Galaxies decomposition with spiral arms – I: 29 galaxies from S4G

Abstract

ABSTRACT Spiral structure can occupy a significant part of the galaxy, but properly accounting for it in photometric decomposition is rarely done. This may lead to significant errors in the parameters determined. To estimate how exactly neglecting the presence of spiral arms affects the estimation of galaxy decomposition parameters, we perform fitting of 29 galaxies considering spiral arms as a separate component. In this study, we utilize 3.6 μm-band images from the S4G survey and use a new 2D photometric model where each spiral arm is modelled independently. In our model, the light distribution both along and across the arm and its overall shape can be varied significantly. We analyse the differences between models with and without spiral arms, and show that neglecting spiral arms in decomposition causes errors in estimating the parameters of the disc, the bulge, and the bar. We retrieve different parameters of the spiral arms themselves, including their pitch angles, widths, and spiral-to-total luminosity ratio, and examine various relations between them and other galaxy parameters. In particular, we find that the spiral-to-total ratio is higher for galaxies with more luminous discs and with higher bulge-to-total ratios. We report that the pitch angle of spiral arms decreases with increasing bulge or bar fraction. We measure the width of the spiral arms to be 53 per cent of the disc scale length, on average. We examine the contribution of the spiral arms to the azimuthally averaged brightness profile and find that spiral arms produce a ‘bump’ on this profile with a typical height of 0.3–0.7 mag.