On the Baryon Fractions in Clusters and Groups of Galaxies

TL;DR

This study investigates how baryon fractions vary across galaxy groups and clusters, revealing a break at ~1 keV and suggesting a universal baryon loss mechanism influenced by preheating.

Contribution

It provides the first comprehensive analysis of baryon fractions across a wide range of systems, identifying a break at ~1 keV and proposing a unified relation for galaxies and clusters.

Findings

Baryon fraction scales with temperature as f_b ∝ T^0.20 above 1 keV.

Below 1 keV, baryon fractions are significantly smaller and follow a different scaling.

A universal baryon loss mechanism is supported by the small scatter in baryon fractions.

Abstract

We present the baryon fractions of 2MASS groups and clusters as a function of cluster richness using total and gas masses measured from stacked ROSAT X-ray data and stellar masses estimated from the infrared galaxy catalogs. We detect X-ray emission even in the outskirts of clusters, beyond r_200 for richness classes with X-ray temperatures above 1 keV. This enables us to more accurately determine the total gas mass in these groups and clusters. We find that the optically selected groups and clusters have flatter temperature profiles and higher stellar-to-gas mass ratios than the individually studied, X-ray bright clusters. We also find that the stellar mass in poor groups with temperatures below 1 keV is comparable to the gas mass in these systems. Combining these results with individual measurements for clusters, groups, and galaxies from the literature, we find a break in the baryon fraction at ~1 keV. Above this temperature, the baryon fraction scales with temperature as f_b \propto T^0.20\pm0.03. We see significantly smaller baryon fractions below this temperature, and the baryon fraction of poor groups joins smoothly onto that of systems with still shallower potential wells such as normal and dwarf galaxies where the baryon fraction scales with the inferred velocity dispersion as f_b \propto \sigma^1.6. The small scatter in the baryon fraction at any given potential well depth favors a universal baryon loss mechanism and a preheating model for the baryon loss. The scatter is, however, larger for less massive systems. Finally, we note that although the broken power-law relation can be inferred from data points in the literature alone, the consistency between the baryon fractions for poor groups and massive galaxies inspires us to fit the two categories of objects (galaxies and clusters) with one relation.