and Cosmology

9. The Universe at High Redshift
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inferred. Apparently, Ω HI ∼ 10 −3 over the whole redshift
interval 0 < z < 5, with perhaps a small redshift
dependence. Compared to the current density of stars,
this neutral hydrogen density is smaller only by a factor
∼ 3. Therefore, the hydrogen contained in DLAs is an
important reservoir for star formation, and DLAs may
represent condensations of gas that turn into “normal”
galaxies once star-formation sets in. Since DLAs have
low metallicities, typically 1/10 of the Solar abundance,
it is quite plausible that they have not yet experienced
much star formation.
The Nature of DLAs. This interpretation is supported
by the kinematical properties of DLAs. Whereas the
fact that the Lyα line is damped implies that its observed
shape is essentially independent of the Doppler
velocity of the gas, velocity information can nevertheless
be obtained from metal lines. Every DLA is
associated with metal absorption line systems, covering
low- and high-ionization species (such as SiII and
CIV, respectively) which can be observed by choosing
the appropriate wavelength coverage of the spectrum.
The profiles of these metal lines are usually split up into
several components. Interpreted as ionized “clouds”,
the velocity range Δv thus obtained can be used as an
indicator of the characteristic velocities of the DLA.
The values of Δv cover a wide range, with a median of
∼ 90 km/s for the low-ionization lines and ∼ 190 km/s
for the high-ionization transitions. The observed distribution
is largely compatible with the interpretation that
DLAs are rotating disks with a characteristic rotational
velocity of v c ∼ 200 km/s, once random orientations
and impact parameters of the line-of-sight to the QSO
are taken into account.
Search for Emission from DLAs. If this interpretation
is correct, then we might expect that the DLAs can also
be observed as galaxies in emission. This, however, is
exceedingly difficult for the high-redshift DLAs. Noting
that they are discovered as absorption lines in the spectrum
of QSOs, we face the difficulty of imaging a highredshift
galaxy very close to the line-of-sight to a bright
QSO (to quote characteristic numbers, the typical
QSO used for absorption-line spectroscopy has B ∼ 18,
whereas an L ∗ -galaxy at z ∼ 3hasB ∼ 24.5). Due to the
size of the point-spread function this is nearly hopeless
from the ground. But even with the resolution of HST, it
is a difficult undertaking. Another possibility is to look
for the Lyα emission line at the absorption redshift,
located right in the wavelength range where the DLA
fully blocks the QSO light. However, as we discussed for
LBGs above, not all galaxies show Lyα in emission, and
it is not too surprising that these searches have largely
failed. To data, only three DLA have been detected in
emission, with two of them seen only through the Lyα
emission line at the trough of the damped absorption
line, but with no observable continuum radiation. This
latter fact indicates that the blue light from DLAs is considerably
fainter than that from a typical LBG at z ∼ 3,
consistent with the interpretation that DLAs are not
strong star-forming objects. One of these three DLAs,
however, is observed to be considerably brighter and
seems to share some characteristics of LBGs, including
a high star-formation rate. In addition, two DLAs have
been detected by [OIII] emission lines. Overall, then,
the nature of high-redshift DLAs is still unclear, due to
the small number of direct identifications.
For DLAs at low redshifts the observational situation
is different, in that a fair fraction of them have
counterparts seen in emission. Whereas the interpretation
of the data is still not unambiguous, it seems that
the low-redshift population of DLAs may be composed
of normal galaxies.
The spatial abundance of DLAs is largely unknown.
The observed frequency of DLAs in QSO spectra is
the product of the spatial abundance and the absorption
cross-section of the absorbers. This product can be compared
with the corresponding quantity of local galaxies:
the detailed mapping of nearby galaxies in the 21-cm
line shows that their abundance and gaseous crosssection
are compatible with the frequency of DLAs
for z 1.5, and falls short by a factor ∼ 2 for the
higher-redshifts DLAs.
9.2.5 Lyman-Alpha Blobs
The search for high-redshift galaxies with narrow-band
imaging, where the filter is centered on the redshifted
Lyα emission line, has revealed a class of objects which
are termed “Lyman-α blobs”. These are luminous and
very extended sources of Lyα emission; their characteristic
flux in the Lyα line is ∼ 10 44 erg/s, and their
typical size is ∼ 100 kpc. Some of these sources show