The effects of dust on the derived photometric parameters of disks and bulges in spiral galaxies

Abstract

Spiral galaxies contain large amounts of interstellar dust, that absorbs and scatters their photons. This results in strong distortions and changes of their observed stellar images from what would be observed in the absence of the dust. Because of this the measured structural parameters of spiral galaxies, and indeed, knowledge of some of the most fundamental physical attributes of galaxies - their stellar distributions - is strongly biased. I present here the results of a study to quantify the effects of dust on the derived photometric parameters of disks (old stellar disks and young stellar
disks) and bulges in spiral galaxies: scale-lengths, axis-ratios, central surface-brightness, effective radii and Sersic indices. The goal of this study is to provide corrections for dust effects to observers by following the procedures and algorithms they use to perform surface brightness photometry of real images of galaxies.

The changes in the derived photometric parameters from their intrinsic values (as seen in the absence of dust) were obtained by fitting simulated images of disks and bulges produced using radiative transfer calculations. The fits to the simulations were performed using GALFIT 3.0.2 data analysis algorithm and the fitted models were the commonly used infinitely thin disks described by exponential, general Sersic and de Vaucouleurs distributions. The analysis was done firstly for disks and bulges seen in isolation (thus quantifying dust and projection effects) and subsequently for the same morphological components seen together (thus quantifying the dust effects on bulge-disk decomposition). This is the first time a systematic and self-consistent quantification of these effects has been performed covering the whole parameter space and all photometric parameters of spiral galaxies and its constituent stellar components. The approach proposed here allows a clear separation of projection effects, dust effects and decomposition effects, through chain corrections.

For single morphological components, I find the young stellar disks to suffer the most severe variation in the photometric parameters due to dust effects. In this context I also present corrections for narrow line (Balmer line) images. Old stellar disks are also significantly affected by dust, in particular when fits are performed with exponential functions. The photometric parameters of bulges are to a lesser extent affected by dust. I also find that the variation of dust corrections with face-on dust opacity and inclination is similar for bulges with different intrinsic stellar emissivities (different Sersic index), with differences manifesting only close to edge-on orientations of the disk. Dust corrections for bulges are found to be insensitive to the choice of the truncation radius and ellipticity of the bulge.

I find that dust effects on the photometric parameters of decomposed disks and bulges increase with the Sersic index of bulge intrinsic volume stellar emissivity distribution and depend on the bulge-to-disk ratio for galaxies with bulge stellar emissivity described by higher Sersic index functions.
All the numerical results are listed in the Appendices and made available to the scientific community.