Tracing the Warm‐Hot Intergalactic Medium at Low Redshift: X‐Ray Forest Observations toward H1821+643

Abstract

We present a high-resolution (=D 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 absorp-tion systems known from UV studies. We detect features withe2 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 s1 of the O vi redshifts. The 1 constraints on the relative abundances of different species imply significant var-iations from system to system in f ðO viÞ, 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 e100 expected in virialized systems, suggesting that the absorption arises in lower density, filamentary structures. At the 2 level, however, the physical constraints are weak, although all of the systems must have temperature T < 106 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 ðO viÞ þ f ðO viiÞ þ f ðO viiiÞ=f ðO viÞ 32 9 for the average ratio of all highly ionized oxygen species to O vi. Combined with estimates of the total column den-