Ca, Fe, and Mg Trends Among and Within Elliptical Galaxies

TL;DR

This study reveals that calcium abundance ratios decrease with galaxy mass in ellipticals, suggesting a possible IMF variation favoring massive stars, and rules out several alternative explanations for this trend.

Contribution

It introduces a new CaHK index to analyze calcium trends and provides evidence supporting IMF variation as a cause for calcium abundance differences.

Findings

[Ca/Fe] decreases with galaxy mass

Ca deficit is global, not radius-dependent

IMF variation may explain calcium trends

Abstract

In a sample of elliptical galaxies that span a large range of mass, a previously unused Ca index, CaHK, shows that [Ca/Fe] and [Ca/Mg] systematically decrease with increasing elliptical galaxy mass. Metallicity mixtures, age effects, stellar chromospheric emission effects, and low-mass initial mass function (IMF) boost effects are ruled out as causes. A [Ca/Fe] range of less than 0.3 dex is sufficient to blanket all observations. Feature gradients within galaxies imply a global Ca deficit rather than a radius-dependent phenomenon. Some, but not all, Type II supernova nucleosynthetic yield calculations indicate a decreasing Ca/Fe yield ratio in more massive supernovae, lending possible support to the hypothesis that more massive elliptical galaxies have an IMF that favors more massive stars. No Type II supernova nucleosynthetic yield calculations show significant leverage in the Ca/Fe ratio as a function of progenitor metallicity. Therefore, it seems unlikely that the Ca behavior can be explained as a built-in metallicity effect, and this argues against explanations that vary only the Type II to Type Ia supernova enrichment ratio.