The Mgb- relation of elliptical galaxies at z 0.37

We derive absorption indices of ellipticals in clusters at z=0.37 frommedium-resolution spectroscopy together with kinematical parameters. Thesegalaxies exhibit a relationship between the Mgb linestrength and their internalvelocity dispersion similar to local dynamically hot galaxies. But for anygiven sigma, Mgb of the distant ellipticals is significantly lower than themean value of the nearby sample. The difference of Mgb between the two samplesis small and can be fully attributed to the younger age of the distant stellarpopulations in accordance with the passive evolution model. The low reductionof Mgb at a look-back time of about 5 Gyrs requires that the bulk of the starsin cluster ellipticals have formed at very high redshifts of z_f>2. For themost massive galaxies, where the reduction is even lower, z_f probably exceeds4. Unlike most methods to measure the evolution of ellipticals usingluminosities, surface brightnesses or colours, the Mgb linestrength does notdepend on corrections for extinction and cosmic expansion (K-correction) andonly very little on the slope of the initial mass function. The combination ofa kinematical parameter with a stellar population indicator allows us to studythe evolution of very similar objects. In addition, the good mass estimateprovided by sigma means that the selection criteria for the galaxy sample as awhole are well controlled. (abriged)