The sky around the quasar QSO J2246-6015
ESO’s Very Large Telescope probes growth of galaxies
Astronomers using ESO’s Very Large
Telescope have spotted a distant galaxy hungrily snacking on nearby gas.
Gas is seen to fall inwards towards the galaxy, creating a flow that
both fuels star formation and drives the galaxy’s rotation. This is the
best direct observational evidence so far supporting the theory that
galaxies pull in and devour nearby material in order to grow and form
stars. The results will appear in the 5 July 2013 issue of the journal
Science.
Astronomers have always suspected that galaxies grow by pulling in
material from their surroundings, but this process has proved very
difficult to observe directly. Now ESO’s Very Large Telescope has been
used to study a very rare alignment between a distant galaxy [1]
and an even more distant quasar — the extremely bright centre of a
galaxy powered by a supermassive black hole. The light from the quasar
passes through the material around the foreground galaxy before reaching
Earth, making it possible to explore in detail the properties of the
gas around the galaxy [2]. These new results give the best view so far of a galaxy in the act of feeding.
“This kind of alignment is very rare and it has allowed us to make unique observations,”
explains Nicolas Bouché of the Research Institute in Astrophysics and
Planetology (IRAP) in Toulouse, France, lead author of the new paper. “We
were able to use ESO’s Very Large Telescope to peer at both the galaxy
itself and its surrounding gas. This meant we could attack an important
problem in galaxy formation: how do galaxies grow and feed star
formation?”
Galaxies quickly deplete their reservoirs of gas as they create new
stars, and so must somehow be continuously replenished with fresh gas to
keep going. Astronomers suspected that the answer to this problem lay
in the collection of cool gas from the surroundings by the gravitational
pull of the galaxy. In this scenario, a galaxy drags gas inwards, which
then circles around the galaxy, rotating with it before falling in.
Although some evidence of such accretion had been observed in galaxies
before [3], the motion of the gas and its other properties had not been fully explored up to now.
The astronomers used two instruments known as SINFONI and UVES [4],
both of which are mounted on ESO’s VLT at the Paranal Observatory in
northern Chile. The new observations showed both how the galaxy itself
was rotating, and revealed the composition and motion of the gas outside
the galaxy.
“The properties of this vast volume of surrounding gas were
exactly what we would expect to find if the cold gas was being pulled in
by the galaxy,” says co-author Michael Murphy (Swinburne University of Technology, Melbourne, Australia). “The
gas is moving as expected, there is about the expected amount and it
also has the right composition to fit the models perfectly. It’s like
feeding time for lions at the zoo — this particular galaxy has a
voracious appetite, and we’ve discovered how it feeds itself to grow so
quickly.”
Astronomers have already found evidence of material around galaxies
in the early Universe, but this is the first time that they have been
able to show clearly that the material is moving inwards rather than
outwards, and also to determine the composition of this fresh fuel for
future generations of stars. Without the quasar’s light to act as a
probe this surrounding gas would be undetectable.
“In this case we were lucky that the quasar happened to be in
just the right place for its light to pass through the infalling gas.
The next generation of extremely large telescopes will enable studies
with multiple sightlines per galaxy and provide a much more complete
view,” concludes co-author Crystal Martin (University of California Santa Barbara, USA).
Notes
[1] This galaxy was detected in the 2012 redshift z ~ 2 SINFONI survey called the SINFONI Mg II Program for Line Emitters (SIMPLE).
The quasar in the background is called QSO J2246-6015, or HE 2243-60
and the galaxy itself lies at a redshift of 2.3285 — meaning that we are
seeing it when the Universe was just about two billion years old.
[2] When the quasar light passes through the gas
clouds some wavelengths are absorbed. The patterns of these absorption
fingerprints can tell astronomers much about the motions and chemical
composition of the gas. Without the quasar in the background far less
information would have been be obtained — the gas clouds do not shine
and are not visible in direct images.
[3] Some hints that galaxies were being fed from
material around them were found in earlier data, including that from the
SAURON survey (LINK to SAURON ANN of 3 July).
[4] SINFONI is the Spectrograph for INtegral Field
Observations in the Near Infrared, while UVES is the Ultraviolet and
Visual Echelle Spectrograph. Both are mounted on ESO’s Very Large
Telescope. SINFONI revealed the motions of the gas in the galaxy itself
and UVES the effects of the gas around the galaxy on the light coming
from the more distant quasar.
More information
This research was presented in a paper
entitled “Signatures of Cool Gas Fueling a Star-Forming Galaxy at
Redshift 2.3”, to appear in the 5 July 2013 issue of the journal Science.
The team is composed of N. Bouché (CNRS; IRAP, France), M. T. Murphy
(Swinburne University of Technology, Melbourne, Australia), G. G.
Kacprzak (Swinburne University of Technology, Australia; Australian
Research Council Super Science Fellow), C. Péroux (Aix Marseille
University, CNRS, France), T. Contini (CNRS; University Paul Sabatier of
Toulouse, France), C. L. Martin (University of California Santa
Barbara, USA), M. Dessauges-Zavadsky (Observatory of Geneva,
Switzerland).
ESO is the foremost intergovernmental astronomy organisation in
Europe and the world’s most productive ground-based astronomical
observatory by far. It is supported by 15 countries: Austria, Belgium,
Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy,
the Netherlands, Portugal, Spain, Sweden, Switzerland and the United
Kingdom. ESO carries out an ambitious programme focused on the design,
construction and operation of powerful ground-based observing facilities
enabling astronomers to make important scientific discoveries. ESO also
plays a leading role in promoting and organising cooperation in
astronomical research. ESO operates three unique world-class observing
sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO
operates the Very Large Telescope, the world’s most advanced
visible-light astronomical observatory and two survey telescopes. VISTA
works in the infrared and is the world’s largest survey telescope and
the VLT Survey Telescope is the largest telescope designed to
exclusively survey the skies in visible light. ESO is the European
partner of a revolutionary astronomical telescope ALMA, the largest
astronomical project in existence. ESO is currently planning the
39-metre European Extremely Large optical/near-infrared Telescope, the
E-ELT, which will become “the world’s biggest eye on the sky”.
Links
Contacts
Nicolas BouchéObservatoire Midi-Pyrénées–IRAP
Toulouse, France
Tel: +33 5 61 33 27 87
Cell: +33 7 51 51 46 83
Email: Nicolas.Bouche@irap.omp.eu
Michael Murphy
Centre for Astrophysics and Supercomputing, Swinburne University of Technology
Melbourne, Australia
Tel: +61 3 9214 5818
Cell: +61 405 214 461
Email: mmurphy@swin.edu.au
Richard Hook
ESO education and Public Outreach Department
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
