MODERN COSMOLOGY

Quantifying large-scale structure 81

Figure 2.14. The flattening of the redshift–space correlation function is quantified by
the quadrupole-to-monopole ratio,ξ 2 /ξ 0. This quantity is positive where fingers-of-God
distortion dominates, and is negative where coherent infall dominates. The full curves show
model predictions forβ= 0 .3, 0.4 and 0.5, withσp= 4 h−^1 Mpc (full), plusβ= 0 .4 with
σp=3, 4 and 5h−^1 Mpc (chain). At large radii, the effects of fingers-of-God are becoming
relatively small, and values ofβ 0 .4 are clearly appropriate.

The quadrupole-to-monopole ratio should be a clear indicator of coherent infall.
In linear theory, it is given by

ξ 2

ξ 0

=f(n)

4 β/ 3 + 4 β^2 / 7

1 + 2 β/ 3 +β^2 / 5

,

wheref(n)=( 3 +n)/n(Hamilton 1992). On small and intermediate scales, the
effective spectral index is negative, so the quadrupole-to-monopole ratio should
be negative, as observed.
However, it is clear that the results on the largest scales are still significantly
affected by finger-of-God smearing. The best way to interpret the observed effects
is to calculate the same quantities for a model. To achieve this, we use the
observed APM 3D power spectrum, plus the distortion model discussed earlier.
This gives the plots shown in figure 2.14. The free parameter isβ, and this has
a best-fit value close to 0.4, approximately consistent with other arguments for a
universe with
= 0 .3 and a small degree of large-scale galaxy bias.

2.7.8 Galaxy formation and biased clustering

We now come to the difficult question of the relation between the galaxy
distribution and the large-scale density field. The formation of galaxies must be