Observations of the MIssing Baryons in the warm-hot intergalactic medium (1806.08395v1)

Abstract: It has been known for decades that the observed number of baryons in the local universe falls about 30-40% short of the total number of baryons predicted by Big-Bang Nucleosynthesis, as inferred from density fluctuations of the Cosmic Microwave Background and seen during the first 2-3 billion years of the universe in the so called Lyman-alpha Forest. A theoretical solution to this paradox locates the missing baryons in the hot and tenuous filamentary gas between galaxies, known as the warm-hot intergalactic medium. However, it is difficult to detect them there because the largest by far constituent of this gas - hydrogen - is mostly ionized and therefore almost invisible in far-ultraviolet spectra with typical signal-to-noise ratios. Indeed, despite the large observational efforts, only a few marginal claims of detection have been made so far. Here we report observations of two absorbers of highly ionized oxygen (OVII) in the high signal-to-noise-ratio X-ray spectrum of a quasar at redshift >0.4. These absorbers show no variability over a 2-year timescale and have no associated cold absorption, making the assumption that they originate from the quasar's intrinsic outflow or the host galaxy's interstellar medium implausible. The OVII systems lie in regions characterized by large (x4 compared to average) galaxy over-densities and their number (down to the sensitivity threshold of our data), agrees well with numerical simulation predictions for the long-sought warm-hot intergalactic medium (WHIM). We conclude that the missing baryons have been found.

• The paper confirms two high-significance OVII absorbers in the WHIM, resolving a major fraction of the missing baryon problem.
• It combines high signal-to-noise X-ray spectroscopy with numerical simulations to validate observations against Λ-CDM predictions.
• Findings suggest that WHIM baryons, with 0.1-0.2 solar metallicity, could account for 9-40% of the total baryons expected in the universe.

Detection of Missing Baryons in the Warm-Hot Intergalactic Medium

In the accompanying paper, the authors examine a longstanding discrepancy between predicted and observed baryonic matter in the universe. It is well-documented that the observed baryon content accounts for only 60-70% of the expected value derived from Big Bang nucleosynthesis. This missing baryon problem suggests the presence of baryonic matter in a form not easily detectable by conventional observational techniques. The paper resolves a significant portion of this baryon discrepancy by identifying the warm-hot intergalactic medium (WHIM), a diffuse and ionized component of the intergalactic medium (IGM), as a repository for these missing baryons.

The authors leverage high signal-to-noise X-ray spectroscopy from XMM-Newton observations of the blazar 1ES 1553+113, which reveal intervening absorption features corresponding to OVII ions. These ions are a reliable tracer for WHIM gas at temperatures between $10^5.7$ to $10^6.2$ K, the temperature range where a substantial fraction of the supposed missing baryons are expected to exist. The absorption lines in question exhibit a high statistical significance and are detected at redshifts consistent with being part of the WHIM, rather than intrinsic features of the blazar or its local environment.

The comprehensive analysis confirms two OVII He-α absorbers at redshifts $z=0.4339$ and $z=0.3551$, marking the systems as unlikely candidates for intrinsic blazar absorption or a host galaxy interstellar medium origin. The authors substantiate these findings through a combination of numerical simulations and an extensive paper of signal-to-noise criteria, showing that the detected numbers of absorbers align well with the cosmological Λ-CDM paradigm predictions. Furthermore, cross-correlation with galaxy over-densities lends further credence to the classification of these absorbers as belonging to WHIM structures.

From a cosmological perspective, the identification and characterization of these WHIM baryons are crucial for completing the baryon inventory of the universe. The paper deduces that the WHIM baryons, under the confines of acceptable metallicity ($0.1-0.2$ solar), could contribute to the bulk of the remaining hidden baryonic matter, equating to $9-40\%$ of the total baryons predicted by cosmological models.

In terms of implications, the detection of these OVII lines provides a pivotal tool for tracing diffuse baryons in the intergalactic medium. The work confirms the theoretical anticipation that a significant fraction of baryonic matter resides in a tenuous, shock-heated phase undetectable to optical and ultraviolet surveys. These observations reinforce the role of sophisticated X-ray spectroscopy as a powerful asset in the elucidation of cosmic baryon cycles and galaxy formation theories.

Future advancements in X-ray observatories with improved sensibilities and resolution will further augment the probing of such tenuous, high-temperature gas, allowing for even more precise quantifications of the WHIM component. This capability will enhance our understanding of galactic feedback mechanisms and their role in the distribution of baryonic matter at cosmological scales. The detection of WHIM represents not merely an end to the search for missing baryons but potentially a new chapter in our comprehension of intergalactic dynamics and baryon lifecycle evolution.