The Low-Mass End of the Initial Mass Function in Massive Early-Type-Galaxies

I present a spectroscopic method to constrain the low-mass end of the Initial Mass Function (IMF) from integrated optical spectra of massive early-type galaxies. I use the new set of non-degenerate optical spectroscopic indices defined in Spiniello et al. (MNRAS 438: 1483, 2014) which are strong in cool giants and dwarfs and almost absent in main sequence stars. The use of these spectroscopic indicators together with an extended version of the simple stellar populationmodels by Conroy and van Dokkum (ApJ 747: 69, 2012) allows me to decouple the IMF from stellar population age, metallicity and elemental abundance, and thus to constrain the low-mass end of the IMF slope. I present evidence that the low-mass end of the IMF is not universal. A clear trend of steepening IMF slope with increasing velocity dispersion from 150 to 310km s(-1) is found in stacked Sloan Digital Sky Survey (SDSS) galaxies spectra with varying velocity dispersions.