Abstract
Recent evidence based independently on spectral line strengths and dynamical
modelling point towards a non-universal stellar Initial Mass Function (IMF),
implying an excess of low-mass stars in the most massive elliptical galaxies.
Here we show that a time-independent bottom-heavy IMF is compatible neither
with the observed metal-rich populations found in giant ellipticals nor with
the number of stellar remnants observed within these systems. We suggest a
two-stage formation scenario involving a time-dependent IMF to reconcile these
observational constraints. In this model, an early strong star-bursting stage
with a top-heavy IMF is followed by a more prolonged stage with a bottom-heavy
IMF. Such model is physically motivated by the fact that a sustained high star
formation will bring the interstellar medium to a state of pressure,
temperature and turbulence that can drastically alter the fragmentation of the
gaseous component into small clumps, promoting the formation of low-mass stars.
This toy model is in good agreement with the different observational constrains
on massive elliptical galaxies, such as age, metallicity, alpha-enhancement,
M/L, or the mass fraction of the stellar component in low-mass stars.
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