Hamburger Sternwarte - Research|
Quasar Absorption Lines and the Intergalactic Medium
Immediately after Quasi-stellar Objects (QSOs or quasars)
were recognized as extragalactic sources in 1963
it was noticed that as their light travels to Earth any intervening matter will
leave its signature on the spectra. We have to dinstinguish between a diffuse medium
causing a uniform decrease of the quasar continuum (Gunn-Peterson effect)
and isolated structures producing discrete absorption lines (Fig. 1). These spectral
imprints provide a wealth of information, allowing us to study the detailed physical
conditions in the absorbing medium, such as its temperature, kinematics, chemical
composition, and background radiation.
Fig. 1: Schematic illustration of the cumulative absorption spectra due to
cosmologically distributed gas which intersects the line of sight from Earth to
the QSO (from Ed Janssen, ESO)
The basic observational properties of QSO absorption lines were established
in the late 1970s and early 1980s with ground-based telescopes of the 4-m class.
But now the new generation of telescopes (both ground-based and from space) provides
high resolution, high signal-to-noise data that contain a wealth of information
about the cosmological distribution of the baryonic matter and its physical
properties over a wide range of redshifts, up to z ~ 6. Fig. 2 shows a typical quasar
spectrum with many of the common features.
Fig. 2: Typical Spectrum of a quasar showing the quasar continuum and intrinsic
emission lines as well as the absorption features caused by the intergalactic
The quasar itself produces a relatively flat continuum and broad emission lines. In
some cases there are intrinsic absorption lines originating in the immediate vicinity
of the quasar. However, the vast majority of absorption lines are produced by the
intergalactic medium. The numerous absorption lines blueward the Ly α emission are
mostly Ly α lines at different redshifts zabs <
zQSO. The observed wavelengths scales in accordance with
λobs = λrest(1 + zabs),
where λrest = 1215.67 Å. The region redward of the Ly α
emission will be populated only by absorption due to other chemical transitions with
Spectra for wavelengths λobs > 3200 Å are observable with
ground-based telescopes, whereas spectra at shorter wavelengths must be obtained with
telescopes above the atmosphere. In the UV the wavelength limit arises from the cutoff
in the coatings of the optics. The Hubble Space Telescope (HST) has been used for
QSO spectroscopy in the wavelength range 1150 - 3200 Å. For shorter wavelength the
Far Ultraviolet Spectroscopic Explorer (FUSE) made available the region between 912 and
1150 Å. The Milky Way ist opaque between 912 Å and 62 Å (= 0.2 keV).
In the X-rays the Chandra X-ray Observatory and XMM-Newton are being used to study quasar
absorption lines, though with poor resolution of λ/Δλ = 500.
Historically, absorption systems with N(H I) < 1017.2 cm-2
have been called Ly α forest line, those with 1017.2 cm-2
< N(H I) < 1020.3 cm-2 are Lyman limit systems, and
those with N(H I) > 1020.3 cm-2 are damped Ly α
systems. The column density N gives the integrated volume density of the
material along the line of sight.
II. Analysis of Absorption Line Spectra
Absorption spectra are relatively easy to interpret, since the lines arise from atoms
in the ground state. With high resolution observations (i.e. FWHM < 25 km/s) of quasars
it is, at least in principle, possible to consider the physical conditions and kinematics
of the absorbing structures. However, the separation of the various effects that
determine the shapes of the spectral features is a difficult task. The absorption
profiles observed for the different species are caused by a combination of the spatial
distribution of the absorbing material, the macroscopic velocity, temperature,
metallicity, and abundance pattern. The ionization structure is determined by the gas
densities and by the UV radiation field, i.e. the extragalactic radiation due to
the quasar background and stellar photons escaped from galaxies.
The analysis of absorption lines requires assumptions to be made concerning
the shapes of the line profiles. Ideally, the physical properties of the absorbing
material should be characterized imposing as little prejudice as possible. The classical
methods are the Voigt profile fitting or the curve-of-growth approach depending on the
resolution of the spectra. The absorption line profile is described by the Voigt
integral, a convolution of the Lorenzian profile for an individual atom with a
function describing the broadening due to the distribution of the atomic velocity
(generally assumed to be Maxwellian). The resulting cross-section for absorbtion reads
σ(ν) = σ0 H(a,x), where H(a,x) denotes the Hjerting function
and σ0 the cross-section at line center.
Though it is possible to account for multiple absorption components even in blended
features, the analysis of complex line ensembles may be ambiguous. For example,
the existence of turbulent velocity fields will mimic multiple absorption lines leading
to a simplistic interpretation. Voigt profiles should be regarded as a convenient
parameterization of the data, but even a perfect fit may be a poor representation of
the true physical scenario.
III. Physical Properties of the Intergalactic Medium
Theoretical progress in the early 1990s leads to sophisticated cosmological
simulations allowing to predict the physical properties of the intergalactic medium
from initial conditions of a given structure formation model.
observational studies become an important referee for the model construction
and the cosmological implications.
Fig. 3: Numerical hydrodynamics simulation of the Ly α forest at z = 3.
The isosurfaces represent baryons at ten times the mean cosmic density and are color
coded to the gas temperature (dark blue = 30,000 K, light blue = 300,000 K). At the
bottom a single random slice through the cube is shown, with the baryonic overdensity
represented by a rainbow (from black = minimum to red = maximum). Additionally the He II
mass fraction is shown with a wire mesh (from Zhang et al. 1997).
Fig. 4: An evolutionary sequence of the H I column density (from Zhang et al. 1997).
Fig. 5: Two simple kinematic models illustrating the line formation in a
differentially moving ensemble of absorbing clouds.
Fig. 6: An evolutionary sequence of the baryon overdensity distribution with
superimposed velocity field (from Zhang et al. 1997).
Individuals involved in the QSO absorption line research are
A synopsis of research highlights is presented in the following sections: