The global mass function of M 15
1
ESO/ST-ECF, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany e-mail: apasqual@eso.org
2
ESA, Space Telescope Operations Division, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: gdemarchi@rssd.esa.int
3
INAF, Osservatorio Astronomico di Roma, Via di Frascati 33, 00040 Monte Porzio Catone (RM), Italy e-mail: pulone@coma.mporzio.astro.it
4
Osservatorio Astronomico di Cagliari, Strada 54, Poggio dei Pini, 09012 Capoterra, Cagliari, Italy e-mail: sbrigas@eso.org
Received:
14
August
2003
Accepted:
21
July
2004
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 ~
from the cluster
centre. The data confirm that the cluster MF has a characteristic mass
of ~0.3 
, as obtained by Paresce & De Marchi ([CITE])
for a stellar field at 
from the centre. By combining the
present data with those published by other authors for various radial
distances (near the centre, at 
and at ),
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: 
for stars more
massive than 0.8 
, 
for 
and 
at smaller masses (Salpeter's IMF
would have 
). The best fitting model also suggests that the
cluster mass is ~
and that the
mass-to-light ratio is on average 
, with 
in the core. A large amount of mass (~
) 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.
Key words: globular clusters: general / globular clusters: individual: M 15
© ESO, 2004