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

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 A133 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 2σ upper limit on Lyα absorption of log(N _{H I} /cm^-2) < 13.7, assuming a Doppler parameter of b = 20 km s^-1. As this sightline is ∼1100 pkpc (0.7R _vir) from the center of A133, 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.