The SWELLS survey - V. A Salpeter stellar initial mass function in the
  bulges of massive spiral galaxies
  Recent work has suggested that the stellar initial mass function (IMF) is not
universal, but rather is correlated with galaxy stellar mass, stellar velocity
dispersion, or morphological type. In this paper, we investigate variations of
the IMF within individual galaxies. For this purpose, we use strong lensing and
gas kinematics to measure independently the normalisation of the IMF of the
bulge and disk components of a sample of 5 massive spiral galaxies with
substantial bulge components taken from the SWELLS survey. We find that the
stellar mass of the bulges are tightly constrained by the lensing and kinematic
data. A comparison with masses based on stellar population synthesis models
fitted to optical and near infrared photometry favors a Salpeter-like
normalisation of the IMF. Conversely, the disk masses are less well constrained
due to degeneracies with the dark matter halo, but are consistent with Milky
Way type IMFs in agreement with previous studies. The disks are submaximal at
2.2 disk scale lengths, but due to the contribution of the bulges, the galaxies
are baryon dominated at 2.2 disk scale lengths. Globally, our inferred IMF
normalisation is consistent with that found for early-type galaxies of
comparable stellar mass (> 10^11 M_sun). Our results suggest a non-universal
IMF within the different components of spiral galaxies, adding to the
well-known differences in stellar populations between disks and bulges.
