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 \sim 7 ^ { \prime } from the cluster centre .
The data confirm that the cluster MF has a characteristic mass of \sim 0.3 M _ { \odot } , as obtained by Paresce & De Marchi ( 2000 ) for a stellar field at 4 \aas@@fstack { \prime } 6 from the centre .
By combining the present data with those published by other authors for various radial distances ( near the centre , at 20 ^ { \prime \prime } and at 4 \aas@@fstack { \prime } 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 suggests that the GMF should resemble a segmented power-law with the following indices : x \simeq 0.8 for stars more massive than 0.8 M _ { \odot } , x \simeq 0.9 for 0.3 - 0.8 M _ { \odot } and x \simeq - 2.2 at smaller masses ( Salpeter ’ s IMF would have x = 1.35 ) .
The best fitting model also suggests that the cluster mass is \sim 5.4 \times 10 ^ { 5 } M _ { \odot } and that the mass-to-light ratio is on average M / L _ { V } \simeq 2.1 , with M / L _ { V } \simeq 3.7 in the core .
A large amount of mass ( \sim 44 \% ) 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 .