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.
