and Cosmology

5.6 AGNs and Cosmology
at very high z is to be expected because the SMBHs
in the center of galaxies first need to form, and this obviously
happens in the first ∼ 10 9 years after the Big
Bang.
5.6.3 Quasar Absorption Lines
The optical/UV spectra of quasars are characterized by
strong emission lines. In addition, they also show absorption
lines, which we have not mentioned thus far.
Depending on the redshift of the QSOs, the wavelength
range of the spectrum, and the spectral resolution, QSO
spectra may contain a large variety of absorption lines.
In principle, several possible explanations exist. They
may be caused by absorbing material in the AGN itself
or in its host galaxy, so they have an intrinsic origin.
Alternatively, they may arise during the long journey
between the QSO and us due to intervening gas along
the line-of-sight. We will see that different kinds of absorption
lines exist, and that both of these possibilities
indeed occur. The analysis of those absorption lines
which do not have their origin in the QSO itself provides
information about the gas in the Universe. For
this purpose, a QSO is basically a very distant bright
light source used for probing the intervening gas.
This gas can be either in intergalactic space or is correlated
with foreground galaxies. In the former case,
we expect that this gas is metal-poor and thus consists
mainly of hydrogen and helium. Furthermore, in order
to cause absorption, the intergalactic medium must not
be fully ionized, but needs to contain a fraction of neutral
hydrogen. Gas located closer to galaxies may be
expected to also contain appreciable amounts of metals
which can give rise to absorption lines.
The identification of a spectral line with a specific
line transition and a corresponding redshift is,
in general, possible only if at least two lines occur
at the same redshift. For this reason, doublet transitions
are particularly valuable, such as those of MgII
(λ = 2795 Å and λ = 2802 Å), and CIV (λ = 1548 Å
and λ = 1551 Å). The spectrum of virtually any QSO at
high (emission line) redshift z em shows narrow absorption
lines by CIV and MgII at absorption line redshifts
z abs < z em . If the spectral coverage extends to shorter
wavelengths than the observed Lyα emission line of
the QSO, numerous narrow absorption lines exist at
λ obs λ obs (Lyα) = (1 + z em ) 1216 Å. The set of these
absorption lines is denoted as the Lyman-α forest. In
about 15% of all QSOs, very broad absorption lines
are found, the width of which may even considerably
exceed that of the broad emission lines.
Classification of QSO Absorption Lines. The different
absorption lines in QSOs are distinguished by classes
according to their wavelength and width.
• Metal systems: In general these are narrow absorption
lines, of which MgII and CIV most frequently
occur (and which are the easiest to identify). However,
in addition, a number of lines of other elements
exist (Fig. 5.39). The redshift of these absorption
lines is 0 < z abs < z em ; therefore they are caused by
intervening matter along the line-of-sight and are
not associated with the QSO. Normally a metal system
consists of many different lines of different ions,
219
Fig. 5.39. Spectrum of the QSO 1331+17
at z em = 2.081 observed by the Multi-
Mirror Telescope in Arizona. In the
spectrum, a whole series of absorption
lines can be seen which have all been
identified with gas at z abs = 1.776. The
corresponding Lyα line at λ ≈ 3400 Å is
very broad; it belongs to the damped Lyα
lines