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Spiral structure can occupy a significant part of the galaxy, but properlyaccounting for it in photometric decomposition is rarely done. This may lead tosignificant errors in the parameters determined. To estimate how exactlyneglecting the presence of spiral arms affects the estimation of galaxydecomposition parameters, we perform fitting of 29 galaxies considering spiralarms as a separate component. In this study, we utilize 3.6$\mu$m-band imagesfrom the S$^4$G survey and use a new 2D photometric model where each spiral armis modeled independently. In our model, the light distribution both along andacross the arm can be varied significantly, as well as its overall shape. Weanalyze the differences between models with and without spiral arms, and showthat neglecting spiral arms in decomposition causes errors in estimating theparameters of the disk, the bulge, and the bar. We retrieve differentparameters of the spiral arms themselves, including their pitch angles, widths,and spiral-to-total luminosity ratio, and examine various relations betweenthem and other galaxy parameters. In particular, we find that thespiral-to-total ratio is higher for galaxies with more luminous discs and withhigher bulge-to-total ratios. We report that the pitch angle of spiral armsdecreases with increasing bulge or bar fraction. We measure the width of thespiral arms to be 53\% of the disc scale length, on average. We examine thecontribution of the spiral arms to the azimuthally-averaged brightness profileand find that spiral arms produce a ``bump'' on this profile with a typicalheight of 0.3--0.7 mag.

Galaxies decomposition with spiral arms -- II: 29 galaxies from S$^4$G