Non-parametric decompositions of disc galaxies in S4G using DISKFIT

Abstract

We present photometric models of 532 disc galaxies in 3.6μm images from the Spitzer Survey of Stellar Structure in Galaxies (S4G) using the non-parametric DISKFIT algorithm. We first test DISKFIT's performance on 400 synthetic S4G-like galaxy images. DISKFIT is unreliable in the bulge region, but accurately disentangles exponential discs from Ferrers bars farther out as long as their position angles differ by more than 5°. We then proceed to model the S4G galaxies, successfully fitting 489 of them using an automated approach for initializing DISKFIT, optimizing the model and deriving uncertainties using a bootstrap-resampling technique. The resulting component geometries and surface brightness profiles are compared to those derived by Salo et al. using the parametric model GALFIT. We find generally good agreement between the models, but discrepancies between best-fitting values for individual systems are often significant: the choice of algorithm clearly impacts the inferred disc and bar structure. In particular, we find that DISKFIT typically assigns more light to the bar and less light to the disc relative to the Ferrers and exponential profiles derived using GALFIT in the bar region. Given DISKFIT's reliability at disentangling these components in our synthetic images, we conclude that the surface brightness distributions of barred S4G galaxies are not well-represented by these functional forms. The results presented here underscore the importance of validating photometric decomposition algorithms before applying them to real data and the utility of DISKFIT's non-parametric approach at measuring the structure of discs and bars in nearby galaxies.