Shape asymmetries and lopsidedness-radial-alignment in simulated galaxies

Galaxies are observed to be lopsided, meaning that they are more massive and more extended along one direction than the opposite. However, the galaxies generated in cosmological simulations are much less lopsided, inconsistent with observations. In this work, we provide a statistical analysis of the lopsided morphology of 2148 simulated isolated satellite galaxies generated by TNG50-1 simulation, incorporating the effect of tidal fields from halo centres. We study the radial alignment (RA) between the major axes of satellites and the radial direction of their halo centres within truncation radii of $3R_h$, $5R_h$ and $10R_h$. According to our results, RA is absent for all these truncations. 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. If the satellites are truncated within radii of $3R_h$ and $5R_h$ with $R_h$ being the stellar half mass radius, 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. Within a larger truncated radius of $10R_h$, the number of satellites with axial ratios $a_-/a_+ <1.0$ is about $10\%$ more than that with $a_-/a_+ > 1.0$. Therefore, the tidal fields from halo centres play a minor role in the generation of lopsided satellites. The lopsidedness radial alignment (LRA), i.e., an alignment of long semi-major-axes along the radial direction of halo centres, is further studied. No clear evidence of LRA is found in our sample within the framework of $\Lambda$CDM Newtonian dynamics. In comparison, the LRA can be naturally induced by the external fields from the central host galaxy in Milgromian dynamics. (See paper for full abstract)Comment: 16 pages, 12 figures, 3 tables, submitted to MNRA