The Initial Mass Function of Early-type Galaxies: no correlation with
  [Mg/Fe]
  The Initial Mass Function (IMF) of early-type galaxies (ETGs) has been found
to feature systematic variations by both dynamical and spectroscopic studies.
In particular, spectral line strengths, based on gravity-sensitive features,
suggest an excess of low-mass stars in massive ETGs, i.e. a bottom-heavy IMF.
The physical drivers of IMF variations are currently unknown. The abundance
ratio of alpha elements, such as [Mg/Fe], has been suggested as a possible
driver of the IMF changes, although dynamical constraints do not support this
claim. In this letter, we take advantage of the large SDSS database. Our sample
comprises 24,781 high-quality spectra, covering a large range in velocity
dispersion (100<sigma0<320 km/s) and abundance ratio (-0.1<[Mg/Fe]<+0.4). The
large volume of data allows us to stack the spectra at fixed values of sigma0
and [Mg/Fe]. Our analysis -- based on gravity-sensitive line strengths -- gives
a strong correlation with central velocity dispersion and a negligible
variation with [Mg/Fe] at fixed sigma0. This result is robust against
individual elemental abundance variations, and seems not to raise any apparent
inconsistency with the alternative method based on galaxy dynamics.
