The Dust Attenuation Law in Galaxies

Understanding the properties and physical mechanisms that shape dust attenuation curves in galaxies is one of the fundamental questions of extragalactic astrophysics, with a great practical significance for deriving the physical properties of galaxies, such as the star formation rate and stellar mass. Attenuation curves result from a combination of dust grain properties, dust content, and the spatial arrangement of dust and different populations of stars. In this review we assess the current state of the field, paying particular attention to the importance of extinction curves as the building blocks of attenuation laws. We introduce a quantitative framework to characterize and compare extinction and attenuation curves, present a theoretical foundation for interpreting empirical results, overview an array of observational methods, and review the observational state of the field at both low and high redshift. Our main conclusions are: Attenuation curves exhibit a large range of slopes, from curves with shallow (Milky Way-like) slopes to those exceeding the slope of the SMC extinction curve. The slopes of the curves correlate strongly with the effective optical opacities, in the sense that galaxies with low dust column density (lower visual attenuation) tend to have steeper slopes, whereas the galaxies with high dust column density have shallower (grey) slopes. Galaxies appear to exhibit a diverse range of 2175A UV bump strengths, but on average have suppressed bumps compared to the average Milky Way extinction curve. Theoretical studies indicate that variations in bump strength may result from similar geometric and radiative transfer effects that drive the correlation between the slope and the dust column.