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Systematic variation of the stellar initial mass function in early-type galaxies


Much of our knowledge of galaxies comes from analysing the radiation emitted by their stars, which depends on the present number of each type of star in the galaxy. The present number depends on the stellar initial mass function (IMF), which describes the distribution of stellar masses when the population formed, and knowledge of it is critical to almost every aspect of galaxy evolution. More than 50 years after the first IMF determination1, no consensus has emerged on whether it is universal among different types of galaxies2. Previous studies indicated that the IMF and the dark matter fraction in galaxy centres cannot both be universal3,4,5,6,7, but they could not convincingly discriminate between the two possibilities. Only recently were indications found that massive elliptical galaxies may not have the same IMF as the Milky Way8. Here we report a study of the two-dimensional stellar kinematics for the large representative ATLAS3D sample9 of nearby early-type galaxies spanning two orders of magnitude in stellar mass, using detailed dynamical models. We find a strong systematic variation in IMF in early-type galaxies as a function of their stellar mass-to-light ratios, producing differences of a factor of up to three in galactic stellar mass. This implies that a galaxy’s IMF depends intimately on the galaxy's formation history.

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Figure 1: Differentiating between the stellar and dark matter with integral-field stellar kinematics.
Figure 2: Systematic variation of the IMF in early-type galaxies.


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M.C. acknowledges support from a Royal Society University Research Fellowship. This work was supported by the rolling grants ‘Astrophysics at Oxford’ from the UK Research Councils. R.L.D. acknowledges support from Christ Church College, Oxford University, and from the Royal Society in the form of a Wolfson Merit Award. S.K. acknowledges support from the Royal Society Joint Projects Grant. R.M.M. is supported by the Gemini Observatory. T.N. and M. Bois acknowledge support from the DFG Cluster of Excellence ‘Origin and Structure of the Universe’. M.S. acknowledges support from a STFC Advanced Fellowship. N.S. and T.A.D. acknowledge support from an STFC studentship.

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Correspondence to Michele Cappellari.

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Cappellari, M., McDermid, R., Alatalo, K. et al. Systematic variation of the stellar initial mass function in early-type galaxies. Nature 484, 485–488 (2012).

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