Stellar populations in the centres of brightest cluster galaxies

TL;DR

This study analyzes the stellar populations in 49 brightest cluster galaxies, revealing subtle differences from normal ellipticals and linking younger ages to cooling-flow clusters, to understand their formation and evolution.

Contribution

It provides detailed SSP-parameter measurements for BCGs and compares them with normal ellipticals, highlighting differences in metallicity and alpha-abundance ratios.

Findings

BCGs have higher metallicity and alpha-abundance than normal ellipticals.

No significant difference in index-velocity dispersion relations between BCGs and ellipticals.

Approximately 26% of BCGs are younger than 6 Gyr, often in cooling-flow clusters.

Abstract

This paper is part of a series devoted to the study of the stellar populations in brightest cluster galaxies (BCGs), aimed at setting constraints on the formation and evolution of these objects. We have obtained high signal-to-noise ratio, long-slit spectra of 49 BCGs in the nearby Universe. Here, we derive Single Stellar Population (SSP)-equivalent ages, metallicities and alpha-abundance ratios in the centres of the galaxies using the Lick/IDS system of absorption line indices. We systematically compare the indices and derived parameters for the BCGs with those of large samples of ordinary elliptical galaxies in the same mass range. We find no significant differences between the index-velocity dispersion relations of the BCG data and those of normal ellipticals, but we do find subtle differences between the derived SSP-parameters. The BCGs show, on average, higher metallicity ([Z/H]) and alpha-abundance ([E/Fe]) values. We analyse possible correlations between the derived parameters and the internal properties of the galaxies (velocity dispersion, rotation, luminosity) and those of the host clusters (density, mass, distance from BCG to X-ray peak, presence of cooling flows), with the aim of dissentangling if the BCG properties are more influenced by their internal or host cluster properties. The SSP-parameters show very little dependence on the mass or luminosity of the galaxies, or the mass or density of the host clusters. Of this sample, 26 per cent show luminosity-weighted ages younger than 6 Gyr, probably a consequence of recent - if small - episodes of star formation. In agreement with previous studies, the BCGs with intermediate ages tend to be found in cooling-flow clusters with large X-ray excess.