Clusters of galaxies consist of galaxies and a large mass of hot gas trapped in a gravitational well composed mostly of dark matter. The nature of dark matter is one of the outstanding problems in current astronomy. The best way to map the dark matter distribution in a cluster of galaxies is to measure the radial profile of the temperature and density of the hot gas using X-ray observations. These observations show that the dominant observable mass is in the X-ray emitting gas. Galaxies are a minor constituent of clusters; flotsum in a larger gravitational sea. A key assumption in modelling the X-ray results is that the cluster is in hydrostatic equilibrium. This assumption can only be tested by mapping the velocity distribution of the hot cluster gas. Only high resolution X-ray spectroscopy can map the velocity distribution in the hot intra-cluster medium and probe the mass distribution and dark matter fraction of the cluster medium.
Clusters of galaxies show strong evolution and the most luminous clusters in the current epoch, are also the most numinous. Observations show that many clusters are merging and the observed evolution is probably because of the merger of sub-clusters. Optical studies of clusters are confused by the intrinsic evolution of the member galaxies. The X-ray band offers the only means for studying the evolution of clusters over a large range of redshift.
Direct estimates of abundances in the hot gas observed from galaxies and clusters of galaxies can be made from X-ray spectra. Such direct measurements are possible because both the free-free continuum and the resonant emission lines of the medium-z elements are observable in the X-ray band. X-ray spectroscopy is the only way to determine the chemical enrichment of the universe simultaneously for all elements with atomic weights between Carbon and Zinc ( Z=6 to 30).
The chemical evolution of the universe is dominated by supernova remnants within each galaxy. The shock heated gas from a supernova explosion emits primarily a line rich X-ray spectrum that can be used to directly determine the supernova abundance pattern and its interaction with the surrounding interstellar medium.
The X-ray sky is dominated by the summed emission from accretion onto a large population of super-massive black holes accreting material from a host galaxy. These so called active galactic nuclei (AGN) have hard power-law X-ray spectra that dominate the overall bolometric luminosity of the system. The primary emission from the central AGN engine is only directly observed in the X-ray band.
The X-rays light-up the surrounding AGN environment and X-ray irradiation can be used to determine the viewing geometry and location of the inflowing material. Fluorescent X-ray iron K line emission and Compton reflection are seen in the X-ray spectra of many AGN. Other systems highly absorbed because they are observed through a molecular torus and only observed above 10 keV where the opacity is lower. X-ray observations of AGN are the most direct method for viewing the interaction of the super-massive black hole with the surrounding galaxy and determining the overall system geometry.
Recent ASCA observations have shown that in many AGN the iron line is broadened and shaped by doppler and gravitational shifts caused by the motion of the reflecting gas close to the deep potential well of the black hole. X-ray observations of the broad iron K line and reverberation effects should allow a solution for the mass and rotation of the massive black hole, and by observing a large sample over a wide range of luminosities study the growth and evolution of the underlying black hole population. X-ray astronomy provides the only method to directly observe matter within a few Schwarzschild radii of black holes and test General Relativity in the strong field limit.
Accretion onto white dwarfs, neutron stars and black holes occurs in binary systems. X-ray observations of these systems at high spectral resolution will allow the first radial velocity measurements using X-ray emission lines to determine mass functions, in particular in those systems where X-ray pulsars are not found. X-ray radial velocity measurements will for the first time determine the mass distribution of black holes, neutron stars, and white dwarfs over a large sample of binary systems.
The stellar coronae of nearby stars are many orders of magnitude more active than our sun, and massive flares are commonly seen. The high plasma temperatures require magnetic fields, both to provide an energy source and to confine the hot plasma. X-ray astronomy allows the study of magnetic reconnection events that are many orders of magnitude more energetic than those found in the solar corona.
Coronally active stars are found in close binary systems and the rotational broadening and orbital velocity of the underlying stars can be used to "doppler image" the location of the coronal material. By combining X-ray Doppler imaging with the plasma temperature and density diagnostics it will be possible to fully map the coronal structures in these active systems. Only in the X-ray band can the coronae of nearby stars be directly observed.
The youngest star forming regions are buried deep in molecular clouds and are only observable in the infra-red and in the X-ray band, where the optical depths are less than unity. The most rapidly rotating young stars are also the most coronally active and X-ray observations can be used to find and study young coronally active stars. X-ray studies of star forming regions provide insight into the formation and evolution of magnetic dynamos in young stars.
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