Lopsided and Bulging Distribution of Satellites Around Paired Halos. I. Observational Measurements and Comparison with Halo-based Models

We investigate the angular distribution of satellite galaxies in and around pairs of galaxy groups in the Sloan Digital Sky Survey (SDSS) and compare the results with the satellite distribution in pairs of dark matter halos by constructing mock catalogs that account for the same selection effects as the observational sample. We find that the angular distribution of satellites in both SDSS and the mock catalog exhibits a pronounced tendency toward lopsidedness, with satellites preferentially located between the two central galaxies. Additionally, there is a significant bulging distribution characterized by a higher concentration of satellites along the line connecting the two centrals compared to those found perpendicular to it. The lopsided and bulging distributions strengthen as pair separation and halo mass increase. The mock catalog successfully reproduces the observational results across all cases considered. We find that the lopsided and bulging distribution of satellites can largely be explained by overlapping two randomly selected halos matched in mass to the actual halo pairs, along with their surrounding satellite distribution, provided that the alignment between the orientations of the halos and the line connecting the halo pairs is taken into account. This suggests that the angular distribution of satellites is a natural consequence of the formation and evolution of large-scale structure in a Λ cold dark matter universe, eliminating the need to introduce other physical origins.