Asymmetry Revisited: The Effect of Dust Attenuation and Galaxy Inclination

Abstract

Abstract Dust attenuation of an inclined galaxy can cause additional asymmetries in observations, even if the galaxy has a perfectly symmetric structure. Taking advantage of the integral field spectroscopic data observed by the Sloan Digital Sky Survey-IV Mapping Nearby Galaxies at the Apache Point Observatory survey, we investigate the asymmetries of the emission-line and continuum maps of star-forming disk galaxies. We define new parameters, A a and A b , to estimate the asymmetries of a galaxy about its major and minor axes, respectively. Comparing A a and A b in different inclination bins, we attempt to detect the asymmetries caused by dust. For the continuum images, we find that A a increases with the inclination, while A b is a constant as inclination changes. Similar trends are found for g − r, g − i, and r − i color images. The dependence of the asymmetry on inclination suggests a thin dust layer with a scale height smaller than the stellar populations. For the Hα and Hβ images, neither A a nor A b shows a significant correlation with inclination. Also, we do not find any significant dependence of the asymmetry of E(B − V) g on inclination, implying that the dust in the thick disk component is not significant. Compared to the SKIRT simulation, the results suggest that the thin dust disk has an optical depth of τ V ∼ 0.2. This is the first time that the asymmetries caused by the dust attenuation and the inclination are probed statistically with a large sample. Our results indicate that the combination of the dust attenuation and the inclination effects is a potential indicator of the 3D disk orientation.