X‐Ray Absorption by the Low‐Redshift Intergalactic Medium: A Numerical Study of the Λ Cold Dark Matter Model

Using a hydrodynamic simulation of a LCDM universe, we investigate the "X-rayforest" absorption imprinted on the spectra of background quasars by theintervening intergalactic medium (IGM). In agreement with previous studies, wefind that OVII and OVIII produce the strongest absorption features. The strongoxygen absorbers that might be detectable with Chandra or XMM-Newton arise ingas with T ~ 10^6 K and overdensities delta >~ 100 that are characteristic ofgalaxy groups. Future X-ray missions could detect weaker oxygen absorptionproduced by gas with a wider range of temperatures and the lower densities ofunvirialized structures; they could also detect X-ray forest absorption by C,N, Ne, Fe, and possibly Si. If the IGM metallicity is 0.1 solar, then thepredicted number of systems strong enough for a ~5\sigma detection with Chandraor XMM-Newton is extremely low, though scatter in metallicity would increasethe number of strong absorbers even if the mean metallicity remained the same.Our simulation reproduces the high observed incidence of OVI absorbers (in theUV), and the most promising strategy for finding the X-ray forest is to searchat the redshifts of known OVI systems, thus reducing the signal-to-noisethreshold required for a significant detection. However, while many OVIabsorbers have associated OVII or OVIII absorption, the OVI systems trace onlythe low temperature phases of the X-ray forest, and a full accounting of thestrong OVII and OVIII systems will require a mission with the anticipatedcapabilities of Constellation-X. The large effective area of the XEUS satellitewould make it an extremely powerful instrument for studying the IGM, measuringX-ray forest absorption by a variety of elements and revealing the shock-heatedfilaments that may be an important reservoir of cosmic baryons.Comment: 41 pages including 16 figures, replaced with version accepted by ApJ. Includes clarification of several points and brief discussion of recent observational result