Effects on Galaxy Evolution: Pair Interactions versus Environment

Abstract: In a hierarchical universe, mergers may be an important mechanism not only in increasing the mass of galaxies but also in driving the color and morphological evolution of galaxies. We use a large sample of ~1000 simulated galaxies of stellar mass greater than 10^9.6 solar masses (for ~4800 observations at multiple redshifts) from a high-res (0.46 h^{-1} kpc) cosmological simulation to determine under what circumstances being a member of a pair influences galaxy properties at z <= 0.2. We identify gravitationally bound pairs, and find a relative fraction of blue-blue, red-red, and blue-red pairs that agrees with observations (Lin et al. 2010). Pairs tend to avoid the extreme environments of clusters and void centres. While pairs in groups can include galaxies that are both blue, both red, or one of each color, in the field it is rare for pair galaxies to both be red. We find that physically bound pairs closer than 250 h^{-1} kpc tend to have higher sSFRs than the galaxy population as a whole. However, the sSFR of a bound galaxy relative to galaxies in a comparable local density environment (determined by the distance to the fifth nearest neighbor, rho_5), differs depending on the local density. In regions of high rho_5 the bound population has a higher fraction of star-forming (bluer) galaxies, whereas there is very little difference between bound and unbound galaxies in low rho_5 regions. This effect on the star-forming fraction may be driven by the higher fraction of bound HI-rich galaxies compared to unbound galaxies, particularly at high local densities. It appears that being in a pair has an incremental, but not overwhelming, effect on the star formation rate of the paired galaxies, compared to the more pronounced trend where galaxies overall have low sSFR (are red) in clusters and higher sSFR (blue) at the centre of voids. This trend depends most strongly on rho_5.(abridged)