Ultraviolet Perspectives on Diffuse Gas in the Largest Cosmic Structures
  The past decade has seen an explosion of discoveries and new insights into
the diffuse gas within galaxies, galaxy clusters, and the filaments composing
the Cosmic Web. A new decade will bring fresh opportunities to further this
progress towards developing a comprehensive view of the composition, thermal
state, and physical processes of diffuse gas in the Universe. Ultraviolet (UV)
spectroscopy, probing diffuse 10^4-10^6 K gas at high spectral resolution, is
uniquely poised to (1) witness environmental galaxy quenching processes in
action, such as strangulation and tidal- and ram-pressure stripping, (2)
directly account for the baryon content of galaxy clusters in the cold-warm
(T<10^6 K) gas, (3) determine the phase structure and kinematics of gas
participating in the equilibrium-regulating exchange of energy at the cores of
galaxy clusters, and (4) map cold streams and filaments of the Cosmic Web that
feed galaxies and clusters. With a substantial UV undertaking beyond the Hubble
Space Telescope, all of the above would be achievable over the entire epoch of
galaxy cluster formation. Such capabilities, coupled with already-planned
advancements at other wavelengths, will transform extragalactic astronomy by
revealing the dominant formation and growth mechanisms of gaseous halos over
the mass spectrum, settling the debate between early- and late-time metal
enrichment scenarios, and revealing how the ecosystems in which galaxies reside
ultimately facilitate their demise.
