Bars and the symmetry of star formation patterns in the discs of spiral galaxies

We present a test for the degree of symmetry in the distribution of the H a lpha brightness along the arms of a sample of spiral galaxies. The test con sists of deriving the cross-correlation function of the Hot brightness as a function of curvilinear distance along pairs of opposed arms, after unfold ing the arms geometrically. Our results reveal a significantly greater degr ee of symmetry in the: non-barred population than in the barred. We derive parameters for both bar strength and bar ellipticity, and compare these wit h the derived cross-correlations to strengthen this conclusion. We suggest that density waves are a probable cause for the appearance of global, i.e. disc wide, two-fold symmetry in spiral discs. Comparison with published wor k on abundance gradients in the discs of barred and non-barred galaxies ind icates that, as for the abundances, mixing in the spiral disc as a result o f the bar potential may well be responsible for our observation that strong er bars are related to reduced two-fold symmetry in the distribution of sta r-forming regions along the spiral arms.