The space between field galaxies is also not empty.
Observational evidence: observations of Lyman-alpha absorption line systems in the spectra of distant quasars.
Formation of galaxies is not an efficient process.
Densest absorbing systems: disk of proto-galaxies.
Study of chemical abundance in the IGM and its importance.
The ‘missing baryon’ problem: recent estimates of the mass density of the baryonic component of
galaxies is (Bell et al. 2003).
Theory: baryonic mass density predicted by primordial nucleosynthesis and the spectrum of perturbations in the cosmic microwave background radiation is
Conclusion: most of the remaining gas is in the form of diffuse intergalactic gas.
Theoretical predictions of the ionized Warm-Hot Intergalactic Medium (WHIM).
Theoretical problem or technological limitations?
Gravitational heating of the intergalactic gas.
Results from simulations.
Expected observational features.
Artistic view of the X-ray radiation from AGN through the WHIM. The absorption spectrum in the inset. Credit: NASA/ESA.
The distribution of the IGM follows the dark matter “cosmic web”.
What is the “cosmic web”.
Density evolution of the IGM and the large-scale structure.
Consider an uniform distribution of sources with luminosity and the flux density
where is the observed frequency.
We assume a constant comoving density of sources, with evolving luminosity.
The proper number density is more appropriate when dealing with sources in the diffuse IGM.
The emissivity of the IGM is its luminosity per unit proper volume: .
The background intensity reads:
In a standard cosmological model:
Optical depth of the gas at observed frequency due to the absorption by IGM along the line of sight up to redshift . Absorption coefficient for radiation:
In the case of an absorption line, the function describes its line proﬁle.
Finally, it is interesting to write the optical depth again assuming a standard cosmology,
as a function of the cosmological parameter:
M. S. Longair, ”Galaxy Formation”, Second Edition, Springer
Nicastro et al. (2008), “Missing Baryons and the Warm-Hot Intergalactic Medium“, Science v.319, p.55
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