Left panel: A far-ultraviolet (1500 A) image of the luminous Sc galaxy M101 obtained by UIT during the Astro-2 mission. A 5' bar is shown for scale.

Right panel: A simulation of a galaxy with the same structure but 10X higher surface brightness observed with a blue filter by the Keck 10-m telescope at a redshift z = 1.5 in a 10-hour exposure with 0.5" FWHM seeing against a sky background of 22.5 mag per square arcsecond. Pixels are 0.2" square. A 2" bar is shown for scale. The simulation is not easily recognizable as a normal spiral galaxy. Its asymmetries are emphasized; it appears distorted and fragmented. High surface brightness star-forming associations in its disk have taken on the appearance of nearby "companions."

[Simulation by R. W. O'Connell and G. D. Becker, from Marcum et al. "An Ultraviolet/Optical Atlas of Bright Galaxies," ApJ Supp, 2000.]