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Recent studies have found that the oldest and most luminous galaxies in theearly Universe are surprisingly compact, having stellar masses similar topresent-day elliptical galaxies but much smaller sizes. This finding hasattracted considerable attention as it suggests that massive galaxies havegrown by a factor of ~five in size over the past ten billion years. A key testof these results is a determination of the stellar kinematics of one of thecompact galaxies: if the sizes of these objects are as extreme as has beenclaimed, their stars are expected to have much higher velocities than those inpresent-day galaxies of the same mass. Here we report a measurement of thestellar velocity dispersion of a massive compact galaxy at redshift z=2.186,corresponding to a look-back time of 10.7 billion years. The velocitydispersion is very high at 510 (+165, -95) km/s, consistent with the mass andcompactness of the galaxy inferred from photometric data and indicatingsignificant recent structural and dynamical evolution of massive galaxies. Theuncertainty in the dispersion was determined from simulations which include theeffects of noise and template mismatch. However, we caution that some subtlesystematic effect may influence the analysis given the low signal-to-noiseratio of our spectrum.

A high stellar velocity dispersion for a compact massive galaxy at z=2.2