Exploring the bridge between A3556 and A3558 in the Shapley Supercluster

TL;DR

This study uses Chandra and optical/IR data to analyze the bridge between galaxy clusters A3556 and A3558, revealing that excess X-ray emission is due to a filament of galaxies, advancing understanding of intergalactic medium structures.

Contribution

It provides a detailed analysis of point sources in the bridge region, linking X-ray emission to galaxy filaments and improving interpretation of intergalactic medium observations.

Findings

Point source count exceeds average field population.

Point sources resemble galaxy distribution, indicating a filament.

X-ray excess explained by galaxy filament, density estimated at 150 times cosmic baryon density.

Abstract

Looking at the region connecting two clusters is a promising way to identify and study the Warm-Hot Intergalactic Medium. Observations show that the spectrum of the bridge between A3556 and A3558 has a stronger soft X-ray emission than the nearby region. Suzaku observations could not discriminate the origin of the extra emission. In this work we analyze a dedicated Chandra observation of the same target to identify point sources and characterize the background emission in the bridge. We find that the count number of the point sources is much higher than average field population (using CDFS~4~Ms as a reference). Moreover, the shape of the cumulative distribution resembles that of galaxy distribution suggesting that the point sources are galaxies in a filament. The Suzaku extra emission is well explained by the high abundance of point sources identified by Chandra. Furthermore, we used optical/IR observations of point sources in the same field to estimate the density of the putative filament as rho~150 rho_b$, below Suzaku sensitivity.