Intergalactic Matter

The spatial structure, thermodynamic properties and chemical composition of the warm intergalactic gas is studied using quasar absorption lines and X-ray observations of the hot intercluster gas. In comparison with simulations we can investigate the early stages of galaxy formation and its back reaction on the cosmic environment.

Cosmology with the Lyman alpha forest

Neutral hydrogen is widely distributed over the cosmos, absorbing the radiation of distant quasars. This manifests itself as "forest" of absorption lines in the spectra of these quasars. Studying the distribution of this intergalactic matter allows us to constrain the formation of large-scale structures in the universe. The identification of heavy element absorption lines in quasar spectra provides us with insights into the enrichment of intergalactic gas with the products of stellar nucleosynthesis. We have analysed a large sample of high resolution quasar spectra obtained with the Very Large Telescope (VLT) from the ESO and applied these data for a multitude of cosmological applications.

In the immediate vicinity of luminous quasars, the absorption spectra experience a remarkable change. Hard UV radiation from the quasars ionises the remaining neutral hydrogen and reduces the absorption. Thus, the intergalactic medium in the vicinity of quasars becomes more transparent: This is the so-called "Proximity Effect". We have searched for this phenomenon in a large sample of VLT quasar spectra, and for the first time we could significantly detect the Proximity Effect in all individual quasar spectra. This permitted us to determine the mean intensity of the metagalactic UV radiation field.

Below we show the high-resolution spectrum of the quasar HE 0940-1050, obtained with the UVES spectrograph at the ESO-VLT. The range of the hydrogen Lyman forest lines is indicated. Other absorption lines are due to heavier elements ("metals").

The proximity effect of Quasar

If a foreground quasar lies apparently close to the line of sight of another background quasar, a "transverse" Proximity Effect might be expected under certain circumstances (however, this effect was not observed until now). We have combined optical and ultraviolet spectra to estimate the "spectral hardness" of the ionising UV radiation field as a function of redshift. We found that this hardness is particularly high close to each of the considered foreground quasars. This is the first detection of the "transverse Proximity Effect in spectral hardness", stating that the hard UV radiation of individual quasars is detectable even over cosmological distances.

The schematic picture explains the line-of-sight and transverse proximity effects. Each quasar creates a bubble of highly ionized hydrogen around itself, which can be traced from an absorption line analysis of the background quasar 1.

HE 0940-1050 Quasars ionization 

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