Do Galaxies Follow Darwinian Evolution? (Forwarded)

ESO Education and Public Relations Dept.

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Contacts:

Olivier Le Fèvre
Laboratoire d'Astrophysique de Marseille, France
Phone: +33 4 91 05 59 85

Christian Marinoni
Centre de Physique Théorique, Marseille, France
Phone: +33 4 91 26 95 35

For Immediate Release: 6 December 2006

ESO Science Release 45/06

Do Galaxies Follow Darwinian Evolution?

VLT Survey Provides New Insight into Formation of Galaxies

Using VIMOS on ESO's Very Large Telescope, a team of French and Italian
astronomers have shown the strong influence the environment exerts on the
way galaxies form and evolve. The scientists have for the first time
charted remote parts of the Universe, showing that the distribution of
galaxies has considerably evolved with time, depending on the galaxies'
immediate surroundings. This surprising discovery poses new challenges for
theories of the formation and evolution of galaxies.

The 'nature versus nurture' debate is a hot topic in human psychology. But
astronomers too face similar conundrums, in particular when trying to
solve a problem that goes to the very heart of cosmological theories: are
the galaxies we see today simply the product of the primordial conditions
in which they formed, or did experiences in the past change the path of
their evolution?

In a large, three-year long survey carried out with VIMOS [1], the Visible
Imager and Multi-Object Spectrograph on ESO's VLT, astronomers studied
more than 6,500 galaxies over a wide range of distances to investigate how
their properties vary over different timescales, in different environments
and for varying galaxy luminosities [2]. They were able to build an atlas
of the Universe in three dimensions, going back more than 9 billion years.

This new census reveals a surprising result. The colour-density relation,
that describes the relationship between the properties of a galaxy and its
environment, was markedly different 7 billion years ago. The astronomers
thus found that the galaxies' luminosity, their initial genetic
properties, and the environments they reside in have a profound impact on
their evolution.

"Our results indicate that environment is a key player in galaxy
evolution, but there's no simple answer to the 'nature versus nurture'
problem in galaxy evolution," said Olivier Le Fèvre from the Laboratoire
d'Astrophysique de Marseille, France, who coordinates the VIMOS VLT Deep
Survey team that made the discovery. "They suggest that galaxies as we see
them today are the product of their inherent genetic information, evolved
over time, as well as complex interactions with their environments, such
as mergers."

Scientists have known for several decades that galaxies in the Universe's
past look different to those in the present-day Universe, local to the
Milky Way [3]. Today, galaxies can be roughly classified as red, when few
or no new stars are being born, or blue, where star formation is still
ongoing. Moreover, a strong correlation exists between a galaxy's colour
and the environment it resides in: the more sociable types found in dense
clusters are more likely to be red than the more isolated ones.

By looking back at a wide range of galaxies of a variety of ages, the
astronomers were aiming to study how this peculiar correlation has evolved
over time.

"Using VIMOS, we were able to use the largest sample of galaxies currently
available for this type of study, and because of the instrument's ability
to study many objects at a time we obtained many more measurements than
previously possible," said Angela Iovino, from the Brera Astronomical
Observatory, Italy, another member of the team.

The team's discovery of a marked variation in the 'colour-density'
relationship, depending on whether a galaxy is found in a cluster or
alone, and on its luminosity, has many potential implications. The
findings suggest for example that being located in a cluster quenches a
galaxy's ability to form stars more quickly compared with those in
isolation. Luminous galaxies also run out of star-forming material at an
earlier time than fainter ones.

They conclude that the connection between galaxies' colour, luminosity and
their local environment is not merely a result of primordial conditions
'imprinted' during their formation -- but just as for humans, galaxies'
relationship and interactions can have a profound impact on their
evolution.

A high resolution image and its caption is available on this page:
http://www.eso.org/outreach/press-re...hot-45-06.html

More information

Results from this study are published in volume 458 (1) of Astronomy &
Astrophysics, "The VIMOS VLT Deep Survey: The build-up of the
colour-density relation", by O. Cucciati et al. The paper is available
from the A&A web site.

The Survey team consists of O. Cucciati, A. Iovino, L. Guzzo, S. Temporin
(INAF-Observatory of Brera, Italy), C. Marinoni (Centre de Physique
Théorique, Marseille, France), O. Ilbert, B. Maranno, A. Bongiorno
(University of Bologna, Italy), O. Le Fèvre, A. Pollo, L. Tresse, V. Le
Brun, C. Adami, S. Arnouts, A. Mazure, S. de la Torre (CNRS-Université de
Provence, France), S. Bardelli, G. Zamorani, A. Cappi, E. Zucca, M.
Bolzonella, P. Ciliegi, R. Merighi, L. Pozzetti (INAF-Observatory of
Bologna, Italy), P. Franzetti, B. Meneux, M. Scodeggio, D. Bottini, B.
Garilli, D. Maccagni, S. Foucaud, D. Vergani (IASF-INAF, Italy), H.J.
McCracken, Y. Mellier (Institut d'Astrophysique de Paris, France), L.
Scaramella, A. Zanichelli, G. Vettolani, M. Bondi, L. Gregorini (IRA-INAF,
Italy), J.P. Picat, T. Contini, I. Gavignaud, R. Pello, F. Lamareille, G.
Mathez, D. Rizzo (Institut d'Astrophysique de l'Observatoire
Midi-Pyrenées), M. Arnaboldi, M. Radovich, G. Busarello, P. Merluzzi, V.
Ripepi (Observatory of Capodimonte, Italy), S. Charlot (MPIA, Germany) and
S. Paltani (Integral Science Data Centre, Switzerland)

INAF has also made a press release available in Italian.

Notes

[1] The Visible Multi-Object Spectrograph VIMOS is a multi-mode instrument
on Melipal, the third Unit Telescope of the Very Large Telescope array at
ESO's Paranal Observatory. In operation since 2003, VIMOS can provide both
images and astronomical spectra at visible wavelengths over wide fields of
view. In its multi-object mode, it can record up to 1,000 spectra at a
time.

[2] The VIMOS VLT Deep Survey (VVDS) is a breakthrough spectroscopic
survey which will provide, when finished, a complete picture of galaxy and
structure formation over a very broad redshift range (0 z 5), over
sixteen square degrees of the sky in four separate fields.

[3] Because of the time taken for light to reach an observer on Earth over
the vast distances of the cosmos, astronomers studying distant galaxies
are in fact observing conditions in the Universe's past. The region closer
to Earth, local to our own Milky Way Galaxy, is thus often referred to as
the 'present-day' Universe.


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United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25

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