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 ) 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: , 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.