Confirmation of X-ray Absorption by Warm-hot Intergalactic Medium in the Sculptor Wall

Abstract

In a previous paper, we reported a 3σ detection of an absorption line from the warm-hot intergalactic medium (WHIM) using the Chandra and XMM X-ray grating spectra of the blazar H2356-309, the sight line of which intercepts the Sculptor Wall, a large-scale superstructure of galaxies at z ~ 0.03. To verify our initial detection, we obtained a deep (500 ks), follow-up exposure of H2356-309 as part of the Cycle-10 Chandra Large Project Program. From a joint analysis of the Cycle-10 and previous (Cycle-8) Chandra grating data we detect the redshifted O VII WHIM line at a significance level of 3.4σ, a substantial improvement over the 1.7σ level reported previously when using only the Cycle-8 data. The significance increases to 4.0σ when the existing XMM grating data are included in the analysis, thus confirming at higher significance the existence of the line at the redshift of the Sculptor Wall with an equivalent width of 28.5 ± 10.5 mÅ (90% confidence). We obtain a 90% lower limit on the O VII column density of 0.8 × 10[superscript 16] cm[superscript –2] and a 90% upper limit on the Doppler b parameter of 460 km s–1. Assuming the absorber is uniformly distributed throughout the ~15 Mpc portion of the blazar's sight line that intercepts the Sculptor Wall, that the O VII column density is ≈2 × 10[superscript 16] cm[superscript –2] (corresponding to b gsim 150 km –1 where the inferred column density is only weakly dependent on b), and that the oxygen abundance is 0.1 solar, we estimate a baryon over-density of ~30 for the WHIM, which is consistent with the peak of the WHIM mass fraction predicted by cosmological simulations. The clear detection of O VII absorption in the Sculptor Wall demonstrates the viability of using current observatories to study WHIM in the X-ray absorption spectra of blazars behind known large-scale structures.