The Redshifts of Galaxies Around Arp 220 and the Serendipitous Discovery of Three Star-Forming Dwarf Galaxies at z~0.5

Youichi Ohyama (Tohoku Univ), Yoshiaki Taniguchi (Tohoku Univ), J. E. Hibbard (NRAO), and William D. Vacca (IfA).
The Astronomical Journal, 117, 2617 (1999) (pdf, 1.1Mb) (ADS)
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 in the two-dimensional spectra obtained at one of the slit positions. All three galaxies exhibit an [O II] emission line. The spectrum of the brightest galaxy (r~24.4) shows other strong emission lines: Mg II, Hbeta, and [O III]. The emission-line properties of these galaxies as well as their intrinsically low luminosities (R ~ -18.4) indicate that they are star-forming dwarf galaxies.

R data obtained from the UH 88" in 6/95
Keck LRIS Spectra obtained from Keck II with W. D. Vacca in 4/97

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