Using a hydrodynamic simulation of a cold dark matter universe with a cosmological constant ( \Lambda CDM ) , we investigate the “ X-ray forest ” absorption imprinted on the spectra of background quasars by the intervening intergalactic medium ( IGM ) , at redshift z \approx 0 .
In agreement with previous studies , we find that O vii and O viii produce the strongest absorption features .
The strong oxygen absorbers that might be detectable with Chandra or XMM-Newton arise in gas with T \sim 10 ^ { 5.5 } { K } -10 ^ { 6.5 } { K } and overdensities \delta \gtrsim 100 that are characteristic of galaxy groups .
Future X-ray missions could detect weaker oxygen absorption produced by gas with a wider range of temperatures and the lower densities of unvirialized structures ; they could also detect X-ray forest absorption by carbon , nitrogen , neon , iron , and possibly silicon .
If the IGM metallicity is Z = 0.1 Z _ { \odot } , as we assume in most of our calculations , then the predicted number of systems strong enough for a \sim 5 \sigma detection with Chandra or XMM-Newton is extremely low .
However , scatter in metallicity increases the number of strong absorbers even if the mean metallicity remains the same , making the predictions somewhat more optimistic .
Our simulation reproduces the high observed incidence of O vi ( 1032Å , 1038Å ) absorbers , and the most promising strategy for finding the X-ray forest is to search at the redshifts of known O vi systems , thus reducing the signal-to-noise threshold required for a significant detection .
However , while many O vi absorbers have associated O vii or O viii absorption , the O vi systems trace only the low temperature phases of the X-ray forest , and a full accounting of the strong O vii and O viii systems will require a mission with the anticipated capabilities of Constellation-X .
The large effective area of the XEUS satellite would make it an extremely powerful instrument for studying the IGM , measuring X-ray forest absorption by a variety of elements and revealing the shock-heated filaments that may be an important reservoir of cosmic baryons .