Diffuse Baryonic Matter beyond 2020

The hot, diffuse gas that fills the largest overdense structures in the Universe — clusters of galaxies and a web of giant filaments connecting them — provides us with tools to address a wide array of fundamental astrophysical and cosmological questions via observations in the X-ray band. Clusters are sensitive cosmological probes. To utilize their full potential for precision cosmology in the following decades, we must precisely understand their physics — from their cool cores stirred by jets produced by the central supermassive black hole (itself fed by inflow of intracluster gas), to their outskirts, where the infall of intergalactic medium (IGM) drives shocks and accelerates cosmic rays. Beyond the cluster confines lies the virtually unexplored warm IGM, believed to contain most of the baryonic matter in the presentday Universe. As a depository of all the matter ever ejected from galaxies, it carries unique information on the history of energy and metal production in the Universe. Currently planned major observatories, such as Astro-H and IXO, will make deep inroads into these areas, but to see the most interesting parts of the picture will require an almost science-fiction-grade facility with tens of m2 of effective area, subarcsecond angular resolution, a matching imaging calorimeter and a super high-dispersion spectrograph, such as Generation-X. 1 OVERVIEW AND RECENT ADVANCES Most of the visible matter in the Universe is in the form of diffuse gas that fills dips and valleys of the Universe’s gravitational potential. It is heated to T ∼ 105 − 108 K by shocks generated by the growth of Large Scale Structure (LSS). At present, we can study only the hottest and densest phase of this matter, found in central regions of galaxy clusters (r < r500 − r200), which comprises only a small fraction of the total. The gas within these regions is close to hydrostatic, and its X-ray observables can be used to estimate the cluster total (dark Radii of the average overdensity of 500 and 200 above the critical density of the Universe matter dominated) masses,1, 2 providing the basis for sensitive cosmological tests (§2). There are deviations from hydrostatic equilibrium, however, observed in clusters undergoing a growth event and in many coolcore clusters, where jets and relativistic plasma from the central supermassive black hole stir the gas. Enormous progress in understanding these phenomena has been made in the past decade with the advent of powerful Xray imaging spectrographs such as XMM and Chandra. XMM has determined that radiative cooling of the dense cores must be compensated by some heating mechanism.3 Chandra provided a leap in angular resolution that has led to the discovery of the ubiquitous AGN-blown, radio-filled bubbles in most cool cores.4–6 It called into question the old paradigm that gravity is the only important source of thermal energy for the intracluster medium (ICM) — apparently, AGN can inject as much mechanical energy into the core gas as it loses via radiative cooling.7–9 Precisely how this injection works, and how much of the cluster volume is affected, is unclear (§2.1). Merging clusters revealed a wealth of gas motion-related phenomena — subcluster infall, shocks, “cold fronts”, cool core sloshing, ram pressure stripping — all deduced indirectly using Chandra’s high-resolution imaging and modest spectral information.10–14 These phenomena await an imaging calorimeter, which will measure the gas velocities directly (§3.1). One of the surprises was the ubiquity, sharpness and symmetry of “cold fronts” and the stability of AGN bubbles, indicating that mixing and instabilities in the ICM are suppressed by some unexpected microphysical properties of the intracluster plasma. But perhaps the most interesting regions of clusters lie beyond the reach of the current X-ray instruments, because of their extremely low surface brightness. These are regions where the intergalactic medium (IGM) that flows along giant filaments of the Cosmic Web meets the intracluster gas. Physical processes in those regions hold the key to a num-