Images online at :http://www.eso.org/outreach/press-rel/pr-2002/pr-04-02.html

Summary

One of the most fundamental tasks of modern astrophysics is the study of
the evolution of the Universe. This is a daunting undertaking that
requires extensive observations of large samples of objects in order to
produce reasonably detailed maps of the distribution of galaxies in the
Universe and to perform statistical analysis.

Much effort is now being put into mapping the relatively nearby space and
thereby to learn how the Universe looks today. But to study its evolution,
we must compare this with how it looked when it still was young. This is
possible, because 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”.

This may sound easy, but it is not. Very distant objects are very dim and
can only be observed with large telescopes. Looking at one object at a
time would make such a study extremely time-consuming and, in practical
terms, impossible. To do it anyhow, we need the largest possible telescope
with a highly specialised, exceedingly sensitive instrument that is able
to observe a very large number of (faint) objects in the remote universe
simultaneously.

The VLT VIsible Multi-Object Spectrograph (VIMOS) is such an instrument.
It can obtain many hundreds of spectra of individual galaxies in the
shortest possible time; in fact, in one special observing mode, up to 6400
spectra of the galaxies in a remote cluster during a single exposure,
augmenting the data gathering power of the telescope by the same
proportion. This marvellous science machine has just been installed at the
8.2-m MELIPAL telescope, the third unit of the Very Large Telescope (VLT)
at the ESO Paranal Observatory. A main task will be to carry out
3-dimensional mapping of the distant Universe from which we can learn its
large-scale structure.

“First light” was achieved on February 26, 2002, and a first series of
test observations has successfully demonstrated the huge potential of this
amazing facility. Much work on VIMOS is still ahead during the coming
months in order to put into full operation and fine-tune the most
efficient “galaxy cruncher” in the world.

VIMOS is the outcome of a fruitful collaboration between ESO and several
research institutes in France and Italy, under the responsibility of the
Laboratoire d’Astrophysique de Marseille (CNRS, France). The other
partners in the “VIRMOS Consortium” are the Laboratoire d’Astrophysique de
Toulouse, Observatoire Midi-Pyrenees, and Observatoire de Haute-Provence
in France, and Istituto di Radioastronomia (Bologna), Istituto di Fisica
Cosmica e Tecnologie Relative (Milano), Osservatorio Astronomico di
Bologna, Osservatorio Astronomico di Brera (Milano) and Osservatorio
Astronomico di Capodimonte (Naples) in Italy.

  • PR Photo 09a/02: VIMOS image of the Antennae Galaxies (centre).
  • PR Photo 09b/02: First VIMOS Multi-Object Spectrum (full field)
  • PR Photo 09c/02: The VIMOS instrument on VLT MELIPAL
  • PR Photo 09d/02: The VIMOS team at “First Light”.
  • PR Photo 09e/02: “First Light” image of NGC 5364
  • PR Photo 09f/02: Image of the Crab Nebula
  • PR Photo 09g/02: Image of spiral galaxy NGC 2613
  • PR Photo 09h/02: Image of spiral galaxy Messier 100
  • PR Photo 09i/02: Image of cluster of galaxies ACO 3341
  • PR Photo 09j/02: Image of cluster of galaxies MS 1008.1-1224
  • PR Photo 09k/02: Mask design for MOS exposure
  • PR Photo 09l/02: First VIMOS Multi-Object Spectrum (detail)
  • PR Photo 09m/02: Integrated Field Spectroscopy of central area of the
  • “Antennae Galaxies”
  • PR Photo 09n/02: Integrated Field Spectroscopy of central area of the
  • “Antennae Galaxies” (detail)

Caption: PR Photo 09a/02: One of the first images from the new VIMOS
facility, obtained right after the moment of “first light” on Ferbruary
26, 2002. It shows the famous “Antennae Galaxies” (NGC 4038/39), the
result of a recent collision between two galaxies. As an immediate outcome
of this dramatic event, stars are born within massive complexes that
appear blue in this composite photo, based on exposures through green,
orange and red optical filtres. PR Photo 09b/02: Some of the first spectra
of distant galaxies obtained with VIMOS in Multi-Object-Spectroscopy (MOS)
mode. More than 220 galaxies were observed simultaneously, an
unprecedented efficiency for such a “deep” exposure, reaching so far out
in space. These spectra allow to obtain the redshift, a measure of
distance, as well as to assess the physical status of the gas and stars in
each of these galaxies. A part of this photo is enlarged as PR Photo
09l/02. Technical information about these photos is available below. Other
“First Light” images from VIMOS are shown in the photo gallery below.

The next in the long series of front-line instruments to be installed on the
ESO Very Large Telescope (VLT), VIMOS (and its complementary,
infrared-sensitive counterpart NIRMOS, now in the design stage) will allow
mapping of the distribution of galaxies, clusters, and quasars during a time
interval spanning more than 90% of the age of the universe. It will let us
look back in time to a moment only ~1.5 billion years after the Big Bang
(corresponding to a redshift of about 5).

