Redshifts of Galaxies Around Arp 220 and Serendipitous Discovery of Three Star-Forming Dwarf Galaxies at Redshift $z \sim$ 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 ($z \sim$ 0.036 -- 0.091) than that of Arp 220 ($z \simeq 0.018$). 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 $z \simeq 0.09$, 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 $z \sim 0.5$, found along one of the slit positions. All three galaxies exhibit an [O ii] \lambda 3727 emission line. The spectrum of the brightest galaxy ($m_R \simeq 24.4$) shows other strong emission lines: Mg ii \lambda 2798, H \beta, [O iii] \lambda 4959, and [O iii] \lambda 5007. The emission-line properties of these galaxies as well as their intrinsically low luminosities ($M_R \geq -18.4$) indicate that they are star-forming dwarf galaxies.