VLT Images Progenitors of Today’s Large Galaxies [1]
Summary
An international team of astronomers [2] has made the deepest-ever
near-infrared Ks-band image of the sky, using the ISAAC multi-mode
instrument on the 8.2-m VLT ANTU telescope.
For this, the VLT was pointed for more than 100 hours under optimal
observing conditions at the Hubble Deep Field South (HDF-S) and obtained
images in three near-infrared filters. The resulting images reveal
extremely distant galaxies, which appear at infrared wavelengths, but are
barely detected in the deepest optical images acquired with the Hubble
Space Telescope (HST).
Astronomer Marijn Franx from the University of Leiden and leader of the
team concludes: “These results demonstrate that very deep observations in
the near-infrared are essential to obtain a proper census of the earliest
phases of the universe. The new VLT images have opened a new research
domain which has not been observationally accessible before”.
The HDF-S is a tiny field on the sky in the southern constellation Tucana
(The Toucan) – only about 1% of the area of the full moon. The NASA/ESA
Hubble Space Telescope (HST) observed it with a total exposure time of
about 1 week, yielding the deepest optical images ever taken of the sky,
similar to those made earlier on the Hubble Deep Field North (HDF-N).
The VLT infrared images of the same field were obtained in the course of a
major research project, the Faint InfraRed Extragalactic Survey (FIRES).
They were made at wavelengths up to 2.3 micron where the HST is not
competitive.
Ivo Labbe, another team member from the University of Leiden, is certain:
“Without the unique capabilities of the VLT and ISAAC we would never have
been able to observe these very remote galaxies. In fact, the image in the
Ks-band is the deepest which has ever been made at that wavelength”.
The optical light emitted by the distant galaxies has been redshifted to
the near-infrared spectral region [3]. Indeed, some of the galaxies found
in the new images are so remote that – due to the finite speed of light –
they are observed as they were when the Universe was still extremely
young, less than 2 billion years old.
From these observations, two interesting conclusions have been drawn so
far. One is that although the newly identified galaxies do not appear to
form stars very actively they probably account for about half the mass of
normal matter present at this epoch. This is in sharp contrast to the
galaxies at this early time found during optical surveys – they are very
blue because of young and hot stars.
Another is that galaxies existed already at that epoch which are clearly
rather large, and some show spiral structure similar to that seen in very
nearby galaxies.
This new important insight is having profound impact on the current
attempts to understand the formation and evolution of galaxies.
Formation and evolution of galaxies
How did galaxies form in the early Universe? How did they evolve and when
did the first stars form in those systems?
These are some of the key questions in present-day astronomy. Thanks to
powerful ground- and space-based telescopes, astronomers are now able to
actively pursue studies in this direction. Recent front-line observational
results are helping them to gain new insights into these fundamental issues.
Light emitted by distant galaxies travels a long time before we observe it
with our telescopes. In this way, astronomers can look back in time and
directly study galaxies as they were when the universe was still very young.
However, this is technically difficult, as the galaxies are extremely faint.
Another complication is that, due to the expansion of the universe, their
light is shifted towards longer wavelengths [3].
In order to study those early galaxies in some detail, astronomers thus need
to use the largest ground-based telescopes, collecting their faint light
during very long integrations. And they must work in the infrared region of
the spectrum which is not visible to the human eye.
The Hubble Deep Field South (HDF-S) was selected to be studied in great
detail with the Hubble Space Telescope (HST) and other powerful telescopes.
The HST images of this field represent a total exposure time of 140 hours.
Many ground-based telescopes have obtained additional photos and spectra, in
particular telescopes at the European Southern Observatory in Chile.
The ISAAC observations
Caption: PR Photo 28a/02 shows a three-colour composite image of the small
sky field observed with the ISAAC multi-mode instrument at VLT ANTU during
the FIRES project. The central field observed by the HST WFPC2-camera is
outlined in white. The photo is a combination of one HST exposure (in the
I-filter at wavelength 0.814 micron; here rendered as blue) and two ISAAC
exposures (Js; 1.24 micron; green – Ks; 2.16 micron; red). A striking variety
of colours is evident, reflecting the different types and distances of the
galaxies in this field. PR Photo 28b/02 is a reproduction of the ISAAC
Ks-image with a total exposure time of 35.6 hours during optimal observing
conditions, and with extraordinary image sharpness, 0.46 arcsec. It is the
deepest image ever obtained in this waveband. The field measures 2.5 x 2.5
arcmin^2; North is up and East is left.
The sky field in the direction of HDF-S observed in the present study (the
Faint InfraRed Extragalactic Survey (FIRES)), measures 2.5 x 2.5 arcmin2. It
is slightly larger than the field covered by the WFPC2 camera on the HST,
but still 100 times smaller than the full moon.
