Old Metal-rich Globular Cluster Populations: Peak Color and Peak Metallicity Trends with Mass of Host Spheroids

TL;DR

This study investigates how the peak color and metallicity of old metal-rich globular clusters relate to the mass of their host galaxies, considering age and element ratio effects, and finds age and alpha-element variations significantly influence observed trends.

Contribution

It demonstrates that age and alpha-element ratio variations can account for a substantial part of the color-mass trend in globular clusters, challenging the assumption that metallicity alone explains it.

Findings

Age trends can contribute up to ~50% of the color trend.

Alpha-element ratio variations explain about 30% of the color trend.

Metallicity variations are likely smaller than previously assumed.

Abstract

We address the problem of the factors contributing to a peak color trend of old metal-rich globular cluster (MRGC) populations with mass of their hosts, early-type galaxies and spheroidal subsystems of spiral ones (spheroids). The color-mass trend is often converted to a metallicity-mass trend under the assumption that age effects are small or negligible. While direct estimates of the ages of MRGC populations neither can rule out nor reliably support the populations' age trend, key data on timing of the formation of spheroids and other indirect evidence imply it in the sense: the more massive spheroid the older on average its MRGC population. We show that the contribution of an allowable age trend of the MRGC populations to their peak color trend can achieve up to ~50 % or so. In this event the comparable value of the color trend, ~30 %, is due to alpha-element ratio systematic variations of the order of Delta[alpha/Fe] ~ 0.1 to 0.2 dex because of a correlation between the [alpha/Fe] ratios and age. Hence a systematic variation of exactly [Fe/H] ratios may turn out to be less significant among the contributors, and its range many times lower, i.e. of the order of Delta[Fe/H] ~ 0.1 or even none, than the corresponding range deduced by assuming no age trend.