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Lopsided galactic disks in IllustrisTNG

A significant fraction of nearby late-type galaxies are lopsided. We study the asymmetry of the stellar component in a sample of well-resolved disky galaxies selected from the last snapshot of the Illustris TNG100 simulation based on their flatness and rotational support. Among 1912 disks, we identify 161 objects with significant asymmetry in terms of the m=1 Fourier mode of the stellar component within (1-2) stellar half-mass radii and describe their properties using three representative examples. The profiles of the m=1 mode typically increase with radius, and the corresponding phase is constant in the asymmetric region, signifying a global distortion. Following the evolution of the lopsided disks over time, we find that their history is rather uneventful and the occurrence of the asymmetry is fairly recent. Only about 1/3 of the lopsided disks experienced any strong interaction recently that could have led to the distortion of their shape: 24% were affected by a more massive object and 9% underwent a gas-rich merger. Still, a majority of lopsided disks show a significant increase in their recent star formation rate. The most frequent mechanism for the formation of lopsided disks thus seems to be asymmetric star formation probably related to gas accretion, although the distortions in the gas and stars are not strongly correlated. This picture is supported by the finding that the lopsided population on average contains more gas, has higher star formation rate, lower metallicity and bluer color than the remaining disks. These correlations are similar to those seen in real galaxies, even though the fraction of simulated lopsided disks (8%) is much lower than in observations (30%). The observed correlation between the presence of the asymmetry and a bar is not reproduced either. These discrepancies may be due to overquenching or insufficient resolution of IllustrisTNG simulations.