Statistics of redshift periodicities

212 Current Issues in Cosmology
480
420
360
velocity (km/s)
300
240
180
120
60
0
−60
−120
−180
0 0.8 1.6 2.4 3.2 4 4.8 5.6 6.4 7.2 8
distance (Mpc)
Figure 17.3 Velocity–distance diagram for 103 galaxies within 500 km s −1 ,inthe
galactocentric frame of reference.
higher velocity dispersions for Local Groups, with or without biasing (Governato
et al. 1997).
The remarkably small dispersion ( ∼72 km s −1 ,asithappens) of the residuals in
relation to the running apex was also remarked on by KM, who tabulated the velocity
residuals, but for some reason did not plot them. This deficiency is remedied in Fig.
17.4, which reveals clear evidence for structure (the residuals are plotted both raw
and smoothed). A value ∼15 km s −1 was taken for the high-frequency cut-off,
although it is difficult to estimate. Peaks are clearly evident and are consistent with
a periodicity ∼36 km s −1 , relative to the running apex.
Figure 17.5 shows the outcome of a search for periodicity in the KM data using
a fixed solar vector. It turns out that there is a periodicity 35.2 km s −1 when one
subtracts out a velocity component corresponding to an apex
V ⊙ = 220 kms −1 , l ⊙ = 100.0 ◦ , b ⊙ =−17.0 ◦
This again illustrates one of the problems in analyzing the phenomenon: While
the periodicity is readily observable in high-quality data sets, it is not always clear
which vector is “real,” and which are “ghosts” or “harmonics,” or even whether a
single, fixed vector is involved.