1 Introduction

Surface brightness distributions of external galaxies have been studied for many years but a reliable decomposition into structural components is often difficult to obtain. The reliability of models and techniques has been questioned by several authors (Kent 1986; Schombert & Bothun 1987; Byun & Freeman 1995), and there are several points that need further study and refinement. First, brightness distributions have often been studied as one-dimensional (1-D) radial profiles extracted by averaging along elliptical annuli; such ellipses can deviate considerably from the actual isophotes, especially in highly inclined systems with a luminous bulge. The result provides therefore a distorted profile of the surface brightness along the major axis. Second, the seeing must be properly taken into account (cf., Schombert & Bothun 1987), especially when studying the central regions where brightness gradients are highest. Third, the choice among the parametric forms of brightness distributions (exponentials, Hubble and de Vaucouleurs laws, and so on) does not depend on any physical argument, but only on their ability to fit the data. Finally the effects of internal extinction should be taken into account.

The new near-infrared (NIR) panoramic detectors have made imaging in the 1 to 2.5 $\mu$m regime rather straightforward with sensitivity and accuracy comparable to those attainable at optical wavelengths. The NIR bandpasses have been advocated to be the ideal ones to study the characteristics of the galactic backbones, that is the stellar populations which make up the mass distribution in a galaxy (e.g., Rix & Rieke 1993). The reason for this is twofold: first, the extinction is lower, by a factor of ten between the B and K bandpasses; and second, the emission of old stellar populations peaks in the NIR.

In this context, we have undertaken a program of NIR imaging of bright spiral galaxies with measured rotation curves. These images are used to decompose the luminosity distribution into bulge and disk components and then to analyze their contribution to the observed rotation curves. To overcome the methodological drawbacks mentioned above, we have developed a parametric technique to fit a two-dimensional (2-D) bulge + disk distribution to the entire image, which takes into account the effect of seeing. In addition, generalizing Kent's non-parametric approach, we have developed an iterative algorithm to deduce from the 2-D brightness distribution the contributions of bulge and disk, again taking into account the seeing. In this first paper, we report the results of the structural decomposition for a restricted sample of early-type spirals; a subsequent paper will describe the inferred mass distributions. Such systems appear well suited for studies of this kind, mainly because of the relative smoothness of their disks and their (reputedly) lower internal extinction. On the other hand, due to the low gas content, rotation in Sa's is not measured as far out as for later types, and hence they are not quite as effective for exploring dark matter properties. In several cases it has been noted that the presence of a dominant bulge might severely complicate the picture and the analysis (Kent 1988); these are galaxies with luminous bulges and slowly rising rotation curves, and some of them are also included in our sample.