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Propriétés des absorbants Lyman-alpha à grand décalage spectral

Céline Péroux
Abstract : In recent years, an extremely successful method to observationally
study early stages of galaxy formation has been provided by the study
of quasar absorbers. Quasar absorption lines are systems intercepting
our line-of-sight to a given quasar and thus produce a feature in the
quasar spectrum. Damped Lyman-alpha systems (hereafter DLAs) have
N(HI) > 2 * 10^20 atoms cm^-2, and were originally thought to be the
precursors of present day disk galaxies but there is evidence that
they may be dominated by gas-rich proto-dwarf galaxies representing
the basic building blocks of hierarchical growth of structure. Since
their detection is independent of their size, shape, and covering
factor, they provide a unbiased method with which to study early
galaxies. DLAs are a subset of Lyman-limit Systems) (hereafter LLS)
which have hydrogen column densities N(HI) > 1.6 * 10^17 atoms
cm^-2. At z < 1, they are probably associated with galactic
halos. Finally, the Lyman-alpha forest is composed of many small
column density systems ranging from N(HI) = 10^12 to 1.6 * 10^17 atoms

This thesis presents a sample of 66 bright z > 4 quasars observed with
the 4 m Cerro Tololo Inter-American Observatory telescope and the 4.2
m William Herschel telescope. The first part of the study
concentrates on the quasars themselves via the fitting of quasar
continua and the measurement of continuum depression parameters
characterising the mean absorption across the Lyman-alpha forest. The
quasar spectra are then analysed to investigate the absorption systems
they contain. This led to the discovery of 26 new DLAs, 34 LLS and
many associated metal lines which enables the analysis of the
evolution of the column density distribution, f(N,z), and the total
mass in high-column density neutral hydrogen quasar absorbers. The
observed number of LLS per unit redshift is used to constrain f(N,z)
below the DLA limit in the range N(HI) = 1.6 * 10^17 to 2 * 10^20
atoms cm^-2. The joint analysis shows unambiguously that f(N,z)
deviates significantly from a single power law and that a Gamma-law
distribution of the form f(N,z) = (f_*/N_*)(N/N_*)^(-beta) exp(-N/N_*)
provides a better description of the observations. These results are
further used to determine the amount of neutral gas contained in both
DLAs and in systems with N(HI) > 2 * 10^19 atoms cm^-2
("sub-DLAs"). In the redshift range 2 - 3, 85% of the neutral H I +
He II mass density) is in DLAs, however we find that at z>3.5 this
fraction drops to 55% and that the remaining neutral gas mass lies in
sub-DLAs. After correction of the observed mass in H I for this
"missing" neutral gas the comoving mass density no longer shows any
evidence for a decrease over the range z = 2 - 5. The change in the
column density distribution supports a picture, where at z>3.5, we may
be directly observing the formation of high column density neutral
hydrogen systems from lower column density units. Finally, predictions
on the redshift evolution of the sub-DLAs number density are
presented. Preliminary results from measuring their incidence from
archival UVES echelle data seem in good agreement with our
Complete list of metadatas
Contributor : Celine Peroux <>
Submitted on : Friday, December 12, 2003 - 3:07:10 PM
Last modification on : Wednesday, January 25, 2006 - 4:44:59 PM
Long-term archiving on: : Wednesday, September 12, 2012 - 12:05:11 PM


  • HAL Id : tel-00003969, version 1


Céline Péroux. Propriétés des absorbants Lyman-alpha à grand décalage spectral. Astrophysics [astro-ph]. University of Cambridge, 2001. English. ⟨tel-00003969⟩



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