Mock Observations: Three Different Types of Galaxy Alignment in TNG100 Simulations

Abstract

In this study, galaxy samples have been generated using mock observation techniques based on the results of TNG100-1 simulations to investigate three forms of intrinsic alignment: satellite-central alignment between the orientation of the brightest group galaxies (BGG) and the spatial distribution of their satellites, radial alignment between the satellites' orientation and the direction toward their BGG, as well as direct alignment between the orientation of BGG and that of its satellites. Overall, the predictions of galaxy alignment generally align with observations, although minor discrepancies have been identified. For satellite-central alignment, the alignment strength and color-dependence trends are well replicated by the mock observations. Regarding radial alignment, the signals are weak but discernible, with no apparent color dependence. As for direct alignment, no signal is detected, nor is there any color dependence. We also investigate the alignment dependencies on halo or the BGG properties, and proximity effect. For satellite-central alignment, the predicted alignment signal shows a positive correlation with halo and BGG mass, consistent with observations and previous predictions. Similar correlations have also been observed with the BGG age and metallicity, which merit future observational analysis for confirmation. Proximity effects have been observed for all three types of alignment, with satellites closer to the BGG exhibiting stronger alignment signals. The influence of galaxy definition and shape determination on alignment studies is also analyzed. This study underscores the importance of employing mock observation techniques for a fair comparison between predictions and observations.