Dark matter and IMF normalization in Virgo dwarf early-type galaxies

Abstract

In this work, we analyse the dark matter (DM) fraction, f<SUB>DM</SUB>, and mass-to-light ratio mismatch parameter, δ<SUB>IMF</SUB> (computed with respect to a Milky Way-like initial mass function), for a sample of 39 dwarf early-type galaxies in the Virgo cluster. Both f<SUB>DM</SUB> and δ<SUB>IMF</SUB> are estimated within the central (one effective radius) galaxy regions, with a Jeans dynamical analysis that relies on galaxy velocity dispersions, structural parameters, and stellar mass-to-light ratios from the SMAKCED survey. In this first attempt to constrain, simultaneously, the initial mass function (IMF) normalization and the DM content, we explore the impact of different assumptions on the DM model profile. On average, for an Navarro, Frenk & White (NFW) profile, the δ<SUB>IMF</SUB> is consistent with a Chabrier-like normalization ({δ _IMF}∼ 1), with {f_DM}∼ 0.35. One of the main results of this work is that for at least a few systems the δ<SUB>IMF</SUB> are heavier than the Milky Way-like value (I.e. either top- or bottom-heavy). When introducing tangential anisotropy, larger δ<SUB>IMF</SUB> and smaller f<SUB>DM</SUB> are derived. Adopting a steeper concentration-mass relation than that from simulations, we find lower δ<SUB>IMF</SUB> ( ≲ 1) and larger f<SUB>DM</SUB>. A constant M/L profile with null f<SUB>DM</SUB> gives the heaviest δ<SUB>IMF</SUB> (∼2). In the MONDian framework, we find consistent results to those for our reference NFW model. If confirmed, the large scatter of δ<SUB>IMF</SUB> for dEs would provide (further) evidence for a non-universal IMF in early-type systems. On average, our reference f<SUB>DM</SUB> estimates are consistent with those found for low-σ<SUB>e</SUB> (∼ 100 km s^{-1}) early-type galaxies (ETGs). Furthermore, we find f<SUB>DM</SUB> consistent with values from the SMAKCED survey, and find a double-value behaviour of f<SUB>DM</SUB> with stellar mass, which mirrors the trend of dynamical M/L and global star formation efficiency (from abundance matching estimates) with mass.