The global mass function of M 15

Data obtained with the NICMOS instrument on board the Hubble Space Telescope (HST) have been used to deter- mine the H-band luminosity function (LF) and mass function (MF) of three stellar fields in the globular cluster M 15, located ∼7 � from the cluster centre. The data confirm that the cluster MF has a characteristic mass of ∼0.3 M� , 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 simulta- neously 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 � 0.8 for stars more massive than 0.8 M� , x � 0.9 for 0.3−0.8 Mand x �− 2.2 at smaller masses (Salpeter's IMF would have x = 1.35). The best fitting model also suggests that the cluster mass is ∼5.4 × 10 5 Mand that the mass-to-light ratio is on average M/LV � 2.1, with M/LV � 3. 7i n the core. A large amount of mass (∼44%) is found in the cluster core in the form of stellar heavy remnants, which may be sufficient to explain the mass segregation in M 15 without invoking the presence of an intermediate-mass black hole.