We present a gravitational lensing analysis of the cluster of galaxies MS 1008-1224 ( z = 0.30 ) , based on very deep observations obtained using the VLT with FORS and ISAAC during the science verification phase . We reconstructed the projected mass distribution from the B , V , R and I band data using two different algorithms independently .
The mass maps are remarkably similar , which confirmed that the PSF correction worked well , thanks to the superb quality of the images . The J and K band data ( ISAAC ) were combined with the BVRI ( FORS ) data to measure the photometric redshifts of galaxies inside the ISAAC field and to constrain the redshift distribution of the lensed sources .
This enabled us to scale the gravitational convergence into an accurate mass estimate . The total mass inferred from weak shear is 2.3 \times 10 ^ { 14 } h ^ { -1 } M _ { \odot } on large scales ( within \approx 700 h ^ { -1 } kpc ) which is in good agreement with the X-ray mass .
The Mass-to-light ratios are also in excellent agreement ( 319 h , against 312 h from the X-ray ) .
The measured mass profile is well fit by both Navarro , Frenk and White and isothermal sphere with core radius models although the NFW appears to be slightly better . In the inner regions , the lensing mass is about 2 times higher than the X-ray mass , which supports the long-held view that complex physical processes occuring in the innermost parts of lensing-clusters are responsible for the X-ray/lensing mass discrepancy .
We found that the central part of the cluster is composed of two mass peaks whose the center of mass is located 15 arcsecond north of the cD galaxy .
This provides an explanation for the 15 arcsecond offset between the cD and the center of the X-ray map already reported elsewhere . The optical , X-ray and the mass distributions show that MS 1008-1224 is composed of many subsystems which are probably undergoing a merger .
It is likely that the gas is not in equilibrium in the innermost regions which vitiates the X-ray mass estimate . MS 1008-1224 shows a remarkable case of cluster-cluster lensing .
The photometric redshifts show an excess of galaxies located 30 arcseconds south-west of the cD galaxy at a redshift of about 0.9 .
This distant cluster is lensed by MS 1008-1224 , which enables the detection of many of its galaxies .
Hence , MS 1008-1224 can be viewed as a gravitational telescope facilitating the study of a distant cluster . These results show that detailed investigations of lensing clusters require very deep imaging with sub-arcsecond seeing in multiple bands ( BVRI and JK ) .
Our analysis demonstrates that a thorough investigation of clusters of galaxies and a careful handling of the biases can not be performed without a dataset which fulfills these requirements .
The outstanding capabilities of the VLT at Cerro Paranal make it a unique tool which makes such studies possible .