Scaling relations and the fundamental line of the local group dwarf galaxies

We study the scaling relations between global properties of dwarf galaxies in the local group. In addition to quantifying the correlations between pairs of variables, we explore the 'shape' of the distribution of galaxies in log parameter space using standardized principal component analysis, the analysis is performed first in the 3D structural parameter space of stellar mass M _*, internal velocity V and characteristic radius R _* (or surface brightness μ_*). It is then extended to a 4D space that includes a stellar population parameter such as metallicity Z or star formation rate Ṁ_*. We find that the local group dwarfs basically define a one-parameter 'fundamental line' (FL), primarily driven by stellar mass, M_*. A more detailed inspection reveals differences between the star formation properties of dwarf irregulars (dI's) and dwarf ellipticals (dE's), beyond the tendency of the latter to be more massive. In particular, the metallicities of dI's are typically lower by a factor of 3 at a given M_* and they grow faster with increasing M_*, showing a tighter FL in the 4D space for the dE's. The structural scaling relations of dI's resemble those of the more massive spirals, but the dI's have lower star formation rates for a given M_* which also grow faster with increasing M_*. On the other hand, the FL of the dE's departs from the fundamental plane of bigger ellipticals. While the one-parameter nature of the FL and the associated slopes of the scaling relations are consistent with the general predictions of supernova feedback from Dekel & Woo, the differences between the FL's of the dE's and the dI's remain a challenge and should serve as a guide for the secondary physical processes responsible for these two types.