In this paper , we provide a physical model for the origin of variations in the shapes and bump strengths of dust attenuation laws in galaxies by combining a large suite of cosmological “ zoom-in ” galaxy formation simulations with 3D Monte Carlo dust radiative transfer calculations .
We model galaxies over 3 orders of magnitude in stellar mass , ranging from Milky Way like systems through massive galaxies at high-redshift .
Critically , for these calculations we employ a constant underlying dust extinction law in all cases , and examine how the role of geometry and radiative transfer effects impact the resultant attenuation curves .
Our main results follow .
Despite our usage of a constant dust extinction curve , we find dramatic variations in the derived attenuation laws .
The slopes of normalized attenuation laws depend primarily on the complexities of star-dust geometry .
Increasing fractions of unobscured young stars flatten normalized curves , while increasing fractions of unobscured old stars steepen curves .
Similar to the slopes of our model attenuation laws , we find dramatic variation in the 2175 \text { \AA } ultraviolet ( UV ) bump strength , including a subset of curves with little to no bump .
These bump strengths are primarily influenced by the fraction of unobscured O and B stars in our model , with the impact of scattered light having only a secondary effect .
Taken together , these results lead to a natural relationship between the attenuation curve slope and 2175 \text { \AA } bump strength .
Finally , we apply these results to a 25 Mpc/ h box cosmological hydrodynamic simulation in order to model the expected dispersion in attenuation laws at integer redshifts from z = 0 - 6 .
A significant dispersion is expected at low redshifts , and decreases toward z = 6 .
We provide tabulated results for the best fit median attenuation curve at all redshifts .