The Cosmic Dance of Distant Galaxies (Forwarded)

ESO Education and Public Relations Dept.

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Text with all links and the photos are available on the ESO
Website at URL:

http://www.eso.org/outreach/press-rel/pr-2006/pr-10-06.html
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Contacts:
François Hammer, Hector Flores, and Mathieu Puech
Observatoire de Paris, France
Phone: +33 (0)1 45 07 74 08, +33 (0)1 45 07 75 25,
+33 (0)1 45 07 71 43

For Immediate Release: 15 March 2006

ESO Science Release 10/06

The Cosmic Dance of Distant Galaxies

GIRAFFE at VLT reveals the turbulent life of distant galaxies

Studying several tens of distant galaxies, an international team
of astronomers found that galaxies had the same amount of dark
matter relative to stars 6 billion years ago as they have now.
If confirmed, this suggests a much closer interplay between dark
and normal matter than previously believed. The scientists also
found that as many as 4 out of 10 galaxies are out of balance.
These results shed a new light on how galaxies form and evolve
since the Universe was only half its current age.

"This may imply that collisions and merging are important in
the formation and evolution of galaxies", said François Hammer,
Paris Observatory, France, and one of the leaders of the team
[1].

ESO PR Photo 10a/06
Collision Between Galaxies (Artist's Impression)

Caption: The new results obtained with GIRAFFE on the VLT
seem to show that collisions and merging are important in
the formation and evolution of galaxies. Here, such a
collision is shown in this artist's impression.

The scientists were interested in finding out how galaxies that
are far away -- thus seen as they were when the Universe was
younger -- evolved into the ones nearby. In particular, they
wanted to study the importance of dark matter in galaxies.

"Dark matter, which composes about 25% of the Universe, is a
simple word to describe something we really don't understand,"
said Hector Flores, co-leader. "From looking at how galaxy
rotates, we know that dark matter must be present, as
otherwise these gigantic structures would just dissolve."

In nearby galaxies, and in our own Milky Way for that matter,
astronomers have found that there exist a relation between the
amount of dark matter and ordinary stars: for every kilogram
of material within a star there is roughly 30 kilograms of
dark matter. But does this relation between dark and ordinary
matter still hold in the Universe's past?

This required measuring the velocity in different parts of
distant galaxies, a rather tricky experiment: previous
measurements were indeed unable to probe these galaxies in
sufficient details, since they had to select a single slit,
i.e. a single direction, across the galaxy.

Things changed with the availability of the multi-object
GIRAFFE spectrograph [2], now installed on the 8.2-m Kueyen
Unit Telescope of ESO's Very Large Telescope (VLT) at the
Paranal Observatory (Chile).

In one mode, known as "3-D spectroscopy" or "integrated field",
this instrument can obtain simultaneous spectra of smaller
areas of extended objects like galaxies or nebulae. For this,
15 deployable fibre bundles, the so-called Integral Field Units
(IFUs) , cf. ESO PR 01/02 , are used to make meticulous
measurements of distant galaxies. Each IFU is a microscopic,
state-of-the-art two-dimensional lens array with an aperture
of 3 x 2 arcsec2 on the sky. It is like an insect's eye, with
twenty micro-lenses coupled with optical fibres leading the
light recorded at each point in the field to the entry slit
of the spectrograph.

ESO PR Photo 10b/06
Mapping Distant Galaxies (FLAMES-GIRAFFE/VLT)

Caption: Three examples of results obtained with GIRAFFE on
distant galaxies. In the first column, images of galaxies
as obtained with the Hubble Space Telescope are shown. The
second column is the velocity field as deduced from GIRAFFE
observations: the reddish parts show material moving away
from us with respect to the mean velocity of the galaxy,
while the blue parts are moving towards us. The scale in km/s
is shown on the right. The last column is a map of electron
density per cubic centimetre. The first object corresponds to
a spiral galaxy forming star at a frantic rate of 100 solar
masses per year. The electron density map allows the
astronomers to localise the region of star formation as the
black region on the left. The second object is a galaxy
which is clearly "out of balance" and therefore shows a very
perturbed velocity field. The third object appears to show
an outflow -- matter being ejected perpendicular to the plane
of the galaxy.

