The (galaxy-wide) IMF in giant elliptical galaxies: from top to bottom

Abstract

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