Redshifts of Galaxies around Arp 220 and Serendipitous Discovery of Three Star-forming Dwarf Galaxies at Redshift

We present redshift measurements of four faint galaxies around the archetypal ultraluminous infrared galaxy Arp 220. These galaxies have significantly higher redshifts (z ∼ 0.036 – 0.091) than that of Arp 220 (z ≃ 0.018). Therefore, we conclude that they are background objects 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 ≃ 0.09, as suggested by Heckman et al. [1996, ApJ, 457, 616] on the basis of their associated soft X-ray emission. We also report the serendipitous discovery of three additional galaxies at redshift z ∼ 0.5, found along one of the slit positions. All three galaxies exhibit an [O ii]λ3727 emission line. The spectrum of the brightest galaxy (mR ≃ 24.4) shows other strong emission lines: Mg ii λ2798, Hβ, [O iii]λ4959, and [O iii]λ5007. The emission-line properties of these galaxies as well as their intrinsically low luminosities (MR ≥ −18.4) indicate that they are star-forming dwarf galaxies. Subject headings: galaxies: individual (Arp 220) galaxies: redshift galaxies: groups galaxies: clusters