Black hole without a home (Forwarded)

ESA News
http://www.esa.int

14 September 2005

Black hole without a home

The detection of a super-massive black hole without a massive 'host'
galaxy is the surprising result from a large Hubble and VLT study of
quasars.

This is the first convincing discovery of such an object. One intriguing
explanation is that the host galaxy may be made almost exclusively of
'dark matter'.

A team of European astronomers has used two of the most powerful
astronomical facilities available, the NASA/ESA Hubble Space Telescope and
the ESO Very Large Telescope (VLT) at Cerro Paranal, to discover a bright
quasar without a massive host galaxy.

Quasars are powerful and typically very distant source of huge amounts of
radiation. They are commonly associated with galaxies containing an active
central black hole.
zzz
The team conducted a detailed study of 20 relatively nearby quasars. For
19 of them, they found, as expected, that these super-massive black holes
are surrounded by a host galaxy. But when they studied the bright quasar
HE0450-2958, located some 5000 million light-years away, they could not
find evidence for a host galaxy.

The astronomers suggest that this may indicate a rare case of a collision
between a seemingly normal spiral galaxy and an 'exotic' object harbouring
a very massive black hole.

With masses that are hundreds of millions times bigger than the Sun,
super-massive black holes are commonly found in the centres of the most
massive galaxies, including our own Milky Way. These black holes sometimes
dramatically manifest themselves by devouring matter that they
gravitationally swallow from their surroundings.

The best fed of these objects shine as 'quasars' (standing for
'quasi-stellar object' because they had initially been thought of as
stars).

The past decade of observations, largely with the Hubble telescope, has
shown that quasars are normally associated with massive host galaxies.
However, observing the host galaxy of a quasar is challenging work because
the quasar completely outshines the host and masks the galaxy's underlying
structure.

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

Despite the innovative techniques used, no host galaxy was seen around
HE0450-2958. This means that if any host galaxy exists, it must either be
at least six times fainter than typical host galaxies, or have a radius
smaller than about 300 light-years, i.e. 20 to 170 times smaller than
typical host galaxies (which normally have radii ranging from about 6000
to 50 000 light-years).

"With the powerful combination of Hubble and the VLT we are confident that
we would have been able to detect a normal host galaxy," said Pierre
Magain of the Université de Liège, Belgium.

The astronomers did however detect an interesting smaller cloud of gas
about 2500 light-years wide near the quasar, which they call 'the blob'.
VLT observations show this cloud to be glowing because it is bathed in the
intense radiation coming from the quasar, and not from stars inside the
cloud. Most likely, it is the gas from this cloud that feeds the
super-massive black hole, thereby allowing it to become a quasar.

The left image above shows a strongly disturbed galaxy, with all the signs
of a recent collision, near the quasar. The VLT observations show it to be
forming stars at a great rate.

"The absence of a massive host galaxy, combined with the existence of the
blob and the star-forming galaxy, lead us to believe that we have
uncovered a really exotic quasar," said Frédéric Courbin of the Ecole
Polytechnique Federale de Lausanne, Switzerland.

"There is little doubt that an increase in the formation of stars in the
companion galaxy and the quasar itself have been ignited by a collision
that must have taken place about 100 million years ago. What happened to
the putative quasar host remains unknown."

HE0450-2958 is a challenging case. The astronomers propose several
possible explanations. Has the host galaxy been completely disrupted as a
result of the collision? Has an isolated black hole captured gas while
crossing the disk of a spiral galaxy? This would require very special
conditions and would probably not have caused such a tremendous
disturbance of the neighbouring galaxy as is observed. Further studies
will hopefully clarify the situation.

Another intriguing hypothesis is that the galaxy harbouring the black hole
was almost exclusively made of 'dark matter'. It may be that what is
observed is a normal phase in the formation of a massive galaxy, which in
this case has taken place several 1000 million years later than in most
others.

For more information:

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

Frédéric Courbin/Pascale Jablonka
Laboratoire d'Astrophysique
Ecole Polytechnique Federale de Lausanne, Switzerland
Tel: +41 22 379 2418 / +41 22 379 2469
E-mail: frederic.courbin @ epfl.ch / pascale.jablonka @ obs.unige.ch

Pierre Magain
Institut d'Astrophysique de Geophysique
Universite de Liege, Belgium
Tel: +32 4366 97 53
E-mail: Pierre.Magain @ ulg.ac.be

Lutz Wisotzki
Astrophysikalisches Institut Potsdam, Germany
Tel: +49 331 7499532
E-mail: lwisotzki @ aip.de

Lars Lindberg Christensen
Hubble European Space Agency Information Centre, Garching, Germany
Tel: +49 89 3200 6306
Mobile +49 173 3872 621
E-mail: lars @ eso.org

Henri Boffin
European Southern Observatory
Tel: +49 89 3200 6222
E-mail: hboffin @ eso.org

Ray Villard
Space Telescope Science Institute, Baltimore, USA
Tel: +1 410 338 4514
E-mail: villard @ stsci.edu

The full team is: Pierre Magain, Géraldine Letawe (Univ. Liege, Belgium),
Frederic Courbin, Georges Meylan (Ecole Polytechnique Federale de Lausanne
(EPFL), Switzerland), Pascale Jablonka (EPFL; also affiliated to Univ.
Geneve), Knud Jahnke and Lutz Wisotzki (Astrophysikalisches Institut
Potsdam, Germany).

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IMAGE CAPTIONS:

[Image 1:
http://www.esa.int/esaSC/SEM3JL7X9DE_index_1.html]
This figure shows two images of quasars from a sample of 20 relatively
nearby quasars examined by a team of European astronomers using two of the
most powerful astronomical facilities available, the NASA/ESA Hubble Space
Telescope and the ESO Very Large Telescope (VLT) at Cerro Paranal.

The quasar on the left, HE0450-2958 (distance to its centre, about 5000
million light-years) does not have a massive 'host' galaxy. The quasar
HE1239-2426 on the right (1500 million light-years away), has a normal
'host' galaxy which displays large spiral arms. Although HE1239-2426 is
much closer than HE0450-2958, the host galaxy of the latter would still be
clearly visible if it was as bright as the one of HE1239-2426.

Also seen in the image to the left (above the quasar) is a strongly
disturbed galaxy, showing all the signs of a recent collision. The VLT
observations show it to be forming stars at a frantic rate. Below the
quasar a foreground star is seen.

The images were taken with the Advanced Camera for Surveys on board the
NASA/ESA Hubble Space Telescope and have been scaled to exhibit the same
linear scale.

Credits: NASA/ESA/ESO - Frédéric Courbin (Ecole Polytechnique Federale de
Lausanne, Switzerland) & Pierre Magain (Universite de Liege, Belgium)

[Image 2:
http://www.esa.int/esaSC/SEM3JL7X9DE...html#subhead1]
This image shows the quasar HE0450-2958 after advanced image processing
known as MCS-deconvolution. Thanks to this technique, it is possible to
remove the brilliant glare from the quasar itself.

The most interesting feature in the image is the nearly total absence of
starlight from a host galaxy. The processing also reveals an interesting
smaller cloud of gas next to the quasar, about 2500 light-years wide,
which scientists call 'the blob'. VLT observations show this cloud to be
glowing because it is bathed in the intense radiation coming from the
quasar. Most likely, it is the gas from this blob¹ that feeds the
super-massive black hole, thereby allowing it to shine as a quasar.

Credits: NASA/ESA/ESO - Frédéric Courbin (Ecole Polytechnique Federale de
Lausanne, Switzerland) & Pierre Magain (Universite de Liege, Belgium)