COS Observations of the Cosmic Web: A Search for the Cooler Components of a Hot, X-ray Identified Filament

TL;DR

This study used UV and optical observations to search for cooler gas components in a cosmic web filament associated with a galaxy cluster, finding no evidence of such gas and suggesting the filament's gas is predominantly hot.

Contribution

First observational attempt to detect cold, cool, or warm gas in a filament where hot X-ray emitting gas has been observed, setting upper limits on cooler gas presence.

Findings

No evidence of cold, cool, or warm gas detected in the filament.

Set a 2σ upper limit on Lyα absorption at log(N_HI/cm^-2) < 13.7.

Suggests the filament's gas is predominantly hot or that cooler components are small and clumpy.

Abstract

In the local universe, a large fraction of the baryon content is believed to exist as diffuse gas in filaments. While this gas is directly observable in X-ray emission around clusters of galaxies, it is primarily studied through its UV absorption. Recently, X-ray observations of large-scale filaments connecting to the cosmic web around the nearby ($z=0.05584$) cluster Abell 133 were reported. One of these filaments is intersected by the sightline to quasar [VV98] J010250.2$-$220929, allowing for a first-ever census of cold, cool, and warm gas in a filament of the cosmic web where hot gas has been seen in X-ray emission. Here, we present UV observations with the Cosmic Origins Spectrograph and optical observations with the Magellan Echellette spectrograph of [VV98] J010250.2$-$220929. We find no evidence of cold, cool, or warm gas associated with the filament. In particular, we set a $2\sigma$ upper limit on Ly$\alpha$ absorption of $\log(N_{HI} / \textrm{cm}^{-2}) < 13.7$, assuming a Doppler parameter of $b=20\,\textrm{km}\,\textrm{s}^{-1}$. As this sightline is ${\sim}1100\,\textrm{pkpc}$ ($0.7R_\textrm{vir}$) from the center of Abell 133, we suggest that all gas in the filament is hot at this location, or that any warm, cool, or cold components are small and clumpy. A broader census of this system -- combining more UV sightlines, deeper X-ray observations, and a larger redshift catalog of cluster members -- is needed to better understand the roles of filaments around clusters.