8.1 Definition
A dwarf galaxy may be defined in terms of absolute magnitude and/or
linear size. There is, however, a continuous transition between giant,
average, and dwarf galaxies. The separation must rest on some
arbitrary demarcation lines. As far as we know, lenticulars and
spirals in the Sa-Sc range are all brighter than MT
-16; dwarf
systems fainter than -16 occur only among ellipticals and late-type
spirals Sd-Sm or Magellanic irregulars Im, but of course not all E or
Im systems are dwarfs. For instance, by this definition the Large Magellanic Cloud
(MT = -18.1) is certainly not a dwarf
and the Small Cloud
(MT = -16.0) is a borderline case, but M32
(MT = -15.6) and
NGC 205 (MT = -15.8) qualify as
dwarfs. Similar systems in
the Virgo cluster are NGC 4486B, the bright compact dE0
companion of M87, and
IC 3475, the prototype of the low-luminosity diffuse
ellipticals described by
Reaves (1956),
which are both at MT = -16.0. The
low-luminosity dwarf ellipticals of the Local Group exemplified by the
Sculptor and Fornax systems are much fainter, at MT =
-11.2 and -12.9,
respectively, and the extreme dwarf globular-like systems discovered
by Wilson (1955) and
Zwicky (1957)
are fainter still-for example,
Leo II is at MT = -9.1.
The lower end of the scale is indefinite, and for all we know (or rather do not know) ``pygmy'' systems of even smaller populations down to isolated star-cluster size might exist and remain undetected throughout intergalactic space as Zwicky has often argued (1957).