The similarity of the stellar mass fractions of galaxy groups and clusters

TL;DR

This study analyzes a large sample of galaxy groups and clusters to understand how stellar mass fractions, including intracluster light, vary with total mass, revealing weak dependence and implications for baryon content and feedback processes.

Contribution

It provides a comprehensive observational analysis with a large sample, explicitly includes intracluster light, and compares mass estimates to simulations and lensing, advancing understanding of stellar mass fractions in galaxy systems.

Findings

Stellar mass fraction depends weakly on total mass.

Intracluster light contributes 20-40% to stellar mass.

Galaxy groups have lower baryon fractions than clusters.

Abstract

We employ a large sample of 20171 optically-selected groups and clusters at 0.15 < z < 0.4 in the SDSS to investigate how the stacked stellar mass fraction varies across a wide range of total mass, $M_{500}$. Our study improves upon previous observational studies in a number of important ways, including having a much larger sample size, an explicit inclusion of the intracluster light (ICL) component, and a thorough examination of the accuracy of our total mass estimates via comparisons to simulations and weak lensing observations. We find that the stellar mass fraction depends only weakly on total mass and that the contribution of ICL to the total stellar mass fraction is significant (typically 20-40 per cent). Both of these findings are in excellent accordance with the predictions of cosmological simulations. Under the assumption of a Chabrier (Salpeter) IMF, the derived star formation efficiency ($f_{star}$/$f_{b}$, where $f_b=\Omega_b/\Omega_m$) is relatively low at 8 per cent (14 per cent) and is consistent with the global star formation efficiency of semi-analytic models that reproduce the galaxy stellar mass function. When our measured stellar mass fractions are combined with the observed relation between hot gas mass fraction and total mass from X-ray observations, our results imply that galaxy groups have significantly lower baryon fractions than massive clusters. Ejection of gas due to energetic AGN feedback (most likely at high redshift) provides a plausible mechanism for explaining the trends we observe.