Astronomy Colloquium

We discovered a rare class of giant spiral galaxies at redshift z=0.1-0.6 that rival giant elliptical galaxies in mass and luminosity. These  super spirals have diameters of 50-135 kpc and stellar masses of up to 1E12 solar masses. With star formation rates of 2-100 solar masses per year, they fall on the star-forming main sequence. In most respects, they appear to be scaled-up versions of normal spiral galaxies.  However, a large percentage (36%) are undergoing interactions or mergers. The AGN percentage is also large (9%). We conjecture that super spirals represent the small fraction of giant galaxies that, unlike giant ellipticals, were not transformed by major mergers. High rotation velocities up to 630 km/s at 40 kpc indicate massive dark halos, consistent with the high observed merger rate. Because of their giant sizes and other extreme properties, super spirals present a unique laboratory for studying galaxy scaling laws. The mere existence of such giant disk galaxies constrains the applicability of star formation quenching mechanisms to galaxy evolution.