Observations of the missing baryons in the warm-hot intergalactic medium

Area of Science:

• Cosmology and Astrophysics
• Intergalactic Medium Research
• Baryon Census

Background:

• A long-standing cosmological puzzle is the 30-40% deficit in observed baryons compared to Big Bang nucleosynthesis predictions.
• Theoretical models propose these missing baryons reside in the warm-hot intergalactic medium (WHIM), a hot, tenuous gas between galaxies.
• Detecting the WHIM is challenging due to the low signal-to-noise ratio of ultraviolet spectra, as hydrogen is mostly ionized and thus invisible.

Purpose of the Study:

• To detect and confirm the existence of the missing baryons in the warm-hot intergalactic medium.
• To investigate the properties and location of the warm-hot intergalactic medium using X-ray spectroscopy.

Main Methods:

• Analysis of high-signal-to-noise ratio X-ray spectra of a quasar at redshift > 0.4.
• Identification of absorption features from highly ionized oxygen (O VII) as tracers of the WHIM.
• Assessment of absorber variability and associated cold absorption to rule out alternative origins (e.g., quasar outflows, galactic ISM).

Main Results:

• Detection of two O VII absorbers in the quasar's X-ray spectrum, indicating the presence of hot, ionized gas.
• Absorbers showed no variability over two years and lacked associated cold absorption, ruling out localized sources.
• The O VII systems were found in regions with high galaxy overdensities, consistent with WHIM predictions and simulations.

Conclusions:

• The observed O VII absorbers are attributed to the warm-hot intergalactic medium, providing strong evidence for its existence.
• This discovery resolves the long-standing missing baryon problem by locating the deficit baryonic matter in the WHIM.
• The findings validate numerical simulations of cosmic structure formation and the distribution of baryonic matter in the Universe.