Star formation in close pairs selected from the Sloan Digital Sky survey

The effect of galaxy interactions on star formation has been investigated using Data Release 1 (DR1) of the Sloan Digital Sky Survey (SDSS). Both the imaging and spectroscopy data products have been used to construct a catalogue of nearest companions to a volume limited (0.03 < z < 0.1) sample of galaxies drawn from the main galaxy sample of SDSS. Of the 13973 galaxies in the volume limited sample, we have identified 12492 systems with companions at projected separations less than 300kpc. Star-formation rates for the volume-limited sample have been calculated from extinction and aperture corrected H alpha luminosities and, where available, IRAS data. Specific star formation rates were calculated by estimating galaxy masses from z-band luminosities, and r-band concentration indices were used as an indicator of morphological class. The mean specific star-formation rate is significantly enhanced for projected separations less than 25 kpc. For late-type galaxies the correlation extends out to projected separations of 300 kpc. The specific star-formation rate is observed to decrease with increasing recessional velocity difference, but the magnitude of this effect is small compared to that associated with the projected separation. We also observe a tight relationship between the concentration index and pair separation; the mean concentration index is largest for pairs with separations of approximately 75 kpc and declines rapidly for separations smaller than this. This is interpreted as being due to the presence of tidally-triggered nuclear starbursts in close pairs. Further, we find no dependence of star formation enhancement on the morphological type or mass of the companion galaxy.