A Hungry Quasar Caught in the Act
The VLT Secures Spectacular Image of Distant Gravitational Interaction
31 May 2001
A new image of a distant quasar (the luminous core of an "active" galaxy) shows that it is engaged in a gravitational battle with its neighbouring galaxies. It also provides information on how supermassive black holes present in the center of quasars are fed. Using the FORS2 multi-mode instrument at the ESO 8.2-metre VLT KUEYEN telescope on Paranal (Chile), a team of German astronomers  obtained a spectacular image of the close and complex environment of the distant quasar "HE 1013-2136", located some 10 billion light-years away . The remarkable structures revealed in this photo lend support to the hypothesis that quasar activity is connected to gravitational interaction between galaxies, already at this early epoch of the Universe (about 5 billion years after the Big Bang).
Feeding the Black Hole
"Quasars" (Quasi-Stellar Objects) were first discovered by Dutch-American astronomer Maarten Schmidt in 1963 as distant, energetic objects of star-like appearance. Since then, more than 15,000 quasars have been found and we now know that they are the luminous cores at the heart of distant galaxies.
Such "Active Galactic Nuclei (AGN)" are thought to host Supermassive Black Holes of up to one billion solar masses at their centres. Black Holes represent the densest possible state of matter; if the Earth were to become one, it would measure no more than a few millimetres across. The Black Hole in a galaxy gobbles up the gas and dust of its host, a process that efficiently powers the luminous core that we observe as a point-like "quasar".
A Black Hole must be continuously fed to remain active. During an active phase of typically 100 million years, the Black Hole in a quasar swallows material with a total weight of up to 10 solar masses every year. This may be predominantly in the form of gas and dust that happen to come too close to the hole.
Our own galaxy, the Milky Way, is also very likely to harbour a Black Hole at its center. However, this hole apparently lacks material to swallow and is somewhat starved - in any case it is much less active than some holes in other galaxies.
A key question in connection with the quasar phenomenon is therefore to understand how a large amount of material can be brought towards the center of the host galaxy. Most astronomers believe that disturbances caused by gravitational interaction with neighbouring galaxies constitute a triggering mechanism for fueling the centers of Active Galaxies. The efficiency of such tidal interactions taking place of course depends on how many galaxies are located in the immediate neighbourhood of the quasar as well as on the relative velocities of the quasar host and its companions.
Searching for quasar companions
Since some time, astronomers have therefore been searching for clear evidence for a connection between gravitational interaction and the quasar phenomenon. However, quasars are very bright objects and their light easily outshines all nearby objects. Any companion galaxies and structural features that may indicate interaction are therefore hard to detect.
While observations with the Hubble Space Telescope (HST) have much improved our knowledge of the interaction-activity connection in some relatively nearby quasars, it has been difficult to probe the same phenomenon in more distant quasar environments. Such studies clearly require larger telescopes.
The observations of the quasar HE 1013-2136 presented here result from a new programme that addresses this issue at earlier cosmic epochs. This 17-mag object is seen in the southern constellation Hydra (The Water Snake) and is located at a distance of about 10 billion light years (the redshift is z = 0.785)
ESO Press Photo eso0122 shows an image of HE 1013-2136 and its immediate surroundings, obtained with the FORS2 multi-mode instrument at the 8.2-metre VLT KUEYEN telescope under very good seeing conditions. The image resolution is about 0.6 arcsec, or about 10,000 light-years at the distance of the quasar. The image has been further sharpened by means of image processing software (the Lucy algorithm) in ESO Press Photo eso0122, now also showing the distribution of objects very close to the bright quasar image. This impressively illustrates the light gathering and resolution power of the VLT.
Tidal forces at HE 1013-2136
The quasar is the point-like object at the center of the images. It is embedded within a complex structure that mainly consists of two arc-like and knotty tails extending in different directions. Such tails are well-known from nearby galaxy interactions, c.f. NGC 6872/IC4970 and are a consequence of tidal forces in the gravitational field of the galaxies.
The astronomers believe that the two tidal tails result from a dramatic interaction between the quasar host galaxy and one or more of the close companion galaxies. The longer, southern tail extends over more than 150,000 light years, one-and-a-half times the diameter of the Milky Way. In many respects this distant interaction resembles the well known Antenna Galaxies (see the Hubble image), where two nearby galaxies distort each other in a gravitational dance.
Galaxy mergers in the young Universe
In the case of HE 1013-2136, a number of knots can be seen along both tidal tails. In particular, the object just below the quasar image, most easily seen in the sharpened image (ESO Press Photo eso0122), lies at a projected distance of only 20,000 light years. This is about two-thirds of the distance from the Earth to the center of the Milky Way galaxy. This object is most likely a companion that is interacting with the quasar host.
Recent observations of nearby quasars have revealed that they mostly reside in elliptical galaxies. Numerical simulations suggest that such galaxies can be formed by successive mergers of spiral galaxies.
Klaus Jäger and his colleagues point out that "with the VLT observations of HE 1013-2136, we may be directly witnessing such a dramatic merger of galaxies. The special significance of this observation is the great distance and hence the comparably early time at which this happens, when the Universe was about one third as old as it is now". He adds: "This particular galaxy will most probably evolve into the same type of elliptical quasar host galaxy that we observe much nearer to us, that is, at much later times".
Klaus J. Fricke