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

Abstract

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, H1821+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 OVII 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 OVII line is $(4.1\pm 1.3)\mathrm{m}\mathring{\mathrm{A}}$, which corresponds to an OVII column density of $(1.4\pm 0.4)\times 10^{15}\mathrm{c}{\mathrm{m}}^{-2}$. We constrain the absorbing gas to have a density of $n_{\mathrm{H}}=(1-2)\times 10^{-6}\mathrm{c}{\mathrm{m}}^{-3}$ for a single WHIM filament. We derive ${\Omega }_{\mathrm{b}}(\mathrm{O}\mathrm{VII})=(0.0023\pm 0.0007){\left[{\mathrm{f}}_{\mathrm{O}\mathrm{VII}}Z/{\mathrm{Z}}_{\odot }\right]}^{-1}$ for the cosmological mass density of OVII, assuming that all 17 systems contribute equally.

12 pages, 6 figures, accepted for publication in the Astrophysical Journal

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