and Cosmology

8.1 Redshift Surveys of Galaxies
a region of the sky is chosen in which the survey is performed.
Second, those objects in this region need to be
selected for which spectra should be obtained. In most
cases, for practical reasons the objects are selected according
to their brightness, i.e., spectra are taken of all
galaxies above a certain brightness threshold. The latter
defines the number density of galaxies in the survey, as
well as the required exposure time. To apply the second
criterion, a photometric catalog of sources is required
as a starting point. The criteria may be refined further
in some cases. For instance, a minimum angular extent
of objects may be chosen to avoid the inclusion of stars.
The spectrograph may set constraints on the selection
of objects; e.g., a multi-object spectrograph is often unable
to observe two sources that are too close together
on the sky.
Examples of Redshift Surveys. In the 1980s, the Center
for Astrophysics (CfA) Survey was carried out which
measured the redshifts of more than 14 000 galaxies in
the local Universe (Fig. 8.1). The largest distances of
these galaxies correspond to about cz ∼ 15 000 km/s.
One of the most spectacular results from this survey
was the discovery of the “Great Wall”, a huge structure
in the galaxy distribution (see also Fig. 7.2).
In the Las Campanas Redshift Survey (LCRS), carried
out in the first half of the 1990s, the redshifts of
more than 26 000 galaxies were measured. They are
located in six narrow strips of 80 ◦ length and 1.5 ◦
width each. With distances of up to ∼ 60 000 km/s,
this survey is considerably deeper than the CfA Redshift
Survey. The distribution of galaxies is displayed in
Fig. 8.2, from which we can recognize the typical bubble
or honeycomb structure. Galaxies are distributed
along filaments, which are surrounding large regions in
which virtually no galaxies exist – the aforementioned
voids. The galaxy distribution shows a structure which is
qualitatively very similar to the dark matter distribution
generated in numerical simulations (see, e.g., Fig. 7.12).
In addition, we see from the galaxy distribution that no
structures exist with scales comparable to the extent of
the survey. Thus, the LCRS has probed a scale larger
than that where significant structures of the mass distribution
are found. The survey volume of the LCRS
therefore covers a representative section of the Universe.
A different kind of redshift survey became possible
through the sky survey carried out with the IRAS
satellite (see Sect. 1.3.2). In these redshift surveys of
IRAS galaxies, the selection of objects for which spectra
were obtained was based on the 60 μm flux measured
by IRAS in its (near) all-sky survey. Various
redshift surveys are based on this selection, differing
in the flux limit applied; for example the 2 Jy survey
(hence, S 60 μm ≥ 2 Jy), or the 1.2 Jy survey. The
QDOT and PSCz surveys both have a limiting flux of
S 60 μm ≥ 0.6 Jy, where QDOT observed spectra for one
out of six randomly chosen galaxies from the IRAS
sample, while PSCz is virtually complete and contains
∼ 15 500 redshifts. One of the advantages of the IRAS
Fig. 8.2. The Las Campanas Redshift Survey consists of three
fields each at the North and South Galactic Pole. Each of these
fieldsisastrip1.5 ◦ wide and 80 ◦ long. Overall, the survey contains
about 26 000 galaxies, and the median of their redshift
is about 0.1. The six strips show the distribution of galaxies
on the sphere, and the wedge diagram indicates, for galaxies
with measured redshift, the right ascension versus distance
from the Milky Way, measured in units of 1000 km s −1
311