Detection of the Missing Baryons toward the Sightline of H1821+643

Based on constraints from Big Bang nucleosynthesis and the cosmic microwave background, the baryon content of the high-redshift Universe can be precisely determined. However, at low redshift, about one-third of the baryons remain unaccounted for, which poses the long-standing missing baryon problem. The missing baryons are believed to reside in large-scale filaments in the form of warm-hot intergalactic medium (WHIM). In this work, we employ a novel stacking approach to explore the hot phases of the WHIM. Specifically, we utilize the 470 ks Chandra LETG data of the luminous quasar, H 1821+643, along with previous measurements of UV absorption line systems and spectroscopic redshift measurements of galaxies toward the quasar's sightline. We repeatedly blueshift and stack the X-ray spectrum of the quasar corresponding to the redshifts of the 17 absorption line systems. Thus, we obtain a stacked spectrum with 8.0 Ms total exposure, which allows us to probe X-ray absorption lines with unparalleled sensitivity. Based on the stacked data, we detect an O VII absorption line that exhibits a Gaussian line profile and is statistically significant at the 3.3 sigma level. Since the redshifts of the UV absorption line systems were known a priori, this is the first definitive detection of an X-ray absorption line originating from the WHIM. The equivalent width of the O VII line is (4.1 +/- 1.3) m angstrom, which corresponds to an O VII column density of (1.4 +/- 0.4) x 10(15) Cm-2. We constrain the absorbing gas to have a density of n(H) = ( 1 - 2) x 10(-6) cm(-3) for a single WHIM filament. We derive Omega(b)(O VII) = (0.0023 +/- 0.0007) [f(O VII) Z/Z(circle dot)](-1) for the cosmological mass density of O VII, assuming that all 17 systems contribute equally.