A Cosmic Baby-Boom: Large Population of Galaxies Found in the YoungUniverse with ESO's VLT (Forwarded)

ESO Education and Public Relations Dept.

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Contacts:
Olivier Le Fèvre
Laboratoire d'Astrophysique de Marseille, France
Phone: +33 (0) 4 91 05 59 85

Under embargo until 21 September 2005, 19:00 CEST (17:00 GMT)

ESO Press Release 24/05

A Cosmic Baby-Boom

Large Population of Galaxies Found in the Young Universe with
ESO's VLT

It is one of the major goals of observational cosmology to
trace the way galaxies formed and evolved and to compare it to
predictions from theoretical models. It is therefore essential
to know as precisely as possible how many galaxies were present
in the Universe at different epochs.

This is easier to say than to do. Indeed, if counting galaxies
from deep astronomical images is relatively straightforward,
measuring their distance -- hence, the epoch in the history of
the universe where we see it [1] -- is much more difficult.
This requires taking a spectrum of the galaxy and measuring
its redshift [2].

However, for the faintest galaxies -- that are most likely
the farthest and hence the oldest -- this requires a lot of
observing time on the largest of the telescopes. Until now,
astronomers had thus to first carefully select the candidate
high-redshift galaxies, in order to minimise the time spent on
measuring the distance. But it seems that astronomers were too
careful in doing so, and hence had a wrong picture of the
population of galaxies.

ESO PR Photo 29a/05
New Population of Distant Galaxies

Caption: ESO PR Photo 29a/05 shows a small patch of the sky
surveyed by the VVDS team. This colour-composite image based
on observations made with Megacam at the CFHT indicates a few
of the newly found distant galaxies (encircled ones) based
on VIMOS/VLT data. (credit: LAM-OAMP/CFHT)

It would be better to "simply" observe in a given patch of the
sky all galaxies brighter than a given limit. But looking at
one object at a time would make such a study impossible.

To take up the challenge, a team of French and Italian
astronomers [3] used the largest possible telescope with a
highly specialised, very sensitive instrument that is able to
observe a very large number of (faint) objects in the remote
universe simultaneously.

The astronomers made use of the VIsible Multi-Object
Spectrograph (VIMOS) on Melipal, one of the 8.2-m telescopes of
ESO's Very Large Telescope Array. VIMOS can observe the spectra
of about 1,000 galaxies in one exposure, from which redshifts,
hence distances, can be measured. The possibility to observe
two galaxies at once would be equivalent to using two VLT Unit
Telescopes simultaneously. VIMOS thus effectively multiplies
the efficiency of the VLT hundreds of times.

This makes it possible to complete in a few hours observations
that would have taken months only a few years ago. With
capabilities up to ten times more productive than competing
instruments, VIMOS offers the possibility for the first time
to conduct an unbiased census of the distant Universe.

Using the high efficiency of the VIMOS instrument, the team
of astronomers embarked in the VIMOS VLT Deep Survey (VVDS)
whose aim is to measure in some selected patch of the sky the
redshift of all galaxies brighter than magnitude 24 in the
red, that is, galaxies that are up to 16 million fainter than
what the unaided eye can see.

In a total sample of about 8,000 galaxies selected only on the
basis of their observed brightness in red light, almost 1,000
bright and vigorously star forming galaxies were discovered at
an epoch 1,500 to 4,500 million years after the Big Bang
(redshift between 1.4 and 5).

ESO PR Photo 29b/05
Average Spectra of Distant Galaxies

Caption: ESO PR Photo 29b/05 shows the average spectra of
VVDS galaxies in three redshift, i.e. distances or ages,
ranges (top to bottom): 1.4 to 2.5, 2.5 to 3.5, and 3.5
to 5.0. The main spectral features expected at these
wavelengths are clearly visible in all spectra.

"To our surprise", says Olivier Le Fèvre, from the Laboratoire
d'Astrophysique de Marseille (France) and co-leader of the
VVDS project, "this is two to six times higher than had been
found by previous works. These galaxies had been missed
because previous surveys had selected objects in a much more
restrictive manner than we did. And they did so to accommodate
the much lower efficiency of the previous generation of
instruments."

While observations and models have consistently indicated that
the Universe had not yet formed many stars in the first billion
years of cosmic time, the discovery made by the scientists
calls for a significant change in this picture.

Combining the spectra of all the galaxies in a given redshift
range (i.e. belonging to the same epoch), the astronomers could
estimate the amount of star formed in these galaxies. They find
that the galaxies in the young Universe transform into stars
between 10 and 100 times the mass of our Sun in a year.

"This discovery implies that galaxies formed many more stars
early in the life of the Universe than had previously been
thought", explains Gianpaolo Vettolani, the other co-leader
of the VVDS project, working at INAF-IRA in Bologna (Italy).
"These observations will demand a profound reassessment of
our theories of the formation and evolution of galaxies in a
changing Universe."

It now remains for astronomers to explain how one can create
such a large population of galaxies, producing more stars
than previously assumed, at a time when the Universe was
about 10-20% of its current age.

More Information

These results are reported in the September 22 issue of the
journal Nature (Le Fèvre et al., "A large population of
galaxies 9 to 12 billion years back in the life of the
Universe").

Notes

[1]: Astronomers can "look back in time" by studying remote
objects -- the larger their distance, the longer the light
we now observe has been underway to us, and the longer is
thus the corresponding "look-back time".

[2]: In astronomy, the redshift denotes the fraction by which
the lines in the spectrum of an object are shifted towards
longer wavelengths. The observed redshift of a distant galaxy
gives a direct estimate of the apparent recession velocity as
caused by the universal expansion. Since the expansion rate
increases with distance, the velocity is itself a function
(the Hubble relation) of the distance to the object.

[3]: The team of French and Italian researchers is co-led by
Olivier Le Fèvre, director of the Laboratoire d'Astrophysique
de Marseille (LAM) in France and by Gianpaolo Vettolani (INAF-
IRA, Bologna) in Italy. It has been successful in the study
of the history of galaxies from their formation early in the
life of the Universe until the present epoch by combining
fundamental research with the construction of specialized
instruments uniquely suited to this task. The VIMOS (Visible
Multi-Object Spectrograph) instrument has been designed using
this team's expertise in instrument building, and was build
by a French-Italian consortium financed by ESO, CNRS and the
Provence-Alpes Côte d'Azur region in France, and by CNR, INAF
and the Ministry of Education in Italy.

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