visual light. Photo: ESO
The galaxy Centaurus A (NGC 5128) is one of the most studied objects in the southern sky. The unique appearance of this galaxy was already 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 remain of a cosmic merger between a giant elliptical galaxy, and a smaller spiral galaxy full of dust.

Centaurus A 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). At a distance of merely 11 million light-years, it is also the nearest radio galaxy. The radio emission from the very compact center 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.

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. Observations of the dust emission in the mid-infrared spectral region were carried out with the ISOCAM camera onboard the ESA Infrared Space Observatory. They revealed a structure extending over 5 arcmin (16,500 light-years or 5 kpc), 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.

To look into the very center of the galaxy, the observations must be carried out at wavelengths longer than those of visual light, e.g., in the infrared spectral region. This is because the dust absorbs much less the infrared radiation. Infrared observations of the innermost regions (of Centaurus A (on an arcsec scale) were recently done by a team of astronomers from Italy, UK and USA, by means of the multi-mode ISAAC instrument on the ESO Very Large Telescope (VLT) at Paranal Observatory.

In fact, the team started their infrared studies of this galaxy already in 1997, using the NICMOS camera on board the Hubble Space Telescope (HST). That close view of the galaxy nucleus revealed a thin gaseous disk of material close to the center, which looked very much like an accretion disk that was feeding material into a central black hole. The HST image prompted further spectroscopic observations to probe the rotation of the disk, and thus to measure the mass of the central object.

Two wavelength regions of one of the infrared ISAAC spectra of the center of Centaurus A. The direction of the long spectrograph slit is vertical and the dispersion (wavelength) direction is horizontal; longer wavelengths are towards the right. The two emission lines shown originate in singly ionized Iron and in Hydrogen and both are clearly tilted. This is due to the rapid rotation of the accretion disk surrounding the supermassive black hole in the center of the galaxy. Photo: ESO
The spectroscopic observations required both 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 exposure time for each spectrum was (about) 35 min.

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 leads 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.

The astronomers quickly realized that this enormous mass within the central region cannot be caused by normal stars, as it would then be much more luminous. Instead they conclude that the most conservative explanation for the dark, central mass concentration observed in Centaurus A is indeed a supermassive black hole. The most likely mass of this "central beast" is then about 200 million times the mass of our Sun.

This discovery confirms the 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.