Lopsidedness and Star Formation in Low-z Galaxies

We report on a project using the Fourth Data Release (DR4) of the Sloan Digital Sky Survey to determine the link between large-scale asymmetry in the stellar mass distribution of a galaxy (lopsidedness) and the rate of star formation. We quantify the lopsidedness of the galactic light distribution as the first azimuthal Fourier mode of galaxy light and use this measure as an indicator of the lopsidedness of the galactic mass distribution. An analysis of the DR4 galaxy dataset shows that at low redshift (z < 0.06), a large sample of imaged SDSS galaxies have sufficiently high signal-to-noise ratios, adequate seeing, and low inclination angles that allow an accurate measure of lopsidedness. Lopsided galaxies with mass enhancements of 5-25% are commonplace in this sample. We find a strong link between lopsidedness and star formation, with a smooth transition between two classes of galaxies: a class of lopsided, low-mass, low-concentration galaxies with recent or ongoing star formation, and a class of symmetric, massive, concentrated galaxies with older stellar populations.