Globular cluster kinematics and X-ray emission in the early-type galaxy NGC 1399

Abstract

Context. Of the various observational methods used to address the question of dark matter in early-type galaxies there is the possibility of spatial overlap in using kinematics of stars, clusters and X-ray halos. Aims. To investigate methods for the determination of velocity distribution profiles and deviations from a Gaussian distribution using globular clusters in early-type galaxies, and the implications for dark matter. Methods. The maximum likelihood method together with standard statistical procedures were used to calculate the velocity dispersion. A new “tracer mass estimator” was applied to obtain a mass estimate based on the globular clusters, which is then compared to the mass obtained using X-rays. Results. In NGC 1399 the departures from a Gaussian distribution of the velocities at all radial distances are small. Our conclusion is that in spite of the observations that show that the velocity dispersion decreases between 4 and 10 R e there is evidence that dark matter exists beyond ${\sim} 3\,R_{\rm e}$.