Simona Gallerani

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What can we learn from quasar spectra on the
high-redshift Universe?
Simona Gallerani (Osservatorio Astronomico di
Roma)
In collaboration with T.Choudhury, P. Dayal, X.
Fan, A. Ferrara, A. Maselli, R. Maiolino, R.
Salvaterra
21th July 2009

Roman Young Researchers Meeting

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The early Universe
50 kyr
The Universe is composed by a very hot and dense
mix of particles and radiation
Today
Big Bang
The expansion allows the radiation to decouple
from matter and the first atoms to form
Recombination process
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The first stars
4
Cosmic reionization
100 Myr
1 Gyr
14 Gyr
Overlap stage
Post-overlap stage
Pre-overlap stage
What is the epoch of reionization (EOR)?
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Quasars a class of very bright Active Galactic
Nuclei
Black Hole
Accretion disk
Particularly luminous in the rest frame near
ultra-violet, i.e. close to the Lya emission line
Dust torus
The quasar spectra we observe are strongly
affected by the intervening material along the
lines of sight
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The Lya forest in quasar absorption spectra


?a1216 Å
Keck telescope
REDSHIFT
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QSOs constraints on cosmic reionization
SDSS
20 QSOs
_at_ 5.7ltzlt6.4

Becker et al. (2003)
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Simulating the Ly? forest
Optical depth
(Voigt profile)
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Reionization models
Early Reionization (ERM)
Late Reionization (LRM)
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Models testing
Transmitted flux
Optical depth evolution
Fan et al. (2006) Songaila (2004)
Fan et al. (2002)
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Simulated spectra

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Largest gap width distribution
Observations vs Simulations
Low Redshift (zemlt6)
High Redshift (zemgt6)
SG, Ferrara, Fan, Choudhury (2007)
ERM
LRM
LRM
ERM
The Universe is highly ionized at z6
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Quasars a class of very bright Active Galactic
Nuclei
Black Hole
Accretion disk
Particularly luminous in the rest frame near
ultra-violet, i.e. close to the Lya emission line
Dust torus
The quasar spectra we observe are strongly
affected by the intervening material along the
lines of sight AND BY THE DUST SURROUNDING THE
AGN!
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Dust extinction on quasar spectra
Intrinsic spectrum
Dust absorbed spectrum
Dust absorbs preferentially bluer photons, thus
reddening the spectra
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An example of extincted spectra at z6
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An empirical extinction law for the high redshift
dust
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Conclusions
Quasar absorption spectra provide a huge amount
of information both concerning the ionization
level of the intergalactic medium and the amount
of dust present in the host galaxy.
The analysis of 20 quasars at z6 shows that
observations are consistent with a highly
ionized intergalactic medium at z6.
The analysis of 30 quasars at 4ltzlt6 shows that
the properties of dust at these epochs differ
from the local Universe.