Statistics – Computation
Scientific paper
Aug 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001eso..pres...18.&link_type=abstract
ESO Press Release, 08/2001
Statistics
Computation
Scientific paper
ISAAC Finds "Cool" Young Stellar Systems at the Centres of Active Galaxies
Summary
Supermassive Black Holes are present at the centres of many galaxies, some weighing hundreds of millions times more than the Sun. These extremely dense objects cannot be observed directly, but violently moving gas clouds and stars in their strong gravitational fields are responsible for the emission of energetic radiation from such "active galaxy nuclei" (AGN) .
A heavy Black Hole feeds agressively on its surroundings . When the neighbouring gas and stars finally spiral into the Black Hole, a substantial fraction of the infalling mass is transformed into pure energy. However, it is not yet well understood how, long before this dramatic event takes place, all that material is moved from the outer regions of the galaxy towards the central region.
So how is the food for the central Black Hole delivered to the table in the first place? To cast more light on this central question, a team of French and Swiss astronomers [1] has carried out a series of trailblazing observations with the VLT Infrared Spectrometer And Array Camera (ISAAC) on the VLT 8.2-m ANTU telescope at the ESO Paranal Observatory.
The ISAAC instrument is particularly well suited to this type of observations. Visible light cannot penetrate the thick clouds of dust and gas in the innermost regions of active galaxies, but by recording the infrared light from the stars close to the Black Hole , their motions can be studied. By charting those motions in the central regions of three active galaxies (NGC 1097, NGC 1808 and NGC 5728), the astronomers were able to confirm the presence of "nuclear bars" in all three. These are dynamical structures that "open a road" for the flow of material towards the innermost region.
Moreover, the team was surprised to discover signs of a young stellar population near the centres of these galaxies - stars that have apparently formed quite recently in a central gas disk. Such a system is unstable, however, and will soon disrupt. At some moment, many of those young stars may get too close to the monster in the centre and suffer an unhappy fate...
PR Photo 25a/01 : The active galaxy NGC 1097 (R-band image) PR Photo 25b/01 : The active galaxy NGC 1808 (H-band image) PR Photo 25c/01 : The active galaxy NGC 5728 (K-band image) PR Photo 25d/01 : Schematic drawing of the various structural components mentioned in the text. PR Photo 25e/01 : ISAAC spectrum (2.3 µm) of the central region of NGC 1808 PR Photo 25f/01 : Stellar motions at the centre of NGC 1808 Central black holes in galaxies
ESO PR Photo 25a/01
ESO PR Photo 25a/01 [Preview - JPEG: 400 x 489 pix - 39k] [Normal - JPEG: 800 x 977 pix - 296k]
ESO PR Photo 25b/01
ESO PR Photo 25b/01 [Preview - JPEG: 400 x 499 pix - 40k] [Normal - JPEG: 800 x 997 pix - 168k]
ESO PR Photo 25c/01
ESO PR Photo 25c/01 [Preview - JPEG: 400 x 488 pix - 47k] [Normal - JPEG: 800 x 975 pix - 384k]
Caption : Photos of three active galaxies that were observed with ISAAC during the present programme. They show NGC 1097 (R-band; Photo 25a/01) and the central areas of NGC 1808 (H-band; Photo 25b/01) and NGC 5728 (K-band; Photo 25c/01). The bar-like structures and the luminous centres where the Black Holes are located are well visible - they are discussed in the text. The distances to these galaxies are approximately 55, 35 and 120 million light-years, respectively; the local scales are indicated in the photos. Technical information about these photos is available below.
Recent research with space- and ground-based astronomical telescopes indicate that there are very heavy Black Holes at the centres of most galaxies. There is also general agreement among scientists that many of the closest neighbours of our own Milky Way Galaxy, for example the large spiral Andromeda Galaxy and the peculiar Centaurus A galaxy (cf. ESO PR 04/01 ), do contain central black holes with masses from millions to billions of solar masses [2].
Black Holes have an extremely intense gravitational field and as light can not escape from them, they are dark and invisible. Indeed, with presently available observational tools, they cannot be detected directly, only by effects resulting from interaction with their immediate surroundings.
A small fraction of the black holes in galaxies are thus revealed by the spectacular activity they trigger in the central part of their hosts. Attracted by that heavy object, enormous quantities of gas (mostly hydrogen) spiral inwards towards the black hole. A disk-shaped structure forms in which the gas moves ever faster around the black hole while enormous amounts of energy are radiated at all wavelengths [3]. Getting the food to the Black Hole
A great debate is now going on among scientists about how exactly the black holes are "fed". How is the gas first transported into the disk to fuel the seemingly insatiable supermassive black hole? It is still not well understood how the gas is moved from the outside to within a distance of 1000 light-years of the centre.
Various violent processes have been mentioned in this context, like the merger of galaxies. A fine example of such an event was recently observed at the distant quasar HE 1013-2136 with the ESO Very Large Telescope, cf. ESO PR 13/01. The role of "nuclear bars"
ESO PR Photo 25d/01
ESO PR Photo 25d/01 [Preview - JPEG: 364 x 400 pix - 89k] [Normal - JPEG: 727 x 800 pix - 264k]
Caption : PR Photo 25d/01 is a schematic drawing of the various components of a double-barred galaxy, as mentioned in the text.
Another possibility to move the gas inwards is the presence of bar-like structures at the centres of some galaxies, so-called "nuclear bars" . They look like small versions of the well-known, beautiful large-scale bar-like structures seen in galaxies like NGC 1365 (cf. ESO PR Photos 08a-e/99 ), but the responsible dynamical processes may possibly be somewhat different. Photo 25d/01 shows the various components that are discussed here in a schematic way.
Acting as a gravitational brush, a bar that is thousands of light-years long efficiently "sweeps" the gas in that galaxy towards its core. When sufficient material has collected there, that matter may become dynamically "decoupled", forming a smaller bar at the centre of the larger "primary" bar. Such a "nuclear bar" may then, at least in theory, take over and let the gas move further inwards towards the central black hole.
Until now, nuclear bars have mostly been seen on detailed images as small, elongated structures embedded within the larger primary bars - such structures may ressemble a "Russian doll". In addition, nuclear bars have been detected indirectly due to their gravitational effects, by means of very accurate measurements of the motions of the gas in the central region in a few galaxies.
A first observational campaign by a team of French and Swiss astronomers [1] with the ESO Very Large Telescope (VLT) has now brought new, important insights about these nuclear bars. ISAAC spectra of the innermost regions of three active galaxies
ESO PR Photo 25e/01
ESO PR Photo 25e/01 [Preview - JPEG: 400 x 424 pix - 40k] [Normal - JPEG: 800 x 847 pix - 256k]
ESO PR Photo 25f/01
ESO PR Photo 25f/01 [Preview - JPEG: 400 x 241 pix - 40k] [Normal - JPEG: 800 x 401 pix - 112k]
Caption : PR Photo 25e/01 is a reproduction of a long-slit ISAAC spectrum of the central region of the active galaxy NGC 1808 . It is in the 2.3 µm spectral region and the wavelength increases towards right. Several strong, vertical bands are seen; they are caused by CO-molecules in the atmospheres of the stars in this area. The bright band at the centre corresponds to the nucleus of the galaxy within which the central black hole is located. The characteristic S-shape is a result of the rotation of the stars around this centre, due to the Doppler effect. Technical information about this photo is available below. In the left half of PR Photo 25f/01 , the measured velocities (ordinate) of the stars near the centre of NGC 1808 are plotted at different distances from the nucleus (abscissa). The right hal
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