At the center of most of the major galaxies in the Universe, there is a supermassive black hole. But the question is: which came first ? The black hole, which frantically devours the matter all around it, or the galaxy in which it resides? Thanks to observations made using the Visir camera (1) installed on the ESO's Very Large Telescope, an international team (2) has recently come up with a whole new scenario: black holes may well be "building" the galaxies that host them. These results are published in the November 30, 2009 issue of Astronomy and Astrophysics.

"The question of which came first, the chicken or the egg, a galaxy or the black hole at its center, is one of the most hotly debated subjects in Astrophysics today,” says lead author of the study, David Elbaz, from the Astrophysics of Multi-scale Interactions Laboratory (AIM: a CEA/CNRS/Université Paris 7 Joint Research Unit). Our study suggests that supermassive black holes can trigger a surge of star formation, or even create whole new galaxies. This missing link may help us to understand why galaxies that host the largest black holes also have the most stars."

The team's conclusions are based on careful observation of a very peculiar object, the quasar HE0450-2958, located 5 billion light-years from the Earth. To date, no host galaxy has ever been detected for this quasar (3), commonly known as "the naked quasar" for this reason. Believing that the host galaxy might be hidden behind large amounts of dust, the astrophysicists used Visir, the mid-infrared camera Visir installed on the ESO's Very Large Telescope (VLT). What they actually observed is far different from their original expectations, but something much more surprising: they did not detect any dust clouds. Instead, they discovered that the nearest galaxy to the quasar has been producing stars at an astounding rate.

Although there is not a trace of a star in the area immediately surrounding the quasar, this "companion" galaxy is extremely rich in very young, very bright stars: stars form within it at a rate of approximately 350 Suns a year, a hundred times faster than in a typical galaxy in the Local Universe.

The astrophysicists also discovered a "bridge" of matter between the quasar and its companion galaxy: matter appears to be streaming out of the quasar's black hole towards this galaxy at very high speed. The high-energy injection of this matter into the galaxy suggests that it is the quasar itself that is causing this surge of star formation. If this is the case, the galaxy would have evolved from a cloud of gas hit by the jet of highly energetic particles emerging from the quasar.

"The ‘naked quasar’ and its companion galaxy are bound to merge in the future," David Elbaz explains: "the quasar is moving at a speed of only a few tens of thousands of km/hour with respect to the galaxy and the two objects are only about 22,000 light-years away from each other. Whether or not the quasar is 'naked', it will eventually be "dressed" when it merges with its companion galaxy, the stars in which have mostly been formed by the quasar."

These observations change our understanding of this type of system and allow us to develop a new paradigm. Perhaps this is the missing link that will explain why the mass of black holes is larger in galaxies that contain the most stars (4).

But where does the power of the driver that feeds the supermassive black hole come from? The team explored a new explanation that still needs to be confirmed: the black hole may be fed matter by filaments of intergalactic gas.

The astrophysicists will now search for similar objects in other systems. Future generations of instruments, including the James Webb Space Telescope, operated by NASA/ESA, and in which the CEA-IRFU and the INSU-CNRS team are involved, or Alma, the international millimetric and submillimetric interferometer (Atacama Large Millimeter/submillimeter Array) and the E-ELT (the ESO's European Extremely Large Telescope) in which a number of INSU-CNRS teams are involved, will make it possible to study objects like this that are much further away with a great deal more precision, in the bid to uncover a possible link between the formation of supermassive black holes and galaxy formation in the Distant Universe.

Further information :

This study is published in Astronomy & Astrophysics: “Quasar induced galaxy formation: a new paradigm?” by Elbaz et al.

About the Visir camera :
http://irfu.cea.fr/Phocea/Vie_des_labos/Ast/ast_technique.php?id_ast=1027

More illustrations (photos + film) :
http://www.eso.org/public/outreach/press-rel/pr-2009/pr-46-09.html

(1) The Visir camera was developed by the CEA/IRFU (Institute of Research into the Fundamental Laws of the Universe), the ESO (European Southern Observatory) and the Netherlands Foundation for Research in Astronomy.

(2) The members of this team are: D. Elbaz, E. Pantin, from the Laboratoire d’astrophysique des interactions multi-échelle, (CEA/DSM-CNRS-Université Paris Diderot-Paris 7); K. Jahnke, Max-Planck-Institut für Astronomie, Germany; D. Le Borgne, Institut d’Astrophysique de Paris, (CNRS, Université Pierre et Marie Curie); G. Letawe, Institut Astrophysique et Géophysique, Liège University, Belgium.

(3) The term "Quasar", or "Quasi-star" (also known as "QSO" or "Quasi-Stellar Objects"), refers to the core of a galaxy formed by a supermassive black hole. Its center is intensely bright, hence its name.

(4) The majority of the galaxies in the Local Universe contain a massive black hole at the center, which has a mass equal to around 1/700th of the mass of the stellar bulge. The reason for this relation between the mass of the black hole and stellar mass is one of the most debated subjects in modern Astrophysics.