The Spitzer Survey of Stellar Structure in Galaxies (S$^4$G, Sheth et. al. 2010) is a deep 3.6 and 4.5 $mu$m imaging survey of 2352 nearby ($< 40$ Mpc) galaxies. We describe the S$^4$G data analysis pipeline 4, which is dedicated to 2-dimensional str
uctural surface brightness decompositions of 3.6 $mu$m images, using GALFIT3.0 citep{peng2010}. Besides automatic 1-component Sersic fits, and 2-component Sersic bulge + exponential disk fits, we present human supervised multi-component decompositions, which include, when judged appropriate, a central point source, bulge, disk, and bar components. Comparison of the fitted parameters indicates that multi-component models are needed to obtain reliable estimates for the bulge Sersic index and bulge-to-total light ratio ($B/T$), confirming earlier results citep{laurikainen2007, gadotti2008, weinzirl2009}. In this first paper, we describe the preparations of input data done for decompositions, give examples of our decomposition strategy, and describe the data products released via IRSA and via our web page ({bf tt www.oulu.fi/astronomy/S4G_PIPELINE4/MAIN}). These products include all the input data and decomposition files in electronic form, making it easy to extend the decompositions to suit specific science purposes. We also provide our IDL-based visualization tools (GALFIDL) developed for displaying/running GALFIT-decompositions, as well as our mask editing procedure (MASK_EDIT) used in data preparation. In the second paper we will present a detailed analysis of the bulge, disk, and bar parameter derived from multi-component decompositions.
Recently, Buta etal. (2009) examined the question Do Bars Drive Spiral Density Waves?, an idea supported by theoretical studies and also from a preliminary observational analysis Block etal (2004). They estimated maximum bar strengths Q_b, maximum sp
iral strengths Q_s, and maximum m=2 arm contrasts A_2s for 23 galaxies with deep AAT K_s-band images. These were combined with previously published Q_b and Q_s values for 147 galaxies from the OSUBSGS sample and with the 12 galaxies from Block etal(2004). Weak correlation between Q_b and Q_s was confirmed for the combined sample, whereas the AAT subset alone showed no significant correlations between Q_b and Q_s, nor between Q_b and A_2s. A similar negative result was obtained in Durbala etal. (2009) for 46 galaxies. Based on these studies, the answer to the above question remains uncertain. Here we use a novel approach, and show that although the correlation between the maximum bar and spiral parameters is weak, these parameters do correlate when compared locally. For the OSUBSGS sample a statistically significant correlation is found between the local spiral amplitude, and the forcing due to the bars potential at the same distance, out to 1.6 bar radii (the typical bar perturbation is then of the order of a few percent). Also for the sample of 23 AAT galaxies we find a significant correlation between local parameters out to 1.4 bar radii. Our new results confirm that, at least in a statistical sense, bars do indeed drive spiral density waves.
