La Silla Camera Observes Chandra Deep Field South

ESO PR Photo 02a/03

Caption: PR Photo 02a/03 shows a three-colour composite image of the
Chandra Deep Field South (CDF-S), obtained with the Wide Field Imager
(WFI) camera on the 2.2-m MPG/ESO telescope at the ESO La Silla
Observatory (Chile). It was produced by the combination of about 450
images with a total exposure time of nearly 50 hours. The field
measures 36 x 34 arcmin^2; North is up and East is left. Technical
information is available below.

The combined efforts of three European teams of astronomers, targeting
the same sky field in the southern constellation Fornax (The Oven) have
enabled them to construct a very deep, true-colour image — opening an
exceptionally clear view towards the distant universe.

The image (PR Photo 02a/03) covers an area somewhat larger than the full
moon. It displays more than 100,000 galaxies, several thousand stars and
hundreds of quasars. It is based on images with a total exposure time of
nearly 50 hours, collected under good observing conditions with the Wide
Field Imager (WFI) on the MPG/ESO 2.2m telescope at the ESO La Silla
Observatory (Chile) — many of them extracted from the ESO Science Data
Archive.

The position of this southern sky field was chosen by Riccardo Giacconi
(Nobel Laureate in Physics 2002) at a time when he was Director General
of ESO, together with Piero Rosati (ESO). It was selected as a sky
region towards which the NASA Chandra X-ray satellite observatory,
launched in July 1999, would be pointed while carrying out a very long
exposure (lasting a total of 1 million seconds, or 278 hours) in order
to detect the faintest possible X-ray sources. The field is now known
as the Chandra Deep Field South (CDF-S).

The new WFI photo of CDF-S does not reach quite as deep as the available
images of the "Hubble Deep Fields" (HDF-N in the northern and HDF-S in
the southern sky, cf. e.g. ESO PR Photo 35a/98), but the field-of-view
is about 200 times larger. The present image displays about 50 times
more galaxies than the HDF images, and therefore provides a more
representative view of the universe.

The WFI CDF-S image will now form a most useful basis for the very
extensive and systematic census of the population of distant galaxies
and quasars, allowing at once a detailed study of all evolutionary
stages of the universe since it was about 2 billion years old.

These investigations have started and are expected to provide
information about the evolution of galaxies in unprecedented detail.
They will offer insights into the history of star formation and how
the internal structure of galaxies changes with time and, not least,
throw light on how these two evolutionary aspects are interconnected.

Galaxies in the WFI image

ESO PR Photo 02b/03

Caption: PR Photo 02b/03 contains a collection of twelve subfields
from the full WFI Chandra Deep Field South (WFI CDF-S), centred on
(pairs or groups of) galaxies. Each of the subfields measures 2.5 x
2.5 arcmin^2 (635 x 658 pix2; 1 pixel = 0.238 arcsec). North is up
and East is left. Technical information is available below.

The WFI CDF-S colour image – of which the full field is shown in PR
Photo 02a/03 — was constructed from all available observations in the
optical B-, V- and R-bands obtained under good conditions with the Wide
Field Imager (WFI) on the 2.2-m MPG/ESO telescope at the ESO La Silla
Observatory (Chile), and now stored in the ESO Science Data Archive.

It is the "deepest" image ever taken with this instrument. It covers a
sky field measuring 36 x 34 arcmin^2, i.e., an area somewhat larger
than that of the full moon. The observations were collected during a
period of nearly four years, beginning in January 1999 when the WFI
instrument was first installed (cf. ESO PR 02/99) and ending in October
2002. Altogether, nearly 50 hours of exposure were collected in the
three filters combined here, cf. the technical information below.

Although it is possible to identify more than 100,000 galaxies in the
image — some of which are shown in PR Photo 02b/03 — it is still
remarkably "empty" by astronomical standards. Even the brightest stars
in the field (of visual magnitude 9) can hardly be seen by human
observers with binoculars. In fact, the area density of bright,
nearby galaxies is only half of what it is in "normal" sky fields.
Comparatively empty fields like this one provide an unsually clear
view towards the distant regions in the universe and thus open a
window towards the earliest cosmic times.

Research projects in the Chandra Deep Field South

ESO PR Photo 02c/03 ESO PR Photo 02d/03

Caption: PR Photo 02c-d/03 shows two sky fields within the WFI
image of CDF-S, reproduced at full (pixel) size to illustrate the
exceptional information richness of these data. The subfields measure
6.8 x 7.8 arcmin2 (1717 x 1975 pixels) and 10.1 x 10.5 arcmin^2
(2545 x 2635 pixels), respectively. North is up and East is left.
Technical information is available below.

Astronomers from different teams and disciplines have been quick to join
forces in a world-wide co-ordinated effort around the Chandra Deep Field
South. Observations of this area are now being performed by some of the
most powerful astronomical facilities and instruments. They include
space-based X-ray and infrared observations by the ESA XMM-Newton, the
NASA CHANDRA, Hubble Space Telescope (HST) and soon SIRTF (scheduled
for launch in a few months), as well as imaging and spectroscopical
observations in the infrared and optical part of the spectrum by
telescopes at the ground-based observatories of ESO (La Silla and
Paranal) and NOAO (Kitt Peak and Tololo).