"GIRAFFE on ESO's VLT is the only instrument in the world that
is able to analyze simultaneously the light coming from 15
galaxies covering a field of view almost as large as the full
moon," said Mathieu Puech, lead author of one the papers
presenting the results [3]. "Every galaxy observed in this
mode is split into continuous smaller areas where spectra
are obtained at the same time."

The astronomers used GIRAFFE to measure the velocity fields of
several tens of distant galaxies, leading to the surprising
discovery that as much as 40% of distant galaxies were "out
of balance" -- their internal motions were very disturbed --
a possible sign that they are still showing the aftermath of
collisions between galaxies.

When they limited themselves to only those galaxies that have
apparently reached their equilibrium, the scientists found
that the relation between the dark matter and the stellar
content did not appear to have evolved during the last 6
billions years.

ESO PR Photo 10c/06
Dark Matter and Stellar Mass in Distant Galaxies

Caption: Relation between the total mass of the halo of
distant galaxies (thus including dark matter) and the mass
contained in stars. Red triangles, green squares and blue
dots represent complex kinematics, perturbed rotations
and rotating discs, respectively. Full and dotted lines
represent the relation valid for galaxies in the local
Universe (following Conselice et al., 2005). This plot
shows that all the scatter of the relation is caused by
interloper galaxies with kinematics classified either as
complex or perturbed. Considering only rotating discs, i.e.
galaxies that may have reached equilibrium, the relationship
at moderate redshift is similar (slope, zero point and
scatter) to that in the local universe.

Thanks to its exquisite spectral resolution, GIRAFFE also
allows for the first time to study the distribution of gas as
a function of its density in such distant galaxies. The most
spectacular results reveal a possible outflow of gas and
energy driven by the intense star-formation within the galaxy
and a giant region of very hot gas (HII region) in a galaxy
in equilibrium that produces many stars.

"Such a technique can be expanded to obtain maps of many
physical and chemical characteristics of distant galaxies,
enabling us to study in detail how they assembled their mass
during their entire life," said François Hammer. "In many
respects, GIRAFFE and its multi-integral field mode gives
us a first flavour of what will be achieved with future
extremely large telescopes."

Notes

[1]: The team comprises: François Hammer, Hector Flores,
Mathieu Puech, Chantal Balkowski (GEPI - Observatoire de Paris),
Philippe Amram (LAM - Observatoire Astronomique Marseille-
Provence), Göran Östlin (Stockholm Observatory), Thomas
Marquart (Dept. of Astronomy and Space Physics - Uppsala,
Sweden) and Matthew D. Lehnert (MPE, Germany).

[2]: This complex and unique instrument allows obtaining high-
quality spectra of a large variety of celestial objects, from
individual stars in the Milky Way and other nearby galaxies,
to very distant galaxies. It functions by means of multiple
optical fibres that guide the light from the telescope's focal
plane into the entry slit of the spectrograph. Here the light
is dispersed into its different colours. GIRAFFE and these
fibres are an integral part of the advanced Fibre Large Array
Multi-Element Spectrograph (FLAMES) facility which also includes
the OzPoz positioner and an optical field corrector. It is the
outcome of a collaboration between ESO, Observatoire de Paris-
Meudon, Observatoire de Genève-Lausanne and the Anglo Australian
Observatory (AAO) . More details are available in ESO PR 01/02.

The principle of this instrument involves the positioning in the
telescope's focal plane of a large number of optical fibres. This
is done in such a way that each of them guides the light from one
particular celestial object towards the spectrograph that records
the spectra of all these objects simultaneously. The size of the
available field-of-view is no less than about 25 arcmin across,
i.e. almost as large as the full moon. The individual fibres are
moved and positioned "on the objects" in the field by means of
the OzPoz positioner. See also ESO PR 13/02.

[3]: The results will be published in a series of three papers
in the leading research journal, Astronomy and Astrophysics
(click on the title to access the papers):

"3D spectroscopy with VLT/GIRAFFE - I: The true Tully-Fisher
relationship at z~ 0.6" (Flores H., Hammer F., Puech M. et al.);

"3D spectroscopy with VLT/GIRAFFE - II: Are Luminous Compact
Galaxies merger remnants?" (Puech M., Hammer F., Flores H. et
al.); and

"3D spectroscopy with VLT/GIRAFFE - III: Mapping electron
densities in distant galaxies" (Puech M., Flores H., Hammer
F. & Lehnert M.D.).

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