Exploring the Star Formation History of Elliptical Galaxies: Beyond Simple Stellar Populations with a New Estimator of Line Strengths

TL;DR

This study introduces a new method for measuring stellar population indicators in elliptical galaxies, enabling more accurate star formation history modeling beyond simple stellar populations, revealing correlations with galaxy mass and formation epoch.

Contribution

We propose a Boosted Median Continuum approach for more robust equivalent width measurements and incorporate complex star formation histories to better understand elliptical galaxy evolution.

Findings

New continuum measurement method reduces age-metallicity degeneracy.

Composite models provide consistent fits to spectral lines.

Galaxy age and metallicity correlate with stellar mass and velocity dispersion.

Abstract

(Abridged) We study the stellar populations of 14 elliptical galaxies in the Virgo cluster. We propose an alternative approach to the standard side-band method to measure equivalent widths (EWs). Our Boosted Median Continuum maps the EWs more robustly than the side-band method, minimising the effect from neighbouring absorption lines and reducing the age-metallicity degeneracy. We concentrate on Balmer lines (Hbeta,Hgamma,Hdelta), the G band and the 4000A break as age-sensitive indicators, and on the combination [MgFe] as the main metallicity indicator. We go beyond the standard comparison of the observations with simple stellar populations (SSP) and consider various models to describe the star formation histories, either with a continuous star formation rate or with a mixture of two different SSPs. Composite models are found to give more consistent fits among individual line strengths and agree with an independent estimate using the spectral energy distribution. Our age and metallicity estimates correlate well with stellar mass or velocity dispersion, with a significant threshold around 5E10 Msun above which galaxies are uniformly old and metal rich. In a more speculative way, our models suggest that it is formation **epoch** and not formation timescale what drives the Mass-Age relationship of elliptical galaxies.