Redshifts of galaxies around Arp 220 and serendipitous discovery of three star-forming dwarf galaxies at redshift z similar to 0.5

We present redshift measurements of four faint galaxies around the archetyp al ultraluminous infrared galaxy Arp 220. These galaxies have significantly higher redshifts (z similar to 0.036-0.091) than that of Arp 220 (z simila r or equal to 0.018). Therefore, we conclude that they are background objec ts not physically related to Arp 220. Three of these faint galaxies located to the south of Arp 220 are a group of galaxies (or the brightest members in a cluster of galaxies) at z similar or equal to 0.09, as has been sugges ted previously on the basis of their associated soft X-ray emission. We also report the serendipitous discovery of three additional galaxies at redshift z similar to 0.5, found along one of the slit positions. All three galaxies exhibit an [O II] lambda 3727 emission line. The spectrum of the brightest galaxy (M-R similar or equal to 24.4) shows other strong emission lines: Mg II lambda 2798, H beta, [O III] lambda 4959, and L [O III] lambd a 5007. The emission-line properties of these galaxies as well as their int rinsically low luminosities (M-R greater than or equal to -18.4) indicate t hat they are star-forming dwarf galaxies.