Next Contents Previous   To find a PG, one must have some idea
of what one is searching for. Here we discuss a few of the (more
or less) model independent properties of PGs, and the relation
of these properties to search strategies.  2.1 Surface Density
Suppose that PGs are found only at redshift zPG, and have a
lifetime in a ``recognizable PG phase'' of DeltatPG. Further
suppose that the present-day descendants of PGs possess number
density n0 at the present epoch. The number of PGs visible in a
solid angle Deltaomega can then (neglecting mergers) be written
as  equation1 (1)  where dA is angular diameter distance. Now,
the luminosity density of the Universe (in B) is ~ 2.4 x 108
hLBsmsun Mpc-3 (Felten 1985), and the characteristic luminosity
of galaxies is L*B appeq 1.6 x 1010 h-2 LBsmsun (very similar to
the luminosity of the Milky Way), where h = H0/100 and H0 is in
km s-1 Mpc-1. Hence a reasonable estimate for the local number
density of L* galaxies is n0 appeq 0.015 h3 Mpc-3. The surface
density of suc