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ESO Press Release 02/00

Working with the ESO Very Large Telescope (VLT) at the Paranal Observatory, a group of European
astronomers [1] has just obtained one of the deepest looks into the distant Universe ever made by
an optical telescope.

These observations were carried out in the near-infrared spectral region and are part of an attempt
to locate very distant galaxies that have so far escaped detection in the visual bands.

The first results are very promising and some concentrations of galaxies at very large distances
were uncovered.

Some early galaxies may be in hiding

Current theories hypothesize that more than 80% of all stars ever formed were assembled in
galaxies during the latter half of the elapsed lifetime of the Universe, i.e., during the past 7-8 billion

However, doubts have arisen about these ideas. There are now observational indications that a
significant number of those galaxies that formed during the first 20% of the age of the Universe, i.e.
within about 3 billion years after the Big Bang, may not be visible to optical telescopes.

In some cases, we do not see them, because their light is obscured by dust.

Other distant galaxies may escape detection by optical telescopes because star formation in them
has ceased and their light is mainly emitted in the red and infrared spectral bands. This is because,
while very young galaxies mostly contain hot and blue stars, older galaxies have substantial numbers
of cool and red stars. They are then dominated by an older, “evolved” stellar population that is
cooler and redder. The large cosmic velocities of these galaxies further enhance this effect by
causing their light to be “redshifted” towards longer wavelengths, i.e. into the near-infrared spectral

Observations in the infrared needed

Within the present programme, long exposures in near-infrared wavebands were made with the
Infrared Spectrometer And Array Camera (ISAAC), mounted on ANTU, the first of the four 8.2-m
VLT Unit Telescopes.

A first analysis of the new observations indicates that “evolved” galaxies were already present when
the Universe was only 4 billion years old. This information is of great importance to our
understanding of how the matter in the early Universe condensed and the first galaxies and stars
came into being.

While in the nearby Universe evolved galaxies are preferentially located in denser environments such
as groups and clusters of galaxies, little is currently known about the distribution in space of such
objects at early cosmic epochs.

In order to be able to see such obscured and/or “evolved” galaxies in the early Universe, and to look
for hitherto unknown galaxies beyond the limits of “deep-field” imaging in visible spectral bands, it is
necessary to employ other observing techniques. The astronomers must search for such objects on
large-field, very long-exposure sky images obtained in the near-infrared (NIR, wavelength 1-2 µm)
region of the electromagnetic spectrum and at even longer wavelengths (> 10 µm) in the far-IR and
in the sub-mm range.

Such observations are beyond the capability of the infrared cameras installed on the world’s 4-m
class telescopes. However, the advent of the ISAAC instrument at the 8.2-m ANTU telescope has
now opened new and exciting research opportunities in this direction for European astronomers.

With ISAAC, it is possible to obtain “deep” NIR images in an unprecedentedly wide field of view,
covering a sky area about 7 times larger than with the best instruments previously available on very
large telescopes. Such observations also benefit greatly from the very good optical quality provided
by the active optics control of the VLT, as well as the excellent Paranal site.

The ISAAC/ANTU observations

ESO PR Photo 06a/00

Caption: ESO PR Photo 06a/00 displays a 4.5 arcmin2 area of the “AXAF Deep Field”, as
observed with the ISAAC multi-mode instrument at VLT ANTU in the near-IR K band (at wavelength
2.x µm). The total integration time is 8.5 hours and the limiting magnitude is K = 23.5 per arcsec2
(at S/N-ratio = 3). The pixel size is 0.15 arcsec. North is up and east is left. The “Full-Res” version
maintains the original pixels and is of the highest reproduction quality (least file compression). The
reproduction is “negative”, with dark objects on a light sky, in order to better show the faintest
objects. See also the technical note below.

ESO PR Photo 06b/00

Caption: ESO PR Photo 06b/00 is a composite colour image of the field shown in PR Photo
06a/00. It is a combination of the K-band image from ANTU/ISAAC shown in PR Photo 06a/00 with
two images obtained in the B and R bands with the SUSI-2 optical imager at the New Technology
Telescope (NTT) on La Silla in the framework of the ESO-EIS survey. Note the relatively high density
of red galaxies, visible in the upper right part of this image. The colours of most of these galaxies
are consistent with those of “evolved” galaxies, already present when the Universe was only 4
billions years old. The “Full-Res” version maintains the original pixels and is of the highest
reproduction quality (least file compression).

