Large-Scale Structure of the Universe and Cosmological ...

8 Results from Galaxy Surveys
8.1 Galaxies as Cosmological Tracers
Following the discovery of galaxies as basic objects in our universe [547,322,323],
it became clear that their spatial distribution was not uniform but clustered
in the sky, e.g. [709]. In fact, the Local Supercluster was recognized early on
from two-dimensional maps of the galaxy distribution [184]. The first measurements
ever of the angular two-point correlation function w(θ), done in
the Lick survey [653], established already one of the basic results of galaxy
clustering, that at small scales the angular correlation function w(θ) has a
power-law dependence in θ [see Eq. (625) below].
The first systematic study of galaxy clustering was carried out in the 1970’s
by Peebles and his collaborators. In a truly groundbreaking twelve-paper series
[500,303,503,501,504,505,275,575,576,226,577,227], galaxies were seen for
the first time as tracers of the large-scale mass distribution in the gravitational
instability framework 106 . These works confirmed (and extended) the
power-law behavior of the angular two-point function, established its scaling
with apparent magnitude, and measured for the first time the angular power
spectrum and the three- and four-point functions which were found to follow
the hierarchical scaling wN ∼ w N−1
2 . The theoretical interpretation of these
observations was done in the framework of galaxies that traced the mass distribution
in an Einstein de-Sitter universe 107 .
These results, however, relied on visual inspections of poorly calibrated photographic
plates; i.e. with very crude magnitudes (e.g., Zwicky) or galaxy counts
(e.g., Lick) estimated by eye, rather than by some automatic machine. These
surveys were the result of adding many different adjacent photographic plates
and the uniformity of calibration was a serious issue, since large-scale gradients
can be caused by varying exposure time, obscuration by our galaxy,
and atmospheric extinction. These effects are difficult to disentangle from
real clustering, attempts were made to reduce them with smoothing procedures,
but this could also result in a removal of real large-scale clustering.
More than 20 years after completion of Zwicky and Lick surveys, there were
major technological developments in photographic emulsions, computers and
automatic scanning machines, such as the APM (Automatic Plate Measuring
106 For an exhaustive review of this and earlier work see [210,508].
107 In this case self-similarity plus stable clustering leads to hierarchical scaling in
the highly non-linear regime, although it does not explain why hierarchical amplitudes
are independent of configuration, see Sect. 4.5. These observations were
partially motivated by work on the BBGKY approach to the dynamics of gravitational
instability [548] and also generated a significant theoretical activity that led
to much of the development of hierarchical models. For a recent historical account
of these results and a comparison with current views in the framework of biased
galaxy formation in CDM models see [515].
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