A huge database is currently being created that will help to analyse
the evolution of galaxies in all currently feasible respects. All
participating teams have agreed to make their data on this field
publicly available, thus providing the world-wide astronomical
community with a unique opportunity to perform competitive research,
joining forces within this vast scientific project.

Concerted observations

The optical true-colour WFI image presented here forms an important part
of this broad, concerted approach. It combines observations of three
scientific teams that have engaged in complementary scientific projects,
thereby capitalizing on this very powerful combination of their
individual observations. The following teams are involved in this work:

* COMBO-17 (Classifying Objects by Medium-Band Observations in 17
filters): an international collaboration led by Christian Wolf and
other scientists at the Max-Planck-Institut fuer Astronomie (MPIA,
Heidelberg, Germany). This team used 51 hours of WFI observing time
to obtain images through five broad-band and twelve medium-band
optical filters in the visual spectral region in order to measure
the distances (by means of "photometric redshifts") and star-formation
rates of about 10,000 galaxies, thereby also revealing their
evolutionary status.

* EIS (ESO Imaging Survey): a team of visiting astronomers from the ESO
community and beyond, led by Luiz da Costa (ESO). They observed the
CDF-S for 44 hours in six optical bands with the WFI camera on the
MPG/ESO 2.2-m telescope and 28 hours in two near-infrared bands with
the SOFI instrument at the ESO 3.5-m New Technology Telescope (NTT),
both at La Silla. These observations form part of the Deep Public
Imaging Survey that covers a total sky area of 3 square degrees.

* GOODS (The Great Observatories Origins Deep Survey): another
international team (on the ESO side, led by Catherine Cesarsky)
that focusses on the coordination of deep space- and ground-based
observations on a smaller, central area of the CDF-S in order to
image the galaxies in many differerent spectral wavebands, from
X-rays to radio. GOODS has contributed with 40 hours of WFI time for
observations in three broad-band filters that were designed for the
selection of targets to be spectroscopically observed with the ESO
Very Large Telescope (VLT) at the Paranal Observatory (Chile), for
which over 200 hours of observations are planned. About 10,000
galaxies will be spectroscopically observed in order to determine
their redshift (distance), star formation rate, etc.

Another important contribution to this large research undertaking will
come from the GEMS project. This is a "HST treasury programme" (with
Hans-Walter Rix from MPIA as Principal Investigator) which observes the
10,000 galaxies identified in COMBO-17 — and eventually the entire
WFI-field with HST — to show the evolution of their shapes with time.

Great questions

With the combination of data from many wavelength ranges now at hand,
the astronomers are embarking upon studies of the many different
processes in the universe. They expect to shed more light on several
important cosmological questions, such as:

* How and when was the first generation of stars born?

* When exactly was the neutral hydrogen in the universe ionized the
first time by powerful radiation emitted from the first stars and
active galactic nuclei?

* How did galaxies and groups of galaxies evolve during the past 13
billion years?

* What is the true nature of those elusive objects that are only seen
at the infrared and submillimetre wavelengths (cf. ESO PR 23/02)?

* Which fraction of galaxies had an "active" nucleus (probably with a
black hole at the centre) in their past, and how long did this phase
last?

Moreover, since these extensive optical observations were obtained in
the course of a dozen observing periods during several years, it is
also possible to perform studies of certain variable phenomena:

* How many variable sources are seen and what are their types and
properties?

* How many supernovae are detected per time interval, i.e. what is
the supernovae frequency at different cosmic epochs?

* How do those processes depend on each other?

This is just a short and very incomplete list of questions astronomers
world-wide will address using all the complementary observations. No
doubt that the coming studies of the Chandra Deep Field South — with
this and other data — will be most exciting and instructive!

Other wide-field images

Other wide-field images from the WFI have been published in various ESO
press releases during the past four years — they are also available at
the WFI Photo Gallery. A collection of full-resolution files
(TIFF-format) is available on a WFI CD-ROM.

Technical Information

The very extensive data reduction and colour image processing needed to
produce these images were performed by Mischa Schirmer and Thomas Erben
at the "Wide Field Expertise Center" of the Institut fuer Astrophysik
und Extraterrestrische Forschung der Universitaet Bonn (IAEF) in
Germany. It was done by means of a software pipeline specialised for
reduction of multiple CCD wide-field imaging camera data. This pipeline
is mainly based on publicly available software modules and algorithms
(EIS, FLIPS, LDAC, Terapix, Wifix).

The image was constructed from about 150 exposures in each of the
following wavebands: B-band (centred at wavelength 0.456 nm; here
rendered as blue, 15.8 hours total exposure time), V-band (0.540 nm;
green, 15.6 hours) and R-band (0.652 nm; red, 17.8 hours). Only
images taken under sufficiently good observing conditions (defined
as seeing less than 1.1 arcsec) were included. In total, 450 images
were assembled to produce this colour image, together with about as
many calibration images (biases, darks and flats). More than 2
Terabyte (TB) of temporary files were produced during the extensive
data reduction. Parallel processing of all data sets took about
two weeks on a four-processor Sun Enterprise 450 workstation and
a 1.8 GHz dual processor Linux PC. The final colour image was
assembled in Adobe Photoshop. The observations were performed by
ESO (GOODS, EIS) and the COMBO-17 collaboration in the period
1/1999-10/2002.