The group of European astronomers recently obtained a first “ultra-deep” 4.5 arcmin2 image in the
near-infrared J (wavelength 1.2 µm) and K (2.2 µm) bands, centered in the so-called “AXAF Deep
Field”, cf. PR Photos 06a-b/00.

This area of the sky is remarkably devoid of bright stars and provides a clear view towards the
remote Universe, as there is little obscuring dust in our own Galaxy, the Milky Way, in this direction. It
is therefore uniquely suited to probe the depth of the Universe. It is exactly for this reason that it
was selected for a deep survey to be conducted with the Chandra X-Ray Observatory (CXO) during
the guaranteed observing time of the former ESO Director General, Professor Riccardo Giacconi and
as a deep field of the ESO Imaging Survey (EIS, cf. ESO Press Photos 46a-j/99). The sky field
observed with ISAAC and shown above is near the centre of the WFI image (ESO PR Photo 46a/99);
it is displaced about 3.6 arcmin towards West and 1.0 armin towards North.

As seen on the photos, there are great numbers of faint galaxies in this direction. Those of very red
colour emit most of their light in the infrared spectral region and are particularly interesting since
they may either be highly obscured or contain mostly old stars, as described above.

New research possibilities

With observations as these, ISAAC is now opening a new window towards the distant Universe. The
comparison of the new NIR observations with earlier exposures at other wavelengths provides
unique research opportunities. It is possible to measure the average star formation rate and the
total stellar mass content in galaxies that are heavily obscured and are therefore not observable in
the optical bands and which may constitute a substantial fraction of the primeval galaxy population.

Such measurements will also allow to test current theories of galaxy formation that predict stars to
be gradually assembled into galaxies, and hence envisage a progressive decline in the galaxy
population towards very early cosmic times, in particular within 1-2 billion years after the Big Bang.
Moreover, a comparison of NIR, optical and X-ray images will make it possible to gain new insights
into the nuclear activity at the center of star-forming galaxies. It will become possible to study the
distinct effects due to massive black holes and bursts of star formation.

Concentrations of galaxies at large distances

The relatively large field-of-view of ISAAC allows to gain information about the distribution in space
of the faintest and most distant, evolved galaxies and also about the existence of associations of
distant galaxies.

A first clear example is the concentration of galaxies that appear uniformly yellow in PR Photo
06b/00, apparently tracing a group of galaxies that was already assembled when the Universe was
only 6 billion years old. A confirmation of the distance of a few of these galaxies has already been
obtained by means of spectral observations in the framework of an ESO Large Programme, entitled
“A Stringent Test on the Formation of Early Type and Massive Galaxies” and carried out by another
group of astronomers [2].

A further clear example of a concentration of distant galaxies is seen in the upper right part of PR
Photo 06b/00. The very red colours of several galaxies in this sky area indicate that they are even
more distant, “evolved” galaxies, already present when the Universe was only 1/3 of the current age.


[1] The European team consists of Emanuele Giallongo (Principal
Investigator), Adriano Fontana and Francesco Poli (all at Rome Observatory), Stephane Arnouts and
Sandro D’Odorico (European Southern Observatory, Garching), Stefano Cristiani (ST European
Coordinating Facility, Garching), Nicola Menci (Rome Observatory) and Paolo Saracco (Milan
Observatory). The data analysis was performed at the Milan (P. Saracco) and Rome (A. Fontana, F.
Poli) Observatories.

[2] This programme is conducted Andrea Cimatti (Principal Investigator) and Emanuele Daddi (both
at Arcetri Observatory), Tom Broadhurst, Sandro D’Odorico, Roberto Gilmozzi and Alvio Renzini
(European Southern Observatory), Stefano Cristiani (ST European Coordinating Facility, Garching),
Adriano Fontana, Emanuele Giallongo, Nicola Menci and Francesco Poli (Rome Observatory), Marco
Mignoli, Lucia Pozzetti and Giovanni Zamorani (Bologna Observatory) and Paolo Saracco (Milan

Technical note: The K-band image (PR Photo 06a/00) is the result of 510 min of integration time with
ISAAC at VLT ANTU. The 3-sigma magnitude limit is about K = 23.5 per arcsec2. A J-band image
was also obtained during 200 min of integration, with a 3-sigma limit of J = 25 per arcsec2. The
seeing FWHM (Full Width at Half Maximum) is 0.65 arcsec for both bands. The redshift, estimated on
the basis of the measured colours of the mentioned over-density of yellow galaxies (cf. PR Photo
06b/00), is between 0.6 and 0.7 and that of the red galaxies is between 1 and 1.4.