Dust attenuation in hydrodynamic simulations of spiral galaxies

We study the effects of dust in hydrodynamic simulations of spiral galaxies when different radial metallicity gradients are assumed. SUNRISE, a Monte Carlo radiative transfer code, is used to make detailed calculations of the internal extinction of disc galaxies caused by their dust content. SUNRISE is used on eight different smoothed particle hydrodynamics simulations of isolated spiral galaxies. These galaxies vary mainly in mass and hence luminosity, spanning a range in luminosities from -16 to -22 mag in the B band. We focus on the attenuation in different wavelength bands as a function of the disc inclination and the luminosity of the models, and compare this to observations. Observations suggest different metallicity gradients for galaxies of different luminosities. These metallicity gradients were explored in our different models, finding that the resulting dust attenuation matches observations for edge-on galaxies, but does not show a linear behaviour in log (axialratio) as some observations have suggested. A quadratic law describing the dependence of attenuation on inclination, as proposed by more recent observations, reconciles the attenuation of the simulations at intermediate inclinations with observations. We also compare the total infrared-to-ultraviolet flux ratios for the simulated galaxies with those of the Spitzer Infrared Nearby Galaxies Survey sample and find general agreement. Finally, we compare our results with those from simpler models that do not take into account structures such as spiral arms, finding that the inclusion of substructures on the size scale of spiral arms does not change conclusions about the attenuation dependence on inclination or wavelength.