Missing Baryon Science
Constellation-X will measure these filaments in absorption along the line-of-sight to background AGN, constraining the hot baryon content of the Universe. With >100 filaments detected at z > 0, this will provide the first unambiguous detection of the WHIM.
For decades, it was thought that the dilute gas prevalent in the early universe eventually formed into the galaxies that we see today. However, when a census was taken of the amount of the normal matter in the galaxies around us, only 10% of the baryons known to exist were found. This began an extensive search for the missing baryons, and studies found that the hot gas in galaxy groups and clusters, combined with the cold gas that produces UV absorption lines could account for up to 40% of the known baryon content. The remaining >60% of the normal matter was still undiscovered. Cosmological simulations are in broad agreement that the majority of the baryons exist in the temperature range 105-107.5 K, with most of the material lying in the lower overdensity filaments that connect clusters and groups.
The high temperature of the Warm-Hot phase of the Intergalactic Medium (WHIM) may only be probed with X-ray spectroscopy due to the ionization states involved. High spectral resolution studies with Chandra and XMM have shown the first evidence of detection of the WHIM within the Local Group and a suggestion of higher-redshift filaments. Accounting for the remainder of the WHIM remains a major goal of observational astrophysics.
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