Structure of Disk‐dominated Galaxies. I. Bulge/Disk Parameters, Simulations, and Secular Evolution

(Abridged) A robust analysis of galaxy structural parameters, based on themodeling of bulge and disk brightnesses in the BVRH bandpasses, is presentedfor 121 face-on and moderately inclined late-type spirals. Each surfacebrightness (SB) profile is decomposed into a sum of a generalized Sersic bulgeand an exponential disk. The reliability and limitations of our bulge-to-disk(B/D) decompositions are tested with extensive simulations of galaxy brightnessprofiles (1D) and images (2D). Galaxy types are divided into 3 classesaccording to their SB profile shapes; Freeman Type-I and Type-II, and a third``Transition'' class for galaxies whose profiles change from Type-II in theoptical to Type-I in the infrared. We discuss possible interpretations ofFreeman Type-II profiles. The Sersic bulge shape parameter for nearby Type-Ilate-type spirals shows a range between n=0.1-2 but, on average, the underlyingsurface density profile for the bulge and disk of these galaxies is adequatelydescribed by a double-exponential distribution. We confirm a coupling betweenthe bulge and disk with a scale length ratio r_e/h=0.22+/-0.09, orh_bulge/h_disk=0.13+/-0.06 for late-type spirals, in agreement with recentN-body simulations of disk formation and models of secular evolution. Thisratio increases from ~0.20 for late-type spirals to ~0.24 for earlier types.The similar scaling relations for early and late-type spirals suggestcomparable formation and/or evolution scenarios for disk galaxies of all Hubbletypes.