Following earlier speculation by
Zwicky (1957),
Arp (1965), and
Peebles (1971),
Disney (1976)
proposed the existence of galaxies that lurked
mostly below the level of the sky brightness. The distribution of surface
brightness shows no sign of a turnover down to the limit of current surveys
(McGaugh 1996;
Dalcanton et al. 1997).
This raises the issues of whether
there is a limit to the diffuseness of a stellar system and what the mode
of star formation is in a disk with only a few M
pc-2. Even
if Malin-type disks do not contribute much to the luminosity-weighted
integral of galaxies, they are important laboratories for studying the
distribution of dark matter in large halos
(Navarro, Frenk &
White 1997).
Also, the large size of the gas disks can result in a significant cross
section to quasar absorption, perhaps accounting for some fraction of the
damped Lyman-
systems.