78 An introduction to the physics of cosmology
Figure 2.11.The PD94 compilation of power-spectrum measurements. The upper panel
shows raw power measurements; the lower shows these data corrected for relative bias,
nonlinear effects and redshift-space effects.
(medianz= 0 .12) is as high as nature allows: apart from a tail of low surface
brightness galaxies (inevitably omitted from any spectroscopic survey), the
2dFGRS measure all the galaxies that exist over a cosmologically representative
volume. It is the first to achieve this goal. The fidelity of the resulting map of the
galaxy distribution can be seen in figure 2.12, which shows a small subset of the
data: a slice of thickness 4 degrees, centred at declination− 27 ◦.
An issue with using the 2dFGRS data in their current form is that the
sky has to be divided into circular ‘tiles’ each two degrees in diameter (‘2dF’
=‘two-degree field’, within which the AAT is able to measure 400 spectra
simultaneously; see http://www.aao.gov.au/2df/ for details of the instrument). The
tiles are positioned adaptively, so that larger overlaps occur in regions of high
galaxy density. In this way, it is possible to place a fibre on>95% of all galaxies.