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Guiding the Giant

New ESO Survey Provides Targets for the VLT

4 sierpnia 1998

Giant astronomical telescopes like the ESO Very Large Telescope (VLT) must be used efficiently. Observing time is expensive and there are long waiting lines of excellent research programmes. Thus the work at the telescope must be very well prepared and optimized as much as possible - mistakes should be avoided and no time lost! Astronomers working with the new 8-m class optical/infrared telescopes must base their observations on detailed lists of suitable target objects if they want to perform cutting-edge science. This is particularly true for research programmes that depend on observations of large samples of comparatively rare, distant objects. This type of work requires that extensive catalogues of such objects must be prepared in advance. One such major catalogue - that will serve as a very useful basis for future VLT observations - has just become available from the new ESO Imaging Survey (EIS).

The Need for Sky Surveys

Astronomers have since long recognized the need to carry out preparatory observations with other telescopes in order to "guide" large telescopes. To this end, surveys of smaller or larger parts of the sky have been performed by wide-field telescopes, paving the way for subsequent work at the limits of the largest available ground-based telescopes. For instance, a complete photographic survey of the sourthern sky (declination < -17.5°) was carried out in the 1970's with the ESO 1-metre Schmidt Telescope in support of the work at the 3.6-m telescope at the ESO La Silla observatory.

However, while until recently most observational programmes could rely on samples of objects found on photographic plates, this is no longer possible. New image surveys must match the fainter limiting magnitudes reached by the new and larger telescopes.

Modern digital, multi-colour, deep imaging surveys have thus become an indispensable complement to the 8-m telescopes. The new generation of imaging surveys will, without doubt, be the backbone of future research and are likely to be as long-lived as their earlier counterparts, which have served the astronomical community so well over the past decades. The new surveys are now becoming possible, thanks to the new, extremely light-sensitive CCD-mosaics mounted on wide-field telescopes.

The ESO Imaging Survey (EIS)

A very successful, major step in this direction has recently been taken at ESO. It concerns an imaging survey with the 3.5-m New Technology Telescope (NTT) at La Silla, aimed at defining targets for the first year of operation of the VLT. In addition to serving the future observers, this survey is also public , i.e., the resulting data are made available to all interested parties.

The project is known as the ESO Imaging Survey (EIS). It is supervised by a Working Group with members from the European astronomical community ([1]) that has been responsible for defining the survey strategy and for monitoring the progress.

It has been a major challenge to carry out such a public survey in the very short time available. The work by the EIS Team has involved the survey observations at the NTT, development of a pipeline to process the raw data, advanced data reduction, identification of large samples of astronomically "interesting" targets and, not least, the distribution of images and other survey products before the start of operation of the VLT.

To cope with the ambitious one-year timetable, a novel type of collaboration between ESO and the astronomical communities in the ESO Member States was set up. It has allowed to combine efficiently the scientific and technical expertise of the community with ESO in-house know-how and infrastructure. This model has been very successful and may well set the example for future surveys.

Science Goals of EIS

EIS is in many aspects a novel approach for large-scale, ground-based optical observations, in support of large-telescope science. The speed with which raw EIS data have been converted to deliverable products is quite unprecedented, given the nature and scope of this project. This is a key ingredient for imaging surveys, the main goal of which is to provide target lists for 8-m class telescopes.

EIS consists of two parts: a wide-angle survey ("EIS-wide" ) and a deep, multi-colour survey in four optical and two infrared bands ("EIS-deep" ).

EIS-wide covers four pre-selected patches of sky (spanning the R.A. range from 22 h to 9 h). The main science goals of EIS-wide include the search for distant clusters of galaxies and quasars. In addition, there are important spin-offs in terms of bright and distant galaxies, as well as new information about galactic structure and stellar populations.

