Other
Scientific paper
Mar 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001eso..pres....4.&link_type=abstract
ESO Press Release, 03/2001
Other
Scientific paper
ISAAC Inspects the Center of Centaurus A
Summary
The nearby galaxy Centaurus A harbours a supermassive black hole at its centre .
Using the ISAAC instrument at the ESO Very Large Telescope (VLT) , an international team of astronomers [1] 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 centre of Centaurus A .
PR Photo 08a/01 : Visual image of the centre of Centaurus A . PR Photo 08b/01 : ISAAC spectrum of the centre of Centaurus A . PR Photo 08c/01 : The corresponding rotation curve from which the mass of the black hole was deduced. A well studied galaxy with a hidden center
ESO PR Photo 08a/01
ESO PR Photo 08a/01
[Preview - JPEG: 352 x 400 pix - 160k] [Normal - JPEG: 704 x 800 pix - 376k]
Caption : PR Photo 08a/01 shows a small area in the direction of the heavily obscured centre of the peculiar radio galaxy Centaurus A , as seen in visual light. It measures about 80 x 80 arcsec 2 , or 4400 x 4400 light-year 2 at the distance of this galaxy, and has been reproduced from exposures made with the FORS2 multi-mode instrument at the 8.2-m VLT KUEYEN telescope at Paranal. The full field may be seen in PR Photo 05b/00. Technical information about this photo is available below.
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
ESO PR Photo 08b/01
ESO PR Photo 08b/01
[Preview - JPEG: 400 x 303 pix - 216k] [Normal - JPEG: 800 x 606 pix - 572k] [Hires - JPEG: 2274 x 3000 pix - 4.0M]
Caption : PR Photo 08b/01 shows 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 ([FeII]; rest wavelength 1256.68 nm) and in Hydrogen (Paschen-Beta; 1281.81 nm) 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. The light from the receding edge of the disk is Doppler-shifted towards the red (to the right) and the light from the part of the disk approaching us is shifted to the left. This may be better seen in the inserted enlargements. Therefore the inclined disk shows a tilted spectrum. These motions may be represented in a rotation curve, cf. PR Photo 08c/01 . There are other emitting areas above and below the nucleus, especially in the Paschen-Beta line. Technical information about these photos is available below.
ESO PR Photo 08c/01
ESO PR Photo 08c/01
[Preview - JPEG: 341 x 400 pix - 56k] [Normal - JPEG: 682 x 800 pix - 132k]
Caption : PR Photo 08c/01 shows the rotation curve (velocity vrs. distance from the centre) of the disk surrounding the black hole at the centre of Centaurus A . From the ISAAC spectrum displayed in PR Photo 08b/01 , the `average' gas velocities along the slit direction can be derived. Position `0' on the horizontal axis indicates the exact position of the galaxy nucleus; at the distance of Centaurus A , 1 arcsec corresponds to 55.5 light-years (17 pc). The blue triangles and the red squares correspond to emission lines from singly ionized Iron atoms ([Fe II]) and Hydrogen (Paschen-Beta), respectively. The high velocities are the hallmark of a central black hole. The thick solid line represents the expected velocities, assuming the presence of a 200 million solar-mass black hole at the centre. Technical information about these photos is available below.
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 galaxie
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