Metallicity and colours in galaxy pairs in chemical hydrodynamical simulations.

Using chemical hydrodynamical simulations consistent with a �-CDM model, we study the role played by mergers and interactions in the regulation of the star formation activity, colours and the chemical properties of galaxies in pairs. A statistical analysis of the orbital param- eters in galaxy pairs (r < 100 kpc h 1 ) shows that the star formation (SF) activity correlates strongly with the relative separation and weakly with the relative velocity, indicating that close encounters (r < 30 kpc h 1 ) can increase the SF activity to levels higher than that exhibit in galaxies without a close companion. Analysing the internal properties of interacting systems, we find that their stability properties also play a role in the regulation the SF activity (Perez et al 2005a). Particularly, we find that the passive star forming galaxies in pairs are statistically more stable with deeper potential wells and less leftover gas than active star forming pairs. In order to compare our results with observations, we also build a projected catalog of galaxy pairs (2D-GP: rp < 100 kpc h 1 andV < 350 km s 1 ), constructed by projecting the 3D sam- ple in different random directions. In good agreement with observations (Lambas et al 2003), our results indicate that galaxies with rp < 25 kpc h 1 (close pairs) show an enhancement of the SF activity with respect to galaxies without a close companion. All the properties studied for galaxy pairs are analysed in the 2D and 3D simulated catalogs, allowing us to assess the contamination level introduced by spurious pairs. Con- sistently with observational estimations (Nikolic et al 2004), we find that