Tracing the warm-hot intergalactic medium at low redshift: X-ray forest observations toward H1821+643

We present a high-resolution (lambda/Deltalambda approximate to 500) X-ray spectrum of the bright quasar H1821+643 (z = 0.297), obtained in a 470 ks observation with the Chandra X-Ray Observatory. We search for X-ray absorption by highly ionized metal species, O VII and O VIII in particular, at the redshifts of the six intervening O VI absorption systems known from UV studies. We detect features with greater than or similar to2sigma significance at the predicted O VII and O VIII wavelengths of one O VI system, at the O VII wavelength of a second, and at the Ne IX wavelength of a third. We find two additional features of comparable strength one O VII and one O VIII within 1000 km s(-1) of the O VI redshifts. The 1sigma constraints on the relative abundances of different species imply significant variations from system to system in f (O VI), the fraction of oxygen in the O VI state. The constraints in the two detected O VI systems imply gas overdensities lower than the values delta greater than or similar to 100 expected in virialized systems, suggesting that the absorption arises in lower density, filamentary structures. At the 2sigma level, however, the physical constraints are weak, although all of the systems must have temperature T < 10(6) K to be consistent with upper limits on O VII If we treat our 2 detections of known O VI systems as real, but assume minimal O VII and O VIII in the other systems, we estimate [f (O VI) + f(O VII) + f(O VIII)]/f (O VI) = 32 +/- 9 for the average ratio of all highly ionized oxygen species to O VI Combined with estimates of the total column density of O VI absorption per unit redshift, this ratio implies that the total baryon fraction associated with detected O VI absorbers is Omega(b) (O VI) similar to 0.03 h(70)(-1) 70, a substantial fraction of the baryon density predicted by big bang nucleosynthesis and larger than that associated with stars or with gas detected in 21 cm or X-ray emission. Because of the limited signal-to-noise ratio of the detections, these results must be treated with caution. Nonetheless, the combination of the O VI data with these X-ray forest measurements provides the most direct evidence to date for the pervasive, moderate-density, shock-heated intergalactic medium predicted by leading cosmological scenarios. The high inferred incidence of relatively strong O VII and O VIII absorption implies that some regions of the intergalactic medium are enriched to a level substantially above [O/H] = -1.