Redshifts of Galaxies around Arp 220 and Serendipitous Discovery of Three Star-forming Dwarf Galaxies at Redshift [CLC][ITAL]z[/ITAL][/CLC] ∼ 0.5

Abstract

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 has been suggested previously 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.