Xray Satellites And The Early Universe

Both active nuclei and hot gas in galaxy systems are copious sources of X-rays, giving this piece of the spectrum an important role in tracing their early history. Indeed, X-rays may be the most effective way to find active nuclei in young, gas-rich, and probably dust-rich galaxies. They are obviously the most effective way to find hot gas, and can provide crucial information as to when and how this gas became chemically enriched by galactic outflows. Today's flagship X-ray missions, NASA's Chandra X-ray Observatory and ESA's XMM-Newton, have only begun to exploit their complementary capabilities in probing the early Universe. The exquisite, sub-arcsecond angular resolution of Chandra and the formidable collecting area of XMM-Newton allow different kinds of approaches to important problems. As powerful as these are, because of the long lead times and technological development needed for still more powerful observatories, planning is underway for the next generation of high-energy instruments. As with Chandra and XMM-Newton, complementary approaches may prove to be very fruitful.

NASA is planning a multi-spacecraft mission, Constellation-X, in which the solution to needing a larger collecting area is simply to orbit multiple coordinated spacecraft with identical instruments. Several proposals for the basic design, incorporating as many as four spacecraft, are still in competition. Some of these designs use a framework to be unfolded in orbit, accommodating an 8-meter focal length for the individual X-ray telescopes. They would operate in the L2 region, where Earth occultation and shadow crossings are not issues. The specifications call for spatial resolution no worse than 15" at low energies (trading resolution for collecting area in comparison with Chandra), and spectral resolutions R = A/AA = 300-3000 at energies up to 30keV.

ESA is pursuing a quite different, and likewise very ambitious, approach with the X-ray Evolving Universe Spectroscopy mission (XEUS). No launch date has been set; the mission has not been formally approved, although contracts for detector development were let in 2002. Working from the L2 region, XEUS would deliver images with 2-arcsecond resolution, and use detectors to deliver the extraordinary energy resolution of 6 eV up to 6 keV. XEUS would be able to measure spectra for sources an order of magnitude fainter than Chandra can currently detect in deep surveys. This drives the angular resolution required, to avoid overlapping and confusion of the images of faint sources.

Important goals of these missions relate to galaxy formation and evolution. They should be able to trace the appearance and history (chemical and dynamical) of hot gas in bound galaxy systems, from groups to rich clusters. The metallicity of the intergalactic medium will be followed, using X-ray absorption lines seen against high-redshift quasars. The appearance of these early active nuclei should come into view, and may allow us to follow their growth in mass and spin as these affect their X-ray spectral signatures.

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