Galaxy pairs in the Sloan Digital Sky Survey - VI. The orbital extent of
  enhanced star formation in interacting galaxies
  We use pair and environmental classifications of ~ 211,000 star-forming
galaxies from the Sloan Digital Sky Survey, along with a suite of merger
simulations, to investigate the enhancement of star formation as a function of
separation in galaxy pairs. Using a new technique for distinguishing between
the influence of nearby neighbours and larger scale environment, we find a
clear enhancement in star formation out to projected separations of ~ 150 kpc,
beyond which there is no net enhancement. We find the strongest enhancements at
the smallest separations (especially < 20 kpc), consistent with earlier work.
Similar trends are seen in the simulations, which indicate that the strongest
enhancements are produced in highly disturbed systems approaching final
coalescence, whereas the more modest enhancements seen at wider separations are
the result of starburst activity triggered at first pericentre passage, which
persists as the galaxies move to larger separations. The absence of any net
enhancement beyond 150 kpc provides reassurance that the detected enhancements
are due to galaxy-galaxy interactions, rather than larger scale environmental
effects or potential pair selection biases. A rough census indicates that 66
per cent of the enhanced star formation in our pair sample occurs at
separations > 30 kpc. We conclude that significant interaction-induced star
formation is not restricted to merger remnants or galaxies with close
companions; instead, a larger population of wider separation pairs exhibit
enhanced star formation due to recent close encounters.
