HIGH-RESOLUTION KINEMATICAL MAPPING OF THE CORE OF THE GLOBULAR CLUSTER-M15-EQUIVALENT-TO-NGC-7078

Using the ESO New Technology Telescope, we have obtained five high-res olution integrated-light echelle spectra over the core of the globular cluster M 15 = NGC 7078. We used a 1'' x 8'' slit, with a 1'' offset between each exposure in order to cover a total central area of 5'' x 8''. By taking advantage of the spatial resolution along the slit, we extracted spectra at 120 different locations over apertures similar to 1'' square. The Doppler velocity broadening of the cross-correlation functions (CCFs) of these integrated-light spectra is always less than or equal to 17 km s(-1), at all locations in the 5'' x 8'' area. Our measurements therefore provide no evidence for the velocity dispersion cusp observed by Peterson et al. (1989). These new observations confi rm the predictions of our recent numerical simulations (Dubath et al. 1994a): the CCFs of integrated light spectra taken over such small ape rtures are mostly dominated by the contribution of one or two bright s tars, leading to unreliable estimates of the velocity dispersion. This bias can be reduced by taking the average of all 120 CCFs, normalized in intensity, over the whole 5'' x 8'' central area. This gives equal weight to spatially independent dominant stars of different brightnes s. The mean CCF derived in this way gives a velocity dispersion sigma( p) = 11.7 +/- 2.6 km s(-1). This value is independently confirmed by c omparing our observations with numerical simulations. The individual r adial velocities of the 14 best-resolved (spatially or spectroscopical ly) bright stars are also determined; they give sigma(p) = 14.2 +/- 2. 7 km s(-1), a value consistent with the above determination. Two of th e brightest central stars, separated by 2''.5, have radial-velocity va lues differing by 45.2 km s(-1). These stars may be the origin of prev ious reports in the literature of large rotation or streaming motion a cross the centre of M 15. Our study complements the recent work by Geb hardt et al. (1994), who measured the radial velocities of 216 stars l ocated within 1'.5 of the cluster centre. From 0'.1 to 0'.4, their dat a suggest a constant velocity dispersion of about sigma(p) = 11 km s(- 1) consistent with our result obtained inside 0'.1.