Images of three active galaxies that were observed with ISAAC during the present programme. They show NGC 1097 (top) and the central areas of NGC 1808 (middle) and NGC 5728 (bottom). 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. Photo: ESO

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 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...

Central black holes in galaxies
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, do contain central black holes with masses from millions to billions of solar masses.

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.

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.

The role of "nuclear bars"
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, but the responsible dynamical processes may possibly be somewhat different. The below image shows the various components that are discussed here in a schematic way.

This is a schematic drawing of the various components of a double-barred galaxy, as mentioned in the text. Photo: ESO

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 with the ESO Very Large Telescope (VLT) has now brought new, important insights about these nuclear bars.

ISAAC spectra of innermost regions of three active galaxies
The scientists embarked upon a project with the goal of investigating in detail the motions of stars in the central regions of some active, comparatively "nearby" galaxies. As the innermost regions of such galaxies are usually quite dusty, the observations were carried out in infrared light that penetrates the dust clouds much better than does visible light. Thanks to its high efficiency and excellent imaging quality and spectral resolution, the VLT Infrared Spectrometer And Array Camera (ISAAC) is superbly suited for such work.

Several galaxies with active centres were selected for the first observing runs in 1999 and 2000, among these NGC 1097, NGC 1808 and NGC 5728. Infrared spectra were obtained in the 2.3 µm wavelength region in which a number of molecular spectral bands are seen. They are caused by carbon monoxide (¹²CO) molecules in the atmospheres of the stars located near the centres of the galaxies.

Stellar motions
By measuring the exact wavelengths of these molecular bands, it is possible to determine (from the Doppler effect), first, the mean velocity of the stars and, secondly, the spread in this velocity (known as the "velocity dispersion"; right). The first value reflects the general speed with which the stars move around the central black hole. The second indicates the extent to which the individual stellar motions deviate from that mean value.

The comparison with the flight of a swarm of bees is useful: the mean velocity tells how fast the swarm moves forward as a whole - this is the ordered motion of the group. The second value instead indicates how much (or how fast) the individual bees move around inside the swarm - this is the spread in random velocities among the bees.

Dynamical temperature is another concept defined by velocity dispersion. A warm gas is a gas where the molecules swarm around at high random speeds, while the molecules in a cold gas have low velocity dispersion. Astronomers often borrow this terminology and refer to stellar systems with low velocity dispersions as "dynamically cool systems".

Confirming the "nuclear bars"...
When "mapped" over the entire central area of a galaxy, these stellar velocity values provide detailed information about the gravitational field, and thus the mass distribution in the innermost region of the galaxy.

The ISAAC observations did confirm the presence of "nuclear bars" in NGC 1097, NGC 1808 and NGC 5728. They also showed that these bars are truly "decoupled" stellar systems - their motions are only determined by the mass distribution in that area.

...and discovering a "dynamically cool" stellar system!
However, the astronomers were very surprised to discover that in all three galaxies, the velocity dispersion is decreasing towards the centre, exactly contrary to what is predicted by simple models. The likely reason is the presence in the central region of a "newborn" system of stars whose individual velocities have not yet had time to "heat up".

The project leader, Eric Emsellem explains: "Slower individual stellar motions correspond to a lower 'dynamical temperature' of the stellar system in this innermost region. We interpret this as evidence for a recent infall of gas that was induced by the nuclear bar. This has created a new gaseous disk at the centre of the galaxy, which has given birth to new stars. They all move around the black hole with more or less the same circular velocity as the gas from which they were born".

Agreement between observations and models
This interesting scenario is supported by recent, extensive model computations by the team.

In these computer models, large numbers of "stars" (mass points) move in a model galaxy with both a large and a nuclear bar, as observed in the three galaxies. Herve Wozniak refers to them as "self-consistent N-body simulations" and explains why the team is enthusiastic: "When our models also include star formation in the gas in the central region, a new, "dynamically cool" component of young stars emerges and mixes with the old stellar population".

He goes on: "The light from those young stars is superposed on that from the older ones in that area. Because of this, the overall "velocity dispersion" in the central region is then smaller than what it is further out. This is exactly as we observed in the ISAAC spectra obtained in the present programme".

Eric Emsellem points out that such a "dynamically cold" system is unstable and cannot last very long. "Soon it will "heat up" due to complex dynamical processes. It is quite possible that some of these stars will eventually end up as food for the hungry black hole.."

Prospects
With these new high-resolution infrared observations of the structure and the objects in the innermost regions of active galaxies, ISAAC and the VLT are paving the way for future studies of the processes that take place in the immediate neighbourhood of the central black holes.

More active galaxies will now be observed with this method and it will be interesting to see if the presently discovered "cool" and young stellar systems represent a common phenomenon or not.