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Abstract
The current census of observed baryons in the local universe is still missing a significant fraction of them according to standard big bang nucleosynthesis. Numerical simulations predict that most of the missing baryons are in a hot intergalactic medium, which is difficult to observe through its X-ray emission or the Sunyaev-Zeldovich effect. We show that the next generation of X-ray satellites will be able to detect this gas through the X-ray absorption lines imprinted by its highly ionized metals on the spectrum of a background quasar. For the metallicity typically found in intracluster gas, up to 70% of the baryons produce O VIII absorption lines with an equivalent width 0.1 eV. The spectrum of any high-redshift quasar is expected to show several such lines per unit redshift due to intervening gaseous halos of galaxy groups. These lines will be detectable at a signal-to-noise ratio of 5 after a day of integration with the future Constellation-X telescope for any of the several tens of the brightest quasars across the sky.
Highlights
The best estimate for the sum of the baryonic mass currently observed in stars, neutral hydrogen, and X-ray–emitting gas in clusters of galaxies (Fukugita, Hogan, & Peebles 1998) falls short of the most likely value predicted by standard big bang nucleosynthesis (Burles & Tytler 1998)
Numerical simulations suggest that most of the missing baryons reside in a hot intergalactic medium, which is too rarefied to be detectable through its X-ray emission or the Sunyaev-Zeldovich effect (e.g., Ostriker & Cen 1996; Theuns, Leonard, & Efstathiou 1998)
We have investigated the possibility of using the X-ray absorption spectrum of a background quasar to probe the presence of heavy elements in the outer regions of groups and clusters of galaxies
Summary
The best estimate for the sum of the baryonic mass currently observed in stars, neutral hydrogen, and X-ray–emitting gas in clusters of galaxies (Fukugita, Hogan, & Peebles 1998) falls short of the most likely value predicted by standard big bang nucleosynthesis (Burles & Tytler 1998). Numerical simulations suggest that most of the missing baryons reside in a hot intergalactic medium, which is too rarefied to be detectable through its X-ray emission or the Sunyaev-Zeldovich effect (e.g., Ostriker & Cen 1996; Theuns, Leonard, & Efstathiou 1998) Much of this gas resides in the outskirts of galaxy groups or clusters and is likely to possess a similar metallicity to that observed in the X-ray emission spectrum from the cores of these systems (Mushotzky et al 1996). Observations of the X-ray surface brightness of galaxy clusters are commonly fitted by the isothermal b-model This model assumes that the gas temperature is constant and the radial profile of its mass density is given by. For a thermally broadened line, DvX ϭ [2kT/A(X)mp ]1/2 (Rybicki & Lightman 1979), where A(X) is the atomic number of the element X
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