Persbericht

VLT Studies a Moderately Distant Cluster of Galaxies

The FORS/ISAAC Cluster Deep Field

27 februari 1999

Studies of "Deep Fields" are becoming common practice in astronomy. To mention a few: the two Hubble Deep Fields ( HDF-N and HDF-S , north and south of the celestial equator), the NTT Deep Field, the AXAF Deep Field , the FORS Deep Field. The latter will be observed during FORS1 "guaranteed time" that is available to astronomers from those institutes that built this instrument. All of these sky fields have been selected for being quite 'empty', in the sense that few brighter objects are seen in them. They are thus of the same type ('generic'), with the partial exception of HDF-S that contains a QSO (quasar).

A primary imaging programme for the FORS1 Science Verification programme aims at obtaining multicolour (UBVRI), deep images of a moderate redshift cluster seen in this direction and, if possible, complemented with infrared (JHK) images with ISAAC of the cluster core.

The scientific issues that may be addressed with such observational data are manifold, among others:

• Obtain photometric redshifts (and thus distances) by measuring accurate colours of all galaxies in the field;
• Construct gravitational shear maps , by studying systematic effects in the orientation of axes of the galaxies in the field;
• Measure the magnification effect , i.e. the rate by which the light of gravitationally lensed background objects is amplified by this effect;
• Construct models of the mass distribution in the cluster from these shear maps, the measured magnification and the observed gravitational arcs;
• Study the cluster galaxy population down to faint magnitudes, by means of observed colour-magnitude diagrammes;
• Search for high-redshift, lensed galaxies - this is an indirect method to find extremely distant objects;
• Identify interesting (peculiar) stars in the field.

For all of these scientific applications, ample expertise exists within the ESO community. This ensures a prompt, wide, and qualified exploitation of the data. If time allows, some multi-object spectroscopy (MOS) may also be attempted, including arclets and very red objects.

FORS observations of the cluster MS1008.1-1224

For preliminary test observations, a suitable galaxy cluster was selected according to the following characteristics:

• moderate redshift between 0.3 and 0.8 ;
• well observable in the middle of February (i.e. sky position in the interval R.A. = 6 - 10h);
• have a large velocity dispersion (larger than 1000 km/s) and/or Einstein radius (a measure of the size), and possibly with a known giant arc(s) ;
• not too high galactic latitude, in order to have enough stars to map the Point-Spread Function (PSF; the shape of the exposed image of a star) across the FORS field-of-view.

Several possible candidates were considered, and finally the cluster MS1008.1-1224 was selected. The redshift is z = 0.306; the velocity dispersion is 1054 km/sec, and many arclets are seen within the cluster area.

Many exposures were obtained during FORS Commissioning in January 1999, and were used to produce the colour images below.

The two photos show the intricate structure of this fine cluster and reveal the presence of a substantial number of gravitational effects. The strong gravitational potential of the cluster acts as a gravitational lens and distorts the images of several background galaxies, while magnifying their luminosity. The mapping of this "gravitational shear" and the study of the numerous "gravitational arclets" that are produced by the cluster will allow to determine its mass and mass distribution.

Some of the magnified galaxies may be very distant, lying at very high redshift. For them the cluster works as a "gravitational telescope", facilitating the study of these otherwise very faint primordial galaxies.

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