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Galaxy Pairs in cosmological simulations: Effects of interactions on star formation

We carried out a statistical analysis of galaxy pairs in hydrodynamical Lambda-CDM simulations. We focused on the triggering of star formation by interactions and analysed the enhancement of star formation activity in terms of orbital parameters. By comparing to a suitable sample of simulated galaxies without a nearby companion, we find that close encounters (r<30 kpc/h) may effectively induce star formation. However, our results suggest that the stability properties of systems and the spatial proximity are both relevant factors in the process of triggering star formation by tidal interactions. In order to assess the effects of projection and spurious pairs in observational samples, we also constructed and analysed samples of pairs of galaxies in the simulations obtained in projection. We found a good agreement with observational results with a threshold at rp ~ 25 kpc/h for interactions to effectively enhance star formation activity. For pairs within rp < 100 kpc/h, we estimated a ~27% contamination by spurious pairs, reduced to ~19% for close systems. We also found that spurious pairs affect more strongly high density regions with 17% of spurious pairs detected for low density regions compared to 33% found in high density ones. Also, we analysed the dependence of star formation on environment by defining the usual projected density parameter for both pairs and isolated galaxies in the simulations. We find the expected star formation-local density relation for both galaxies in pairs and without a close companion, with a stronger density dependence for close pairs which suggests a relevant role for interactions in driving this relation.