2.3.3. Luminosity and the Fundamental Plane
A remarkable, but perhaps under-appreciated, feature of elliptical systems is the huge range of luminosity spanned by the class. The luminosity of ellipticals spans a dynamic range of around 50,000 (12 magnitudes), from inactive dwarf systems (MV ~ -11 mag) to giant cD galaxies (MV ~ -23 mag). In contrast, an individual classification ``bin'' along the spiral arm of the tuning fork encompasses a range of perhaps 3-5 mag (type Sb having the smallest dynamic range, and Sd the largest ). The enormous luminosity range encompassed by ellipticals masks an underlying physical order that is not obvious from the classification scheme. Ellipticals show a large scatter in quantitative photometric properties, but as Dressler et al.  and Djorgovski & Davis  first showed, correlations between radius, surface brightness, and velocity dispersion in ellipticals trace out a remarkably well-delineated plane in three-dimensional parameter space. Understanding the origin of this fundamental plane is a key component in studies of the physical morphology of early-type systems. While a large portion of the scatter in the fundamental plane is due to metallicity variations, it is important for the student to bear in mind the remarkable fact that luminous ellipticals are among the most highly enriched galaxies in the Universe (up to several hundred percent solar metallicity in their inner portions, with fairly strong internal gradients). The processes through which old systems may have undergone rapid enrichment at the time of formation are fascinating, and well-described by Arimoto  and references therein.