Definitions
• The redshift z of a line is given by z = λobs/λ0− 1, where λ0 is the wavelength at the absorber rest frame and λobs is the observed one, expressed in vacuum and in the heliocentric frame.
• All the lines belonging to the same absorber form a system. These lines can originate from different transitions of the same element or from different elements, but they are all at the same redshift.
• The lines of each system can span many tens or hundreds of kilometers per second. In this case, the systems can be decomposed in sub-systems called components. The decomposition of the overall profile of a system into components depends mostly on the observational resolution, the S/N and the saturation of the line. At low resolution the total profile of a system often appears as a single absorption that is not resolved and the decomposition is therefore not possible. At high resolution it is often possible to decompose a system even in tens of component (see Fig. 6.3 for a comparison between low and high resolution spectra).
Figure 6.3 The left panel shows two CIV absorption signatures (∼ 3000 km/s apart) in the afterglow spectrum of GRB 021004 observed with VLT-FORSI (resolution R ∼ 1000). The right panel reports the VLT-UVES observation (resolution R ∼ 51500) of the same systems clearly showing that each of them consists of several components. This information is completely lost in the low resolution spectrum.
Origins
• The intervening absorbers, zabs zGRB: They are due to gas cosmologically dis- tributed and found by chance along the line of sight (l.o.s.) linking the observer to the GRB. They are not associated with the GRB environment or its host galaxy. • The associated absorbers, zabs ≡ zGRB: They correspond to the gas found in the
GRB host galaxy and/or its halo, or belonging to the GRB environment itself. The physical, dynamical and chemical state of the medium in the star-forming region hosting the GRB progenitor can be modified by the GRB, through shock waves and ionizing photons. Strong fine structure lines are always observed in the high resolution spectra of these systems. The systems associated with random clouds
of the ISM of the host galaxy along the line of sight show low ionization lines and ground state lines, implying distances from the GRB explosion site larger than a few hundred pc. The distinction between these two kinds of associated absorbers is not always clear cut. Vreeswijk et al. (2007) discovered large variations of FeII fine-structure lines in the spectra of GRB 060418 (z = 1.48) on rest frame time scales of a fraction of an hour, and interpreted them as due to ultraviolet (UV) pumping from the afterglow radiation field. Their detailed modeling of the observed variability suggests a distance of the FeII absorbing cloud from the GRB explosion site of ∼ 1.7 kpc, comparable to the size of a typical galaxy at z ∼ 1.4. This would imply that, at least in this case, the entire galaxy (or a significant fraction of it) is affected by the GRB explosion (see Cameron & Driver 2007 for typical galaxy sizes). 6.2.1 Classification according to the HI content
Hydrogen is the most abundant element in the Universe. The Ly-α hydrogen line has a strong oscillator strength and a wavelength λ = 1215.67 ˚A, useful to investigate the Universe at z > 1.6 from ground-based telescopes. Every system shows an HI absorption when this line is covered by the observed spectral range. The most common classification of the absorbers is therefore based on the hydrogen column density.
6.2.1.1 The Lyman-α forest
Fig. 6.4 shows a part of the VLT-UVES high resolution spectrum of GRB 050730. Several absorption lines are present at wavelengths shorter than the Ly-α absorption associated with the GRB. Most of these lines correspond to the Ly-α transition of the atomic hy- drogen situated between the GRB and the observer, more precisely they are associated with hydrogen overdensities confined along filaments by the dark matter (Cen et al. 1994; Petitjean et al. 1995). All these lines together form the so-called Lyman-α forest. The gas inside the Lyman-α forest is strongly ionized, therefore the neutral gas responsible for the Ly-α absorptions represents only a small portion of the baryonic matter inside these absorbers. It has been shown indeed that the Lyman-α forest contains the bulk of baryonic gas in the Universe (e.g. Rauch et al. 1997).
6.2.1.2 The Lyman-limit systems
Some Ly-α absorptions correspond to neutral hydrogen column densities so high that all the ionizing photons, with wavelengths shorter than 912 ˚A (the Lyman series limit), are
Figure 6.4 Part of the VLT-UVES high resolution spectrum of GRB 050730. The DLA absorption (see Sect. 6.2.1.3) associated with the GRB is clearly visible at ∼ 6000 ˚A, with the corresponding Lyman limit (see Sect. 6.2.1.2) at ∼ 4600 ˚A. The absorptions between the GRB Ly-α absorption and the Lyman limit break form the Lyman-α forest.
stopped by the hydrogen. The GRB afterglow (or QSO) spectrum shows a sudden break and its flux is null for wavelengths shorter than λ = (1 + zabs) × 912 ˚A. An example of this effect is illustrated in Fig. 6.4, with the Ly-α absorption at ∼ 6000 ˚A and the corresponding Lyman break at ∼ 4600 ˚A . These systems are called Lyman-limit systems. They correspond to column densities of 17 < logN(HI) < 20.3 (P´eroux et al. 2003b) and originate probably in border region of forming galaxies (see Prochaska 1999 for a discussion on the origins of these systems).
6.2.1.3 The Damped Lyman-α systems (DLAs)
When the hydrogen column density is large enough, the optical depth becomes very high and the ionizing radiation is absorbed by a thin hydrogen layer. These systems have N(HI) ≥ 2 × 1020cm−2 (Wolfe et al. 1986) and are called Damped Lyman-α systems. DLAs are very interesting. They contain most of the neutral gas of the Universe and are
associated with metal lines, found most of the time at wavelengths larger than the GRB Ly-α. They probably trace the interstellar medium of forming protogalaxies.