Galaxy Inclination and the IRX-β Relation: Effects on UV Star Formation Rate Measurements at Intermediate to High Redshifts

Abstract

At intermediate and high redshifts, measurements of galaxy star formation rates are usually based on rest-frame ultraviolet (UV) data. A correction for dust attenuation, A _UV, is needed for these measurements. This correction is typically inferred from UV spectral slopes (β) using an equation known as “Meurer's Relation.” In this paper, we study this relation at a redshift of 1.5 using images and photometric measurements in the rest-frame UV (HST) through mid-infrared (Spitzer). It is shown that massive star-forming galaxies (above {10}¹⁰ {M}_⊙ ) have dust corrections that are dependent on their inclination to the line of sight. Edge-on galaxies have higher A _UV and infrared excess (IRX = L(IR)/L(UV)) than face-on galaxies at a given β. Interestingly, dust corrections for low-mass star-forming galaxies do not depend on inclination. This is likely because more massive galaxies have more disk-like shapes/kinematics, while low-mass galaxies are more prolate and have more disturbed kinematics. To account for an inclination-dependent dust correction, a modified Meurer's Relation is derived: {A}_UV} =4.43+1.99β -1.73(b/a-0.67), where b/a is the galaxy axis ratio. This inclination dependence of A _UV can be explained by a two-component model of the dust distribution inside galaxies. In such a model, the dust attenuation of edge-on galaxies has a higher contribution from a mixture component (dust uniformly mixed with stars in the diffuse interstellar medium), and a lower contribution from a birth cloud component (near-spherical dust shells surrounding young stars in H II regions) than that of face-on galaxies. The difference is caused by the larger path lengths through disks at higher inclinations.