The star formation timescale of elliptical galaxies -- Fitting [Mg/Fe] and total metallicity simultaneously

Abstract

The alpha element to iron peak element ratio, for example [Mg/Fe], is a commonly applied indicator of the galaxy star formation timescale (SFT) since the two groups of elements are mainly produced by different types of supernovae that explode over different timescales. However, it is insufficient to consider only [Mg/Fe] when estimating the SFT. The [Mg/Fe] yield of a stellar population depends on its metallicity. Therefore, it is possible for galaxies with different SFTs and at the same time different total metallicity to have the same [Mg/Fe]. This effect has not been properly taken into consideration in previous studies. In this study, we assume the galaxy-wide stellar initial mass function (gwIMF) to be canonical and invariant. We demonstrate that our computation code reproduces the SFT estimations of previous studies where only the [Mg/Fe] observational constraint is applied. We then demonstrate that once both metallicity and [Mg/Fe] observations are considered, a more severe "downsizing relation" is required. This means that either low-mass ellipticals have longer SFTs (> 4 Gyr for galaxies with mass below $10^{10}$ M${}_{\odot }$) or massive ellipticals have shorter SFTs ($\approx 200$ Myr for galaxies more massive than $10^{11}$ M${}_{\odot }$) than previously thought. This modification increases the difficulty in reconciling such SFTs with other observational constraints. We show that applying different stellar yield modifications does not relieve this formation timescale problem. The quite unrealistically short SFT required by [Mg/Fe] and total metallicity would be prolonged if a variable stellar gwIMF were assumed. Since a systematically varying gwIMF has been suggested by various observations this could present a natural solution to this problem.

9 pages, 7 figures, A&A, in press. Version 2 added references

References in corpus (17)

• The Atlas3D Project - XXX. Star formation histories and stellar population scaling relations of early-type galaxies
• Population Synthesis in the Blue IV: Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations
• The evolution of CNO isotopes: a new window on cosmic star-formation history and the stellar IMF in the age of ALMA
• The optimally-sampled galaxy-wide stellar initial mass function - Observational tests and the publicly available GalIMF code
• A uniform metallicity in the outskirts of massive, nearby galaxy clusters
• Timing the formation and assembly of early-type galaxies via spatially resolved stellar populations analysis
• Chemical Evolution on the Scale of Clusters of Galaxies: A Conundrum?
• Extreme chemical abundance ratio suggesting an exotic origin for an ultra-diffuse galaxy
• The chemical evolution of galaxies with a variable IGIMF
• The Extremely High Dark Matter Halo Concentration of the Relic Compact Elliptical Galaxy Mrk 1216
• The IGIMF and other IMFs in dSphs: the case of Sagittarius
• AGN feedback and the origin of the $\alpha$ enhancement in early type galaxies - insights from the GAEA model
• The effects of the IMF on the chemical evolution of elliptical galaxies
• The metal enrichment of passive galaxies in cosmological simulations of galaxy formation
• Variations of the Initial Mass Function in Semi-Analytical models
• The Influence of Environment on the Chemical Evolution in Low-mass Galaxies
• Simple interpolation functions for the galaxy-wide stellar initial mass function and its effects in early-type galaxies

Cited by in corpus (2)

• Stellar Population and Elemental Abundance Gradients of Early-type Galaxies
• Chemical evolution history of MaNGA galaxies