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arXiv · 1811.03671

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
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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 (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 Msun) 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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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. 2018-11-08. Galaxy inclination and the IRX-beta relation: Effects on UV star-formation rate measurements at intermediate to high redshifts. https://doi.org/10.3847/1538-4357%2Faaef79

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