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Hamburger Sternwarte - Research


Quasar Absorption Lines and the Intergalactic Medium

I. Introduction

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 material.

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 longer λrest.

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. Consequently, 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:



02/10/02 | rb