1994A&A...290..104D

Using the ESO New Technology Telescope, we have obtained five high-resolution integrated-light echelle spectra over the core of the globular cluster M15=NGC7078. We used a 1"x8" slit, with a 1" offset between each exposure in order to cover a total central area of 5"x8". By taking advantage of the spatial resolution along the slit, we extracted spectra at 120 different locations over apertures ∼1" square. The Doppler velocity broadening of the cross-correlation functions (CCFs) of these integrated-light spectra is always ≤17km/s, at all locations in the 5"x8" area. Our measurements therefore provide no evidence for the velocity dispersion cusp observed by Peterson et al. (1989). These new observations confirm the predictions of our recent numerical simulations (Dubath et al. 1994a): the CCFs of integrated light spectra taken over such small apertures are mostly dominated by the contribution of one or two bright stars, 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"x8" central area. This gives equal weight to spatially independent dominant stars of different brightness. The mean CCF derived in this way gives a velocity dispersion σ_p=11.7±2.6km/s. This value is independently confirmed by comparing our observations with numerical simulations. The individual radial velocities of the 14 best-resolved (spatially or spectroscopically) bright stars are also determined; they give σ_p=14.2±2.7km/s, a value consistent with the above determination. Two of the brightest central stars, separated by 2.5", have radial-velocity values differing by 45.2km/s. These stars may be the origin of previous reports in the literature of large rotation or streaming motion across the centre of M15. Our study complements the recent work by Gebhardt et al. (1994), who measured the radial velocities of 216 stars located within 1.5' of the cluster centre. From 0.1' to 0.4,' their data suggest a constant velocity dispersion of about σ_p=11km/s consistent with our result obtained inside 0.1.'