Stellar velocity dispersion and initial mass function gradients in dissipationless galaxy mergers

Monthly Notices of the Royal Astronomical Society, vol. 499, pp. 559–572

Abstract

ABSTRACT The stellar initial mass function (IMF) is believed to be non-universal among early-type galaxies (ETGs). Parametrizing the IMF with the so-called IMF mismatch parameter αIMF, which is a measure of the stellar mass-to-light ratio of an ensemble of stars and thus of the ‘heaviness’ of its IMF, one finds that for ETGs αe (i.e. αIMF integrated within the effective radius Re) increases with σe (the line-of-sight velocity dispersion σlos integrated within Re) and that, within the same ETG, αIMF tends to decrease outwards. We study the effect of dissipationless (dry) mergers on the distribution of the IMF mismatch parameter αIMF in ETGs using the results of binary major and minor merging simulations. We find that dry mergers tend to make the αIMF profiles of ETGs shallower, but do not alter significantly the shape of the distributions in the spatially resolved σlos–αIMF space. Individual galaxies undergoing dry mergers tend to decrease their αe, due to erosion of αIMF gradients and mixing with stellar populations with lighter IMF. Their σe can either decrease or increase, depending on the merging orbital parameters and mass ratio, but tends to decrease for cosmologically motivated merging histories. The αe–σe relation can vary with redshift as a consequence of the evolution of individual ETGs: based on a simple dry-merging model, ETGs of given σe are expected to have higher αe at higher redshift, unless the accreted satellites are so diffuse that they contribute negligibly to the inner stellar distribution of the merger remnant.

Authors 5

1. Carlo Nipoti corresponding

   University of Bologna

   Affiliation as printed

   Department of Physics and Astronomy, University of Bologna, via Gobetti 93/2, I-40129 Bologna, Italy

2. Carlo Cannarozzo

   National Institute for Astrophysics · University of Bologna

   Affiliation as printed

   Department of Physics and Astronomy, University of Bologna, via Gobetti 93/2, I-40129 Bologna, Italy

   INAF – Astrophysics and Space Science Observatory of Bologna, via Gobetti 93/3, I-40129 Bologna, Italy

3. F. Calura

   National Institute for Astrophysics

   Affiliation as printed

   INAF – Astrophysics and Space Science Observatory of Bologna, via Gobetti 93/3, I-40129 Bologna, Italy

4. Alessandro Sonnenfeld Aachen Leiden Observatory

   Leiden University · Leiden Observatory

   Affiliation as printed

   Leiden Observatory, Leiden University, Niels Bohrweg 2, NL-2333 CA Leiden, the Netherlands

5. Tommaso Treu

   University of California, Los Angeles

   Affiliation as printed

   Department of Physics and Astronomy, University of California, Los Angeles, CA 90095-1547, USA