Asymmetry Revisited: The Effect of Dust Attenuation and Galaxy Inclination

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 H alpha and H beta 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 tau(V) similar to 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.