Black Hole In Search Of A Home

A team of European astronomers hasused two of the most powerful astronomical facilities available, theNASA/ESA Hubble Space Telescope and the ESO Very Large Telescope (VLT)at Cerro Paranal, to confidently claim the discovery of a bright quasarwithout a massive host galaxy. Quasars are powerful and typically verydistant source of prodigious amounts of radiation. They are commonlyassociated with galaxies containing an active central black hole.

Theteam conducted a detailed study of 20 relatively nearby quasars. For 19of them, they found, as expected, that these super massive black holesare surrounded by a host galaxy. But when they studied the brightquasar HE0450-2958, located some 5 billion light-years away, they couldnot find evidence for a host galaxy. This, the astronomers suggest, mayindicate a rare case of a collision between a seemingly normal spiralgalaxy and an exotic object harbouring a very massive black hole.

Withmasses up to hundreds of millions that of the Sun, super massive blackholes are commonly found in the centres of the most massive galaxies,including our own Milky Way. These black holes sometimes dramaticallymanifest themselves by devouring matter that they gravitationallyswallow from their surroundings. The best fed of these shine as quasars(the name quasar is a contraction of quasi-stellar object, as they hadinitially been confused with stars).

The past decade ofobservations, largely with the Hubble telescope, has shown that quasarsare normally associated with massive host galaxies. However, observingthe host galaxy of a quasar is challenging work because the quasarcompletely outshines the host and masks the galaxy’s underlyingstructure.

To overcome this problem, the astronomers devised anew and highly efficient strategy. Combining Hubble’s ultra sharpimages and spectroscopy from ESO’s VLT they observed their sample of 20quasars at the same time as a reference star. The star served as areference pinpoint light source that was used to disentangle the quasarlight from any possible light from an underlying galaxy.

Despitethe innovative techniques used, no host galaxy was seen aroundHE0450-2958. This shows that if any host galaxy exists, it must eitherbe at least six times fainter than typical host galaxies, or have aradius smaller than about 300 light-years, i.e. 20 to 170 times smallerthan typical quasar host galaxies (which normally have radii rangingfrom about 6,000 to 50,000 light-years).

“With the powerfulcombination of Hubble and the VLT we are confident that we would havebeen able to detect a normal host galaxy”, says Pierre Magain(Université de Liège, Belgium), member of the team of astronomers whoconducted the study. “We must therefore conclude that, contrary to ourexpectations, this bright quasar is not surrounded by a massive galaxy”.

Theastronomers did however detect an interesting smaller cloud of gasabout 2,500 light-years wide, which they call “the blob”, just next tothe quasar. VLT observations show this cloud to be glowing because itis bathed in the intense radiation coming from the quasar, and not fromstars inside the cloud. Most likely, it is the gas from this cloud thatfeeds the super massive black hole, thereby allowing it to become aquasar.

In the Hubble image, a strongly disturbed galaxy, showingall the signs of a recent collision, is seen near the quasar. The VLTobservations show it to be forming stars at a frantic rate. “Theabsence of a massive host galaxy, combined with the existence of theblob and the star-forming galaxy, lead us to believe that we haveuncovered a really exotic quasar”, says team member Frédéric Courbin(Ecole Polytechnique Federale de Lausanne, Switzerland). “There islittle doubt that an increase in the formation of stars in thecompanion galaxy and the quasar itself have been ignited by a collisionthat must have taken place about 100 million years ago. What happenedto the putative quasar host remains unknown.”

HE0450-2958constitutes a challenging case. The astronomers propose severalpossible explanations. Has the host galaxy been completely disrupted asa result of the collision? It is hard to imagine how that could happen.Has an isolated black hole captured gas while crossing the disk of aspiral galaxy? This would require very special conditions and wouldprobably not have caused such a tremendous disturbance of theneighbouring galaxy as is observed. Further studies will hopefullyclarify the situation.

Another intriguing hypothesis is that thegalaxy harbouring the black hole was almost exclusively made of darkmatter. It may be that what is observed is a normal phase in theformation of a massive galaxy, which in this case has taken placeseveral billion years later than in most others.

The paper on HE0450-2958 is published in the September 15, 2005 issue of the journal Nature.

Notes:

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.

Theteam is composed of Pierre Magain, Géraldine Letawe (Univ. Liege,Belgium), Frederic Courbin, Georges Meylan (Ecole PolytechniqueFederale de Lausanne (EPFL), Switzerland), Pascale Jablonka (EPFL; alsoaffiliated to Univ. Geneve), Knud Jahnke and Lutz Wisotzki(Astrophysikalisches Institut Potsdam, Germany).