QSO absorption line projects
Quasi-stellar Objects (QSOs or quasars) are among the most distant and
intrinsically most luminous objects in the Universe. They are the only
class of cosmological objects that is bright enough to allow detailed
spectroscopy on a large number of objects out to redshifts of ~5. For
the past 20-30 years QSO spectroscopy has not only provided important
information on the QSOs themselves but also on a very different class
of objects, called QSO absorption line systems. Every QSO spectrum
reveals a plethora of absorption lines, most of which actually have
nothing to do with the QSO itself. Instead they are due to
cosmologically distributed gas which intersects the line of sight from
Earth to the QSO and imprints a distinct absorption signature onto the
spectrum of the QSO. The cartoon below illustrates this scenario.
The intimate relationship of the absorbing gas with the thermal,
chemical and dynamical history of the Universe makes the study of QSO
absorption lines a rich and important source of information for many
aspects of modern cosmology.
My (past and present) collaborators on these projects include:
Dust in high-redshift damped Lyman-alpha absorbers
Dust, and its relationship with the gas phase, are key ingredients in
any recipe for galaxy formation and evolution. Understanding the role
of dust in the damped Lyman-alpha systems is particularly important
because they are thought to comprise a significant fraction of the
high-redshift gas available for star formation. In addition, the
extinction caused by the dust can act as a veil which potentially
hides a significant fraction of QSOs.
We have looked for the effects of dust in damped Lyman-alpha absorbers
observed in the SDSS QSO survey. We
find little evidence of dust in these high-redshift systems but we do
find a hint that background QSOs may be gravitationally lensed by
foreground damped Lyman-alpha absorbers.
The high redshift deuterium abundance and the baryonic density
Observations of the high redshift deuterium abundance offer the most
promising way to measure the baryonic density parameter of the
Universe. However, reliable measurements are hard to derive, and after
several years of effort, only a small number of cosmologically useful
constraints exist. Moreover, these measurements show some trends which
may indicate that hidden systematics have influenced the measurements.
We are currently carrying out a large systematic survey of high
redshift quasars to identify suitable objects to use for this
measurement. The end result should be an unbiassed estimate of the
number of baryons (protons, neutrons, etc) in the universe.
The (foreground) proximity effect
The intensity of the extragalactic UV background is an important
cosmological parameter when trying to understand many processes at
high redshift, such as e.g. the baryonic content of intergalactic
medium, pregalactic collapse, or the completeness of current QSO
surveys. At high redshift, the background is often measured from the
observed underdensity of Lyman-alpha forest absorption lines in the
vicinity of background QSOs (`proximity effect').
Using the same technique as in the large-scale structure project
(below) we were able to show that the strength of the proximity effect
depends on the intrinsic luminosity of the QSO. This result confirms
the previously held assumption that the proximity effect is caused by
the extra ionizing radiation from the QSO itself.
However, we could find only little evidence for the existence of the
foreground proximity effect. This indicates that QSOs do not radiate
isotropically (the same in all directions).
Large-scale structure in the Lyman-alpha forest
According to Cold Dark Matter models of large-scale structure
formation the Intergalactic Medium at high redshift, observed as QSO
absorption lines, is the remnant of the gravitational processes which
formed the filamentary and sheet-like structures which are so striking in
galaxy redshift surveys, such as the 2dFGRS.
We have developed a new method to search for large-scale overdensities
and underdensities in Lyman-alpha forest absorption spectra. We have
applied this technique to the spectra of close group of 10 QSOs and
found significant overdensities both along the line of sight and in
the plane of the sky. Some of these structures are very large and cannot
be traced by any other method.