Like archaeologists, astronomers can then dig deep into those early ages
when the first building blocks of galaxies were still in the process of
formation. They will be able to determine when most of the star formation
occurred in the universe and how it evolved with time. They will analyse how
the galaxies cluster in space, and how this distribution varies with time.
Such observations will put important constraints on evolution models, in
particular on the average density of matter in the Universe.

Mapping the distant universe requires to determine the distances of the
enormous numbers of remote galaxies seen in deep pictures of the sky, adding
depth – the third, indispensible dimension – to the photo. VIMOS offers this
capability, and very efficiently. Multi-object spectroscopy is a technique
by which many objects are observed simultaneously. VIMOS can observe the
spectra of about 1000 galaxies in one exposure, from which redshifts, hence
distances, can be measured [2]. The possibility to observe two galaxies at
once would be equivalent to having a telescope twice the size of a VLT Unit
Telescope. VIMOS thus effectively “increases” the size of the VLT hundreds
of times.

From these spectra, the stellar and gaseous content and internal velocities
of galaxies can be infered, forming the base for detailed physical studies.
At present the distances of only a few thousand galaxies and quasars have
been measured in the distant universe. VIMOS aims at observing 100 times
more, over one hundred thousand of those remote objects. This will form a
solid base for unprecedented and detailed statistical studies of the
population of galaxies and quasars in the very early universe.

The international VIRMOS Consortium

VIMOS is one of two major astronomical instruments to be delivered by the
VIRMOS Consortium of French and Italian institutes under a contract signed
in the summer of 1997 between the European Southern Observatory (ESO) and
the French Centre National de la Recherche Scientifique (CNRS). The
participating institutes are:

in France:

* Laboratoire d’Astrophysique de Marseille (LAM), Observatoire
Marseille-Provence (project responsible)
* Laboratoire d’Astrophysique de Toulouse, Observatoire Midi-Pyrenees
* Observatoire de Haute-Provence (OHP)

in Italy:

* Istituto di Radioastronomia (IRA-CNR) (Bologna)
* Istituto di Fisica Cosmica e Tecnologie Relative (IFCTR) (Milano)
* Osservatorio Astronomico di Capodimonte (OAC) (Naples)
* Osservatorio Astronomico di Bologna (OABo)
* Osservatorio Astronomico di Brera (OABr) (Milano)

VIMOS at the VLT: a unique and powerful combination

VIMOS is installed on the Nasmyth “Focus B” platform of the 8.2-m VLT
MELIPAL telescope, cf. PR Photo 09c/02. It may be compared to four
multi-mode instruments of the FORS-type (cf. ESO PR 14/98), joined in one
stiff structure. The construction of VIMOS has involved the production of
large and complex optical elements and their integration in more than 30
remotely controlled, finely moving functions in the instrument.

In the configuration employed for the “first light”, VIMOS made use of two
of its four channels. The two others will be put into operation in the next
commissioning period during the coming months. However, VIMOS is already now
the most efficient multi-object spectrograph in the world, with an
equivalent (accumulated) slit length of up to 70 arcmin on the sky.

VIMOS has a field-of-view as large as half of the full moon (14 x 16 arcmin2
for the four quadrants), the largest sky field to be imaged so far by the
VLT. It has excellent sensitivity in the blue region of the spectrum (about
60% more efficient than any other similar instruments in the ultraviolet
band), and it is also very sensitive in all other visible spectral regions,
all the way to the red limit.

But the absolutely unique feature of VIMOS is its capability to take large
numbers of spectra simultaneously, leading to exceedingly efficient use of
the observing time. Up to about 1000 objects can be observed in a single
exposure in multi-slit mode. And no less than 6400 spectra can be recorded
with the Integral Field Unit, in which a closely packed fibre optics bundle
can simultaneously observe a continuous sky area measuring no less than 56 x
56 arcsec2.

A dedicated machine, the Mask Manufacturing Unit (MMU), cuts the slits for
the entrance apertures of the spectrograph. The laser is capable of cutting
200 slits in less than 15 minutes. This facility was put into operation at
Paranal by the VIRMOS Consortium already in August 2000 and has since been
extensively used for observations with the FORS2 instrument; more details
are available in ESO PR 19/99.

Caption: PR Photo 09d/02: The VIRMOS team in the MELIPAL control room,
moments after “First Light” on February 26, 2002. From left to right:
Oreste Caputi, Marco Scodeggio, Giovanni Sciarretta , Olivier Le Fevre,
Sylvie Brau-Nogue, Christian Lucuix, Bianca Garilli, Markus Kissler (in
front), Xavier Reyes, Michel Saisse, Luc Arnold and Guido Mancini. PR
Photo 09e/02: The spiral galaxy NGC 5364 was the first object to be
observed by VIMOS. This false-colour near-infrared, raw “First Light”
photo shows the extensive spiral arms. Technical information about this
photo is available below.

VIMOS was shipped from Observatoire de Haute-Provence (France) at the end of
2001, and reassembled at Paranal during a first period in January 2002. From
mid-February, the instrument was made ready for installation on the VLT
MELIPAL telescope; this happened on February 24, 2002. VIMOS saw “First
Light” just two days later, on February 26, 2000, cf. PR Photo 09e/02.
During the same night, a number of excellent images were obtained of various
objects, demonstrating the fine capabilities of the instrument in the
“direct imaging”-mode.

