A Supermassive Black Hole in a Nearby Galaxy

ISAAC Inspects the Center of Centaurus A

A well studied galaxy with a hidden center

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". A fine photo of Centaurus A from the VLT was published last year as PR Photo 05b/00 .

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 centre 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 centre have remained largely unknown, due to the dense dust lane that completely obscures the central part of the galaxy in optical light, cf. PR Photo 08a/01 . 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), centred 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.

Peering through the dust

To look into the very centre 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 [1], 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.

The ISAAC spectra

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, cf. PR Photo 08b/01 . High-speed motions of the gas in this disk were detected (PR Photo 08c/01 ), 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.

A massive black hole

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.

More Information

The research described in this Press Release is reported in a research article ("Peering through the dust: Evidence for a supermassive Black Hole at the Nucleus of Centaurus A from VLT IR spectroscopy"), that will appear in the international research journal the Astrophysical Journal on March 10, 2001. The full article is also available on the web as astro-ph/0011059 .

Note

[1]: The team is composed by Ethan Schreier (Principal Investigator; Space Telescope Science Institute - STScI, Baltimore, USA), Alessandro Marconi (Arcetri Observatory, Italy), Alessandro Capetti (Turin Observatory, Italy), David Axon (University of Hertfordshire, United Kingdom), Anton Koekemoer (STScI, USA) and Duccio Macchetto (ESA/STScI, USA).

Technical information about the photos

PR Photo 08a/01 is reproduced from three exposures, obtained during the night of January 31 - February 1, 2000. It is a composite of three exposures in B (300 sec exposure, image quality 0.60 arcsec; here rendered in blue colour), V (240 sec, 0.60 arcsec; green) and R (240 sec, 0.55 arcsec; red). The field covered corresponds to about 80 x 80 arcsec² (395 x 395 pix², 1 pix = 0.2 arcsec). North is up and East is left. PR Photo 08b+c/01 : The original ISAAC spectra were exposed for 35 min each with an average seeing of 0.5 arcsec. Three spectrograph slits were used, but only one of these is shown here. It was centered on the nucleus of Centaurus A and oriented at 33°, measured counter-clockwise from the North direction. The spectral pixel size is 0.6 Angstrom x 0.15 arcsec (i.e., 14 km/sec x 8.3 light-year). The large and small figures cover 2300 km/s x 1665 light-years and 1150 km/s x 330 light-years, respectively.

Contacts

Alessandro Marconi
Arcetri Observatory
Firenze, Italy
Tel.: +39 055 2752250
email: marconi@arcetri.astro.it

David Axon
University of Hertfordshire
Hatfield, UK
Tel: +44-1707-284607
email: dja@star.herts.ac.uk

Ethan Schreier
Space Telescope Science Institute
Baltimore, USA
Tel: +1-410-338-4740
email: ejs@stsci.edu

Anton Koekemoer
Space Telescope Science Institute
Baltimore, USA