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Giovanni Covone » 13.The intergalactic medium - first part


Evidence for the IGM

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.

Cosmic chemical evolution

A schematic view of the chemical evolution of IGM and ISM.

A schematic view of the chemical evolution of IGM and ISM.


Galactic winds in starburst galaxy M82

M82, irregular galaxy with an intense, the red-glowing outwardly expanding gas. Credit: NASA/ESA.

M82, irregular galaxy with an intense, the red-glowing outwardly expanding gas. Credit: NASA/ESA.


The ‘missing baryon’ problem

The ‘missing baryon’ problem: recent estimates of the mass density of the baryonic component of
galaxies is (Bell et al. 2003).

Measurement: \Omega_s h = (2.0 \pm 0.6) \, \times 10^{-3} \, .

Theory: baryonic mass density predicted by primordial nucleosynthesis and the spectrum of perturbations in the cosmic microwave background radiation is \Omega_B h^2 = 0.0223.

Conclusion: most of the remaining gas is in the form of diffuse intergalactic gas.

Census of baryons in the Universe

Density of baryons in the Universe as a function of redshift. Credit: Nicastro et al. (2008).

Density of baryons in the Universe as a function of redshift. Credit: Nicastro et al. (2008).


Missing baryons: the theoretical scenario

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.

Maps of the soft X-ray intensity of all the gas particles (left panels), and only WHIM (right). Credit: Roncarelli (2006).

Maps of the soft X-ray intensity of all the gas particles (left panels), and only WHIM (right). Credit: Roncarelli (2006).


Observational evidence for the WHIM

Artistic view of the X-ray radiation from AGN through the WHIM. The absorption spectrum in the inset. Credit: NASA/ESA.

Artistic view of the X-ray radiation from AGN through the WHIM. The absorption spectrum in the inset. Credit: NASA/ESA.


IGM and the “cosmic web”

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.

Numerical simulation showing the cosmic distribution of dark matter. Credit: MPA.

Numerical simulation showing the cosmic distribution of dark matter. Credit: MPA.


Background emission of IGM

Consider an uniform distribution of sources with luminosity L {\nu, z} and the flux density

S (\nu_0) = \frac{L}{4 \pi D^2 (1+z)}

Background intensity:
I (\nu_0) \, = \, \int S(\nu_0) \, {\rm d} N = \, \frac{1}{4 \pi} \, \int_0^\infty \frac{L(\nu, z) N_0}{(1+z)} {\rm d} r \, ,

where \nu_0 = \nu (1+z)  is the observed frequency.

We assume a constant comoving density N_0 of sources, with evolving luminosity.

The proper number density N (z) \, = \, N_0 \, (1+z)^3  is more appropriate when dealing with sources in the diffuse IGM.

Background emission of IGM (cont.)

The emissivity of the IGM is its luminosity per unit proper volume: \epsilon (\nu) \, = \, L(\nu, z) \, N(z).

The background intensity reads:
I (\nu_0) \, = \, \int_0^\infty \frac{\epsilon(\nu)}{(1+z)^4} \, {\rm d} r \, .<br />

In a standard cosmological model:
I (\nu_0) \, = \, \frac{c}{4 \pi H_0} \, \int_0^\infty \frac{\epsilon(\nu)}{(1+z)^4 [ (1+z)^2 (\Omega_m z +1) - \Omega_{\Lambda} z (z+2) ]^{1/2}} \, {\rm d} z \, .

Optical depth \tau (\nu_0) of the gas at observed frequency \nu_0 due to the absorption by IGM along the line of sight up to redshift z. Absorption coefficient for radiation: \alpha (\nu)

\tau (\nu_0) \,=\, \int \alpha (\nu) \, {\rm d} l \, = \int \alpha (\nu) \frac{dr}{1+z}

In the case of an absorption line, the function \alpha (\nu) describes its line profile.

Optical depth in standard cosmology

Finally, it is interesting to write the optical depth \tau again assuming a standard cosmology,
as a function of the cosmological parameter:

\tau (\nu_0) \,=\, \frac{c}{H_0} \, \int_0^z \frac{\alpha (\nu) [ \nu_0 (1+z)] {\rm d}z }{(1+z)^2 [ (1+z)^2 (\Omega_m z +1) - \Omega_{\Lambda} z (z+2) ]^{1/2}} \, .

I materiali di supporto della lezione

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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