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Giovanni Covone » 14.The intergalactic medium - part second

IGM absorption lines

Absorption lines seen in the spectrum of a distant quasar. Adapted from Danforth & Shull (2008).

Ionized medium up to z=5

$\sigma (\nu) =\frac{e^2 f}{4 \epsilon_0 m_e c} \, g(\nu - \nu_{\rm Ly}) \,.$

Considering that $g(\nu)$ is a delta function

$\tau (\nu_0) \, = \, 4 \times 10^4 \, h^{-1} \, \frac{N_H (z)}{ \left[ (\Omega_m z +1) \, (1+z)^2 - \Omega_{\Lambda} \, z \, (z+2)]^{1/2} \right] } \, .$

Upper limit from observation in quasars spectra up to $z\sim3$ : $\tau (\nu_0) < 0.1$ .

By inverting the above formula, we find the number density per neutral hydrogen:
$N_H < 10^{-5} m^{-3}$

Compare with typical cosmological densities: $\sim 15 \, {\rm m}^{-3}$

Conclusion: significant amount of hydrogen in IGM is ionized.

Lyman alpha forest

High-resolution spectrum of the bright quasar Q1422+231. Credit: Ellison (2000).

Lyman alpha forest (cont.)

Gunn-Peterson effect

Optical spectra of quasars at about z=6 showing the Gunn-Peterson effect. Credit: Fan et al. (2004).

Optical depth at high-z

Observed evolution of the optical depth. Credit: Fan et al. (2006).

Evolution of the IGM

The volume averaged neutral fraction of the IGM versus redshift. Credit: Fan et al. (2006).

From the “Dark Ages” to the present

Artistic view of the cosmic evolution, with evidence on the Epoch of Reionization. Credit: NASA.

Classification of absorption systems

Lyman alpha forest

$N_{\rm HI} = 10^{16} - 10^{21} \, {\rm m}^{-2}$
numerous, weak lines from low-density hydrogen clouds
identification: proto-galactic clouds

Lyman-limit

$N_{\rm HI} = 10^{21} - 10^{24} \, {\rm m}^{-2}$
identification: extended galactic halos

Damped Lyman alpha (DLA) systems

Rare, strong hydrogen absorption, high column densities: $N_{\rm HI} > 10^{24} \, {\rm m}^{24}$
identification: intervening galaxies
An intervening galaxies often produce both metal and damped Lyman alpha absorptions