Origin of a Bottom-heavy Stellar Initial Mass Function in Elliptical Galaxies
Abstract
We investigate the origin of a bottom-heavy stellar initial mass function (IMF) recently observed in elliptical galaxies by using chemical evolution models with a non-universal IMF. We adopt the variable Kroupa IMF with the three slopes (α1, α2, and α3) dependent on metallicities ([Fe/H]) and densities (ρg) of star-forming gas clouds and thereby search for the best IMF model that can reproduce (1) the observed steep IMF slope (α2 ~ 3, i.e., bottom-heavy) for low stellar masses (m <= 1 M ⊙) and (2) the correlation of α2 with chemical properties of elliptical galaxies in a self-consistent manner. We find that if the IMF slope α2 depends on both [Fe/H] and ρg, then elliptical galaxies with higher [Mg/Fe] can have steeper α2 (~3) in our models. We also find that the observed positive correlation of stellar mass-to-light ratios (M/L) with [Mg/Fe] in elliptical galaxies can be quantitatively reproduced in our models with α2vpropβ[Fe/H] + γlog ρg, where β ~ 0.5 and γ ~ 2. We discuss whether the IMF slopes for low-mass (α2) and high-mass stars (α3) need to vary independently from each other to explain a number of IMF-related observational results self-consistently. We also briefly discuss why α2 depends differently on [Fe/H] in dwarf and giant elliptical galaxies.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- December 2013
- DOI:
- 10.1088/0004-637X/779/1/9
- arXiv:
- arXiv:1311.1633
- Bibcode:
- 2013ApJ...779....9B
- Keywords:
-
- galaxies: abundances;
- galaxies: elliptical and lenticular;
- cD;
- galaxies: evolution;
- galaxies: stellar content;
- Astrophysics - Cosmology and Nongalactic Astrophysics;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 15 pages, 10 figures, accepted for the publication in ApJ