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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.

Galactic winds in starburst galaxy M82

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).

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).

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.

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.

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:

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