The global mass function of M15

Data obtained with the NICMOS instrument on board the Hubble Space Telescope (HST) have been used to determine the H-band luminosity function (LF) and mass function (MF) of three stellar fields in the globular cluster M 15, located similar to7' from the cluster centre. The data confirm that the cluster MF has a characteristic mass of similar to0.3 M-circle dot, as obtained by Paresce & De Marchi (2000) for a stellar field at 4.'6 from the centre. By combining the present data with those published by other authors for various radial distances (near the centre, at 20 and at 4.'6), we have studied the radial variation of the LF due to the effects of mass segregation and derived the global mass function (GMF) using the Michie-King approach. The model that simultaneously best fits the LF at various locations, the surface brightness profile and the velocity dispersion profile suggest that the GMF should resemble a segmented power-law with the following indices: x similar or equal to 0.8 for stars more massive than 0.8 M-circle dot, x similar or equal to 0.9 for 0.3-0.8 M-circle dot and x similar or equal to -2.2 at smaller masses (Salpeter's IMF would have x = 1.35). The best fitting model also suggests that the cluster mass is similar to5.4 x 10(5) M-circle dot and that the mass-to-light ratio is on average M/L-V similar or equal to 2.1, with M/L-V similar or equal to 3.7 in the core. A large amount of mass (similar to44%) is found in the cluster core in the form of stellar heavy remnants, which may be sufficient to explain the mass segregation in M15 without invoking the presence of an intermediate-mass black hole.