Projection effects in cluster mass estimates : the case of MS2137
  We revisit the mass properties of the lensing cluster of galaxies MS2137-23 and assess the mutual agreement between cluster mass estimates based on lensing, X-rays and stellar dynamics. We perform a thorough elliptical lens modelling using arcs in the range 20<R<100kpc and weak lensing (100<R<1000kpc). We confirm that the dark matter distribution is consistent with an NFW profile with high concentration $c=11.7\pm0.6$. We further analyse the stellar kinematics data of Sand etal(2004) with a detailed modelling of the los velocity distribution of stars in the cD galaxy and quantify the small bias due to non-Gaussianity of the LOSVD. After correction, the NFW lens model is still unable to properly fit kinematical data and is twice as massive as suggested by X-rays (Allen etal2001). The discrepancy between projected and tridimensional mass estimates is studied by assuming prolate (triaxial) halos with the major axis oriented toward the line-of-sight. This model well explains the high concentration and the misalignement of 13 deg between stellar and dark matter components. We then calculate the systematic and statistical uncertainties in the relative normalization between cylindric M(< R) and spherical M(< r) mass estimates for triaxial halos. These uncertainties prevent any attempt to couple 2D and 3D constraints without a complete tridimensional analysis. Such asphericity/projection effects should be a major concern for comparisons between lensing and X-rays/dynamics mass estimates. 
Raphael Gavazzi (IAP, LAOMP)