  In the context of modified Newtonian dynamics, the fundamental plane, as the
observational signature of the Newtonian virial theorem, is defined by high
surface brightness objects that deviate from being purely isothermal: the
line-of-sight velocity dispersion should slowly decline with radius as observed
in luminous elliptical galaxies. All high surface brightness objects (e.g.
globular clusters, ultra-compact dwarfs) will lie, more or less, on the
fundamental plane defined by elliptical galaxies, but low surface brightness
objects (dwarf spheroidals) would be expected to deviate from this relation.
This is borne out by observations. With MOND, the Faber-Jackson relation (the
power-law relation between luminosity and velocity dispersion), ranging from
globular clusters to clusters of galaxies and including both high and low
surface brightness objects, is the more fundamental and universal scaling
relation in spite of its larger scatter. Faber-Jackson reflects the presence of
an additional dimensional constant (the MOND acceleration) in the structure
equation.