The first spectra were successfully taken during the night of March 2 – 3,
2002. The slit masks that were used on this occasion were prepared with
dedicated software that also optimizes the object selection, cf. PR Photo
09k/02, and were then cut with the laser machine. From the first try on, the
masks have been well aligned on the sky objects. The first observations with
large numbers of spectra were obtained shortly thereafter.

First accomplishments

Images of nearby galaxies, clusters of galaxies, and distant galaxy fields
were among the first to be obtained, using the VIMOS imaging mode and
demonstrating the excellent efficiency of the instrument, various examples
are shown below.

The first observations of multi-spectra were performed in a selected sky
field in which many faint galaxies are present; it is known as the
“VIRMOS-VLT Deep Survey Field at 1000+02”.

Thanks to the excellent sensitivity of VIMOS, the spectra of galaxies as
faint as (red) magnitude R = 23 (i.e. over 6 million times fainter than what
can be perceived with the unaided eye) are visible on exposures lasting only
15 minutes.

Some of the first observations with the Integral Field Unit were made of the
core of the famous Antennae Galaxies (NGC 4038/39). They will form the basis
for a detailed map of the strong emission produced by the current, dramatic
collision of the two galaxies.

First Images and Spectra from VIMOS – a Gallery

The following photos are from a collection of the first images and spectra
obtained with VIMOS. See also PR Photos 09a/02, 09b/02 and 09e/02,
reproduced above. Technical information about all of them is available
below.

Notes

[1]: This is a joint Press Release of ESO, Centre National de la Recherche
Scientifique (CNRS) in France, and Consiglio Nazionale delle Ricerche (CNR)
and Istituto Nazionale di Astrofisica (INAF) in Italy.

[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.

Technical information about the photos

PR Photo 09a/01: Composite VRI image of NGC 4038/39, obtained on 26 February
2002, in a bright sky (full moon). Individual exposures of 60 sec each;
image quality 0.6 arcsec FWHM; the field measures 3.5 x 3.5 arcmin2. North
is up and East is left. PR Photo 09b/02: MOS-spectra obtained with two
quadrants totalling 221 slits + 6 reference objects (stars placed in square
holes to ensure a correct alignment). Exposure time 15 min; LR(red) grism.
This is the raw (unprocessed) image of the spectra. PR Photo 09e/02: A 60
sec i exposure of NGC 5364 on February 26, 2002; image quality 0.6 arcsec
FWHM; full moon; 3.5 x 3.5 arcmin2; North is up and East is left. PR Photo
09f/02: Composite VRI image of Messier 1, obtained on March 4, 2002. The
individual exposures lasted 180 sec; image quality 0.7 arcsec FWHM; field 7
x 7 arcmin2; North is up and East is left. PR Photo 09g/02: Composite VRI
image of NGC 2613, obtained on February 28, 2002. The individual exposures
lasted 180 sec; image quality 0.7 arcsec FWHM; field 7 x 7 arcmin2; North is
up and East is left. PR Photo 09h/02: Composite VRI image of Messier 100,
obtained on March 3, 2002. The individual exposures lasted 180 sec, image
quality 0.7 arcsec FWHM; field 7 x 7 arcmin2; North is up and East is left.
PR Photo 09i/02: R-band image of galaxy cluster ACO 3341, obtained on March
4, 2002. Exposure 300 sec, image quality 0.5 arcsec FWHM;. field 7 x 7
arcmin2; North is up and East is left. PR Photo 09j/02: Composite VRI image
of the distant cluster of galaxies MS 1008.1-1224. The individual exposures
lasted 300 sec; image quality 0.8 arcsec FWHM; field 5 x 3 arcmin2; North is
to the right and East is up. PR Photo 09k/02: Mask design made with the
VMMPS tool, overlaying a pre-image. The selected objects are seen at the
centre of the yellow squares, where a 1 arcsec slit is cut along the spatial
X-axis. The rectangles in white represent the dispersion in wavelength of
the spectra along the Y-axis. Masks are cut with the Mask Manufacturing Unit
(MMU) built by the Virmos Consortium. PR Photo 09l/02: Enlargement of a
small area of PR Photo 09b/02. PR Photo 09m/02: Spectra of the central area
of NGC 4038/39, obtained with the Integral Field Unit on February 26, 2002.
The exposure lasted 5 min and was made with the low resolution red grating.
PR Photo 09m/02: Zoom-in on small area of PR Photo 09m/02. The strong
emission lines of hydrogen (H-alpha) and ionized sulphur (S II) are seen.

Contacts

Olivier Le Fevre

Laboratoire d’Astrophysique de Marseille

France

Tel.: +33 6 08 90 50 43

email: Olivier.LeFevre@astrsp-mrs.fr

Gianpaolo Vettolani

Istituto di Radioastronomia

Bologna, Italy

Tel.: +39 051 6399369

email: vettolani@ira.cnr.it

Sandro d’Odorico

European Southern Observatory

Garching, Germany

Tel.: +4989-3200-6239

email: sdodoric@eso.org