Shape asymmetries and the relation between lopsidedness and radial alignment in simulated galaxies

Abstract

Galaxies are observed to be lopsided, meaning that they are more massive and more extended along one direction than the opposite. In this work, we provide a statistical analysis of the lopsided morphology of 1780 isolated satellite galaxies generated by TNG50-1 simulation, incorporating the effect of tidal fields from halo centres. The isolated satellites are galaxies without nearby substructures whose mass is over $1\%$ of the satellites within their virial radii. We study the radial alignment (RA) between the major axes of satellites and the radial direction of their halo centres in radial ranges of $0$-$2R_{\rm h}$, $2$-$5R_{\rm h}$ and $5$-$10R_{\rm h}$ with $R_{\rm h}$ being the stellar half mass radius. According to our results, the RA is virtually undetectable in inner and intermediate regions, yet it is significantly evident in outer regions. We also calculate the far-to-near-side semi-axial ratios of the major axes, denoted by $a_-/a_+$, which measures the semi-axial ratios of the major axes in the hemispheres between backwards (far side) and facing (near side) the halo centres. In all the radial bins of the satellites, the numbers of satellites with longer semi-axes on the far side are found to be almost equal to those with longer semi-axes on the near side. Therefore, the tidal fields from halo centres play a minor role in the generation of lopsided satellites. The long semi-major-axes radial alignment (LRA), i.e., an alignment between the long semi-major-axes of satellite galaxies and the radial directions to their halo centres, is further studied. No clear evidence of LRA is found in our sample within the framework of $\Lambda$CDM Newtonian dynamics. Finally, we briefly discuss the possible origins of the asymmetry of galaxies in TNG50-1.