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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).

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).

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 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).

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).

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.

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

A DLA system

Spectrum of the QSO 1331+170 with an absorption around 3370 Angstrom. Credit: Pettini et al. (2004).

Spectrum of the QSO 1331+170 with an absorption around 3370 Angstrom. Credit: Pettini et al. (2004).


Types of QSO Absorption Systems

Spectrum of the quasar PKS 0545+039.

Spectrum of the quasar PKS 0545+039.


Distribution of column densities

Number density distribution of Lyman-alpha absorping systems. Credit: Hu et al. (1995).

Number density distribution of Lyman-alpha absorping systems. Credit: Hu et al. (1995).


Evolution of absorping systems

Cosmic evolution of the number density of absorping systems. Credit: Janknecht et al. (2006).

Cosmic evolution of the number density of absorping systems. Credit: Janknecht et al. (2006).


I materiali di supporto della lezione

Mortlock et al. (2011), Nature, "A luminous quasar at a redshift of z = 7.085", http://arxiv.org/pdf/1106.6088v1

Fan et al. (2006), " Observational constraints on Cosmic Reionization", Annual Review of Astronomy & Astrophysics, vol. 44, p.415

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