Whenever the field was visible from Paranal and the atmospheric conditions
were optimal, ESO astronomers pointed the 8.2-m VLT ANTU telescope in the
direction of this field, taking near-infrared images with the ISAAC
multi-mode instrument. The data were transmitted by Internet to the
astronomers of the team in Europe, who then combined them to construct some
of the deepest infrared astronomical images ever taken from the ground.
Colours and distance
A crucial feature of the new observations is that they were made in three
infrared bands (Js, H, Ks), allowing a 3-dimensional view of a small region
of the Universe. This is because, by comparing the brightness of the
galaxies in these colours with that in optical light, as measured by the
HST, it is possible to estimate their redshifts [3] and thus how long ago
the light we now see has been emitted.
For the reddest of the galaxies the answer is that we are seeing them as
they were when the Universe was only about 2 billion years old.
The nature of the galaxies
PR Photos 28c-d/02 display images of some of the galaxies in the Hubble
Deep Field South, as they appear in different colours, including the V+I
(HST – 0.55 + 0.81 micron; left) visual band, the near-infrared Ks-band (VLT –
2.16 micron; middle), together with optical-to-infrared I,J,K-colour
composites (HST+VLT; right). In PR Photo 28c/02, three very red galaxies,
all at large distances, are found to be very bright in the infrared. The
upper two have compact shapes, whereas the galaxy at the bottom is very
large, comparable to the size of the Milky Way galaxy in which we live.
Red galaxies like these that were found in the present survey are a major
constituent of the Universe at high redshift. Three other galaxies in PR
Photo 28d/02 are equally distant but are bluer and their images are also
extended. There are indications of star formation in some knots in the
rudimentary spiral arms. The large galaxies represent a class never before
seen at this large distance and they look surprisingly similar to giant
spiral galaxies like our Milky Way galaxy.
Two conclusions drawn so far about the nature of these galaxies are
therefore all the more important in the context of formation and evolution
of galaxies.
One is that a few of them are clearly rather large and show spiral structure
similar to that seen in very nearby galaxies, cf. PR Photo 28d/02. It is not
obvious that current theoretical models can easily account for such galaxies
having evolved to this stage so early in the life of the Universe.
Another conclusion is that, in contrast to the galaxies at similar redshifts
(and hence, at this early epoch) found most commonly in surveys at optical
wavelengths, most of the ‘infrared-selected’ galaxies show relatively little
visible star-forming activity. They appear in fact to have already formed
most of their stars and in quantities sufficient to account for at least
half the total luminous mass of the Universe at that time. Given the time to
reach this state they must clearly have formed even earlier in the life of
the Universe and are thus probably amongst the “oldest” galaxies now known.
Rather than being randomly distributed in space, these red galaxies are also
found to prefer company, i.e., they tend to cluster close to each other. In
general terms this can be taken as support for the latest theoretical models
in which galaxies, which consist of “normal” matter, form in the
highest-density regions of the much more pervasive “dark” matter. Although
the latter accounts for most of the mass of the universe, its origin so far
is completely unknown.
These new observations may, therefore, also add new insight into one of the
biggest mysteries currently confronting cosmologists. Marijn Franx agrees,
but also cautions against drawing firm conclusions on this aspect too
quickly: “We now need similar images of a considerably larger region of the
sky. We will soon follow-up these first, tantalizing results with more
observations of other sky fields.”
More information
The information presented in this Press Release is based on a research
article (“Ultradeep Near-Infrared ISAAC Observations of the Hubble Deep
Field South: Observations, Reduction, Multicolor Catalog, and Photometric
Redshifts” by Ivo Labbe et al.) that will soon appear in the research
journal “Astronomical Journal” (cf. astro-ph/0212236). A shorter account
will appear in the December 2002 issue of ESO’s house journal “The
Messenger”. Information, including photos and reduced data, is also
available at the website of the FIRES project.
Notes
[1]: This press release is issued in coordination between ESO, Leiden
Observatory, the Netherlands Research School for Research in Astronomy
(NOVA) and the Netherlands Foundation for Research (NWO). A Dutch-language
version is available here.
[2]: The team consists of Ivo Labbe, Marijn Franx, Natascha M. Foerster
Schreiber, Paul van der Werf, Huub Roettgering, Lottie van Starkenburg, Arjen
van de Wel and Konrad Kuijken (Leiden Observatory, The Netherlands), Gregory
Rudnick (Max-Planck-Institut fuer Astrophysik, Garching, Germany),
Hans-Walter Rix (Max-Planck-Institut fuer Astronomie, Heidelberg, Germany),
Alan Moorwood and Emanuele Daddi (ESO, Garching, Germany) and Pieter G. van
Dokkum (California Institute of Technology, Pasadena, USA).
[3]: 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 remote galaxy provides an estimate of its distance.
Contacts
Marijn Franx
Leiden Observatory
The Netherlands
Phone: ++31-71-527-5870
email: franx@strw.leidenuniv.nl
Ivo Labbe
Leiden Observatory
The Netherlands
Phone: ++31-71-527-5805
email: ivo@strw.leidenuniv.nl
