A. A. Dutton, T. Treu, B. J. Brewer, P. J. Marshall, M. W. Auger, M. Barnabe, D. C. Koo, A. S. Bolton, L. V. E. Koopmans
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 normalization 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 is tightly constrained by the lensing and kinematic data and consistent with that obtained by stellar population synthesis models fits to their colours only by assuming 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 provide evidence for a non-universal IMF within the different components of spiral galaxies, adding to the well-known differences in stellar populations between disks and bulges.
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http://arxiv.org/abs/1206.4310
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