On the Impact of Inclination-Dependent Attenuation on the Derived Star Formation Histories of Disk Galaxies

The physical properties of a galaxy (e.g., its star-formation history and dust content) regulate the distribution of light that is emitted by stars and attenuated by the interstellar gas and dust. This attenuation by dust can have a significant impact on the observed spectral energy distribution (SED) of a disk galaxy, especially when taking into account its viewing angle (i.e., inclination). For example, as the inclination angle of a galactic disk changes from face-on to edge-on (i.e., i = 0 deg to i = 90 deg), the proportion of light that is attenuated along the line of sight increases, due to an increasing column density of dust. Therefore, additional care must be taken when modeling the SED of a disk galaxy to account for any inclination dependence. In this work, we develop and implement an inclination-dependent attenuation prescription into our SED fitting code, Lightning, to more accurately derive the physical properties of disk galaxies. First, we present the details of our SED fitting code, Lightning, as it is the cornerstone of our inclination-dependent analyses. We discuss all of the models in Lightning, which can include contributions from a variety of sources, along with the available algorithms to fit the models to observations. Then, to show the future potential of Lightning, we present several examples using a variety of observational data. Next, to better understand how inclination affects the physical properties of disk galaxies, we apply our prescription on two respective galactic samples to (1) study the impact of inclination-dependent attenuation on derived stellar properties and (2) examine and quantify how commonly used star formation rate (SFR) estimators depend on inclination. For the first application, we compare our inclination-dependent attenuation prescription with a more traditional inclination-independent attenuation prescription. Our results indicate stark statistical differences in the derived optical attenuation and stellar masses, with the traditional attenuation prescription resulting in these properties being underestimated compared to the inclination-dependent attenuation prescription at high inclinations. Therefore, the results from this application suggest that SED fitting assuming inclination-independent attenuation potentially underestimates these properties in highly inclined disk galaxies. For the second application, We find that two commonly used SFR estimators (the hybrid UV+IR and AFUV-beta relations) present clear dependencies on inclination. To quantify these dependencies, we expand the parametric form of the estimators to include an inclination-dependence. We then compare both of these new inclination-dependent estimators to similar inclination-independent relations found in the literature. From this comparison, we find that our inclination-dependent relations result in a reduction in the residual scatter of the derived SFRs of our sample by approximately a factor of two. Therefore, this second application demonstrates that inclination must be considered in SFR estimators to produce more accurate SFR estimates in disk galaxies. Overall, this work provides the crucial steps towards understanding and incorporating the impact of inclination-dependence on the derived star-formation histories of disk galaxies. It additionally presents a novel tool (Lightning) which can be used in future studies to more accurately account for this inclination-dependence.