MODERN COSMOLOGY

Galaxy surveys 323

Figure 11.2. Several optical and near-IR surveys (carried out over the last ten
years) in the depth–solid angle plane. The AB magnitude system is defined as
m(AB)=− 2 .5logfν(nJy)+ 31 .4.

of any source population, as opposed to small-area surveys which mostly probe
the faint end of the LF (L.L∗). In general, the deeper the survey is the more
distant are theL∗objects which can be detected. The combination depth–solid
angle will determine the sampled volume at different redshifts, for a given object
selection method. Obviously, the product (limiting flux×survey area) is kept
approximately constant by observational time constraints. In figure 11.2, we
plot several cosmological surveys which have been carried out over the last ten
years with the aim of mapping the structure in the universe and understanding
its evolution. The Sloan Digital Sky Survey (SDSS) and the Hubble Deep Field
(HDF) represent the two complementary extremes, i.e., a shallow survey covering
a significant fraction of the sky and a very deep pencil beam survey.
For a given depth and survey area, the probed volume is ultimately
determined by theselection function, i.e. the set of criteria which lead to the object
detection. There are basically three different selection methods:

(1) Flux-limited selection. All the sources with a flux greater than a given

threshold,Slim, are included in the sample. The simplicity of this method

leads to a straightfoward computation of the probed volume (however, see

caveats later). IfASis the survey area, the maximum redshift,zmax,atwhich

a source of rest frame luminosityLcan be detected, is given implicitily by

L=Slim 4 πD^2 L(zmax)(11.15). Thus, using (11.19), thesurvey volumeis:

Vmax(z,L)=AS

∫zmax

0

Q(z,
m,
)dz. (11.25)

Note that the K-correction is also involved in this calculation when