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Recent work has suggested that the stellar initial mass function (IMF) is notuniversal, but rather is correlated with galaxy stellar mass, stellar velocitydispersion, or morphological type. In this paper, we investigate variations ofthe IMF within individual galaxies. For this purpose, we use strong lensing andgas kinematics to measure independently the normalisation of the IMF of thebulge and disk components of a sample of 5 massive spiral galaxies withsubstantial bulge components taken from the SWELLS survey. We find that thestellar mass of the bulges are tightly constrained by the lensing and kinematicdata. A comparison with masses based on stellar population synthesis modelsfitted to optical and near infrared photometry favors a Salpeter-likenormalisation of the IMF. Conversely, the disk masses are less well constraineddue to degeneracies with the dark matter halo, but are consistent with MilkyWay type IMFs in agreement with previous studies. The disks are submaximal at2.2 disk scale lengths, but due to the contribution of the bulges, the galaxiesare baryon dominated at 2.2 disk scale lengths. Globally, our inferred IMFnormalisation is consistent with that found for early-type galaxies ofcomparable stellar mass (> 10^11 M_sun). Our results suggest a non-universalIMF within the different components of spiral galaxies, adding to thewell-known differences in stellar populations between disks and bulges.

The SWELLS survey - V. A Salpeter stellar initial mass function in the bulges of massive spiral galaxies