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We describe a statistical approach for measuring the influence that agalaxy's closest companion has on the galaxy's properties out to arbitrarilywide separations. We begin by identifying the closest companion for everygalaxy in a large spectroscopic sample of Sloan Digital Sky Survey galaxies. Wethen characterize the local environment of each galaxy by using the number ofgalaxies within 2 Mpc and by determining the isolation of the galaxy pair fromother neighbouring galaxies. We introduce a sophisticated algorithm forcreating a statistical control sample for each galaxy, matching on stellarmass, redshift, local density and isolation. Unlike traditional studies ofclose galaxy pairs, this approach is effective in a wide range of environments,regardless of how far away the closest companion is (although a very distantclosest companion is unlikely to have a measurable influence on the galaxy inquestion). We apply this methodology to measurements of galaxy asymmetry, andfind that the presence of nearby companions drives a clear enhancement ingalaxy asymmetries. The asymmetry excess peaks at the smallest projectedseparations (< 10 kpc), where the mean asymmetry is enhanced by a factor of 2.0$\pm$ 0.2. Enhancements in mean asymmetry decline as pair separation increases,but remain statistically significant (1-2$\sigma$) out to projected separationsof at least 50 kpc.

Galaxy pairs in the Sloan Digital Sky Survey - XI. A new method for measuring the influence of the closest companion out to wide separations