The observations were conducted in 10 runs in the period July 1997 - March 1998. A total of 36 nights were used for this part of the project. The images obtained cover a total area of 17 square degrees in the near-infrared I-band, reaching limiting magnitude of I ~ 23 and, furthermore, an area of 1.7 square degrees in the B- (blue), V- (green-yellow) and I-bands to a comparable depth.

Altogether, the EIS data set consists of about 6000 science and calibration frames, totaling 96 Gbytes of raw data and over 200 Gbytes of reduced images and derived products.

Some results from EIS

On these images, over one million galaxies were detected and about 250 distant clusters of galaxies were identified, with estimated redshifts in the range 0.2 < z < 1.3 [2]. This is by far the largest sample of distant clusters of galaxies currently available.

In addition, white dwarfs, very-low mass stars/brown dwarfs and high-redshift quasar candidates were identified in the field that lies in the direction of the South Galactic Pole.

All the calibrated images and derived catalogs are now publicly available. They can be examined and/or retrieved through an interface in the EIS release WWW-page built in collaboration with the ESO Science Archive, a prototype for future distribution of data to the ESO community.

A photo of a 25 arcmin wide field from EIS is available on the web as ESO Press Photo eso9824 ; the two versions may be accessed via eso9807.

Future surveys at ESO

The EIS project has been conceived as a pilot project for more ambitious, future wide-field imaging surveys to be conducted by ESO. Together, they will provide the basic framework and infrastructure for the gradual development of the required capabilities for pipeline processing, archiving and data mining.

By January 1999, the ESO/MPIA 2.2-m telescope at La Silla will start regular observations with a wide-field camera capable of imaging in one shot an area of the sky that is larger than the full moon. This telescope will be fully dedicated to wide-field imaging and will be approximately 6 times more efficient than is the NTT for imaging surveys such as EIS.

An even more powerful survey telescope is now planned for the Paranal Observatory, next to the VLT. A Memorandum of Understanding has recently been signed by the Director General of ESO, Professor Riccardo Giaconni and the Director of the Capodimonte Observatory (Naples, Italy), Professor Massimo Capaccioli . According to this, the Capodimonte Observatory will deliver to ESO a wide-field 2.6-m telescope, referred to as the VLT Survey Telescope (VST).

The VST will be over 12 times more efficient than the 2.2-m telescope for survey work. When it goes into operation some years from now, ESO will consolidate its front-line position in wide-field imaging capabilities.

Another survey, the DEep Near Infrared Southern Sky Survey (DENIS), is now being carried out at La Silla. It is a joint European project that is conducted at the 1-m ESO telescope by a consortium of 20 astronomical institutes.

Uwagi

[1] The home institutes of the astronomers involved in EIS include the European Southern Observatory, Osservatorio Astronomico di Trieste (Italy), Leiden Observatory (The Netherlands), Institut d'Astrophysique de Paris (France), Max-Planck Institut für Astrophysik (Germany), Astronomisk Observatorium (Copenhagen, Denmark), Istituto di Radioastronomia del CNR (Bologna, Italy), Landensternwarte Heidelberg-Königstuhl (Heidelberg, Germany), DAEC, Observatoire de Paris-Meudon (France), ESA/ESO Space Telescope-European Coordinating Facility (Garching, Germany), Osservatorio Astronomico di Pino Torinese, Torino (Italy) and Osservatorio Astronomico di Capodimonte (Napoli, Italy).

[2] In astronomy, the redshift (z) 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 or quasar gives a direct estimate of the universal expansion (i.e. the `recession velocity'). Since this expansion rate increases with the distance, the velocity (and thus the redshift) is itself a function (the Hubble relation) of the distance to the object. The indicated redshift interval (0.2 < z < 1.3) corresponds to a distance interval of approx. 3,000 to 7,000 million light-years.

Więcej informacji

Further information about EIS is available at http://www.eso.org/eis. From this site, it is possible to visit the EIS release page and to browse through pictures of the distant Universe and of individual objects, some of which will be observed with the VLT in the future.

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