The Multiphase Intergalactic Medium toward PKS 2155-304

Abstract

We study the cluster of H I and O VI absorption systems and the claimed detection of O VIII absorption from the intergalactic medium at z ≈ 0.0567, associated with a group of galaxies toward the BL Lac object PKS 2155-304. As measured by spectrographs on the Hubble Space Telescope, Far Ultraviolet Spectroscopic Explorer, and Chandra, this system appears to contain gas at a variety of temperatures. We analyze this multiphase gas in a clumpy-infall model. From the absence of C IV and Si III absorption in the Lyα clouds, we infer metallicities less than 2.5%-10% of solar values. The only metals are detected in two O VI absorption components, offset by ±400 km s-1 from the group barycenter (cz ≈ 16,600 km s-1). The O VI components may signify "nearside" and "backside" infall into the group potential well, which coincides with the claimed O VIII absorption. If the claimed O VIII detection is real, our analysis suggests that clusters of strong Lyα and O VI absorbers, associated with groups of galaxies, may be the "signposts" of shock-heated metal-enriched baryons. Through combined UV and X-ray spectra of H I and O VI, O VII, and O VIII, one may be able to clarify the heating mechanism of this multiphase gas.