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

A robust analysis of galaxy structural parameters, based on the modeling of bulge and disk brightnesses in the BVRH bandpasses, is presented for 121 face-on and moderately inclined late-type spirals. Each surface brightness (SB) profile is decomposed into a sum of a generalized Sérsic bulge and an exponential disk. The reliability and limitations of our bulge-to-disk (B/D) decompositions are tested with extensive simulations of galaxy brightness profiles (one-dimensional) and images (two-dimensional). We have used repeat observations to test the consistency of our decompositions. The average systematic model errors are <~20% and <~5% for the bulge and disk components, respectively. The final set of galaxy parameters is studied for variations and correlations in the context of profile type differences and wavelength dependencies. Galaxy types are divided into three classes according to their SB profile shapes: Freeman type I, type II, and a third ``transition'' class for galaxies whose profiles change from type II in the optical to type I in the infrared. Roughly 43%, 44%, and 13% of type I, type II, and transition galaxies, respectively, comprise our sample. Only type I galaxies, with their fully exponential disks, are adequately modeled by our two-component decompositions, and our main results focus on these profiles. We discuss possible interpretations of Freeman type II profiles. The Sérsic bulge shape parameter for nearby type I late-type spirals shows a range between n=0.1 and 2, but, on average, the underlying surface density profile for the bulge and disk of these galaxies is adequately described by a double-exponential distribution. The distribution of disk scale lengths shows a decreasing trend with increasing wavelength, consistent with a higher concentration of old stars or dust (or both) in the central regions relative to the outer disk. We confirm a coupling between the bulge and disk with a scale length ratio <r_e/h>=0.22+/-0.09, or <h_bulge/h_disk>=0.13+/-0.06 for late-type spirals, in agreement with recent N-body simulations of disk formation. This ratio increases from ~0.20 for late-type spirals to ~0.24 for earlier types. These observations are consistent with bulges of late-type spiral galaxies being more deeply embedded in their host disk than earlier type bulges. Bulges and disks can thus preserve a nearly constant r_e/h but show a great range of SB for any given effective radius. The similar scaling relations for early- and late-type spirals suggest comparable formation and/or evolution scenarios for disk galaxies of all Hubble types. In the spirit of Courteau, de Jong, & Broeils but using our new, more extensive database, we interpret this result as further evidence for regulated bulge formation by redistribution of disk material to the galaxy center, in agreement with models of secular evolution of the disk.