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

Weichen Wang, Susan A. Kassin, Camilla Pacifici, Guillermo Barro, Alexander de la Vega, Raymond C. Simons, S. M. Faber, Brett Salmon, Henry C. Ferguson, Pablo G. Perez-Gonzalez, Gregory F. Snyder, Karl D. Gordon, Zhu Chen, Dritan Kodra. 2018. Galaxy Inclination and the IRX-beta Relation: Effects on UV Star Formation Rate Measurements at Intermediate to High Redshifts. Astrophysical Journal 869, 2, DOI: 10.3847/1538-4357/aaef79

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 (beta) 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-circle dot) 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 beta. 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 + 1.99 beta -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.

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