Asymmetry Revisited: The Effect of Dust Attenuation and Galaxy Inclination

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 SDSS-IV MaNGA 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 the $A_b$ 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 $\tau_V\sim0.2$. 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.