Globally, the missing baryons are believed to be in the form of warm-hot intergalactic medium (WHIM). The WHIM is a low-density, shock-heated medium that is concentrated in large-scale, filamentary structures that follow the dark matter distribution. Collapsed objects, such as galaxies & clusters, contain only ∼18% of the baryons, and the dominant fraction of baryons is expected to reside in the warm (T < 10^5 K) and hot (T > 10^5 K) phases of the WHIM. The existence of the warm phases of the WHIM has been confirmed by absorption studies in the UV regime, but only account for 20%–30% of the baryon content of the universe. It is hypothesized that a notable fraction of the missing baryons lie in the hot phases of the WHIM. While the densest and hottest parts of the WHIM have been detected in X-ray emission studies, it is expected that the dominant fraction of the WHIM resides at low densities. To resolve the missing baryon problem, the higher ionization states of heavy elements must be studied, whose absorption transitions are at X-ray wavelengths.

The present study aims to address the missing baryon problem using a spectrum stacking approach. Specifically, we utilize Chandra spectra of luminous quasars (H 1821+643 here) along with a priori redshift measurements of UV (Lyα and O VI) absorption line systems and we co-add (i.e., stack) the X-ray line forest from each absorption line system for major X-ray metal lines. Given the multitude of UV absorption systems in the sightline of quasars, this method increases the effective exposure time by the number of the absorption line systems. Thus, we can statistically probe the low-density gas associated with the WHIM to unprecedentedly sensitive limits.

The spectral fitting confirms the existence of an absorption line of Oxygen VII, with statistical significance of 3.3σ. We emphasize that this detection is not the result of a blind search : we did not employ numerous redshift trials to identify a tentative absorption line. Instead, the redshifts of the UV Ly-α absorption lines, which were used to perform the stacking, were known a priori from UV studies. Therefore, the 3.3σ significance represents the actual confidence level of the detection. To further confirm the statistical significance of this detection, we perform Monte Carlo simulations and assess the possibility that the detection is the result of a chance coincidence : we mimic the observed data by generating a set of 17 random redshifts and perform the analysis. Overall, we obtain 3 random redshifts sets showing a >3.3σ absorption line out of 10^4 sets, which is consistent with the expectation.

For the O VII detection toward the sightline the quasar, we calculated the equivalent width of the line : 4.1 ± 1.3 mÅ. Given that the source is optically thin, and thus the detected absorption line is unsaturated, we can convert the EWO to the column density of the ion using a linear formula. [Lots of assumptions] We estimate that the O VII baryon mass density is Ωb(O VII) = 0.017±0.005. Given that the cosmic baryon mass density is Ωb = 0.045, we conclude that O VII absorbers contribute (38±10)% of the total baryon content of the Universe. This is in good agreement with recent results from IllustrisTNG simulations.