DISCOVERY OF A GAS-RICH COMPANION TO THE EXTREMELY METAL-POOR GALAXY DDO 68

We present H I spectral-line imaging of the extremely metal-poor galaxy DDO 68. This system has a nebular oxygen abundance of only ~3% Z sub([middot in circle]), making it one of the most metal-deficient galaxies known in the local volume. Surprisingly, DDO 68 is a relatively massive and luminous galaxy for its metal content, making it a significant outlier in the massemdashmetallicity and luminosityemdashmetallicity relationships. The origin of such a low oxygen abundance in DDO 68 presents a challenge for models of the chemical evolution of galaxies. One possible solution to this problem is the infall of pristine neutral gas, potentially initiated during a gravitational interaction. Using archival Hi spectral-line imaging obtained with the Karl G. Jansky Very Large Array, we have discovered a previously unknown companion of DDO 68. This low-mass (M sub(HI) = 2.8 x 10 super(7) M sub([middot in circle])), recently star-forming (SFR sub(FUV) = 1.4 x 10 super(-3) M sub([middot in circle]) yr super(-1), SFR sub(H alpha ) < 7 x 10 super(-5) M sub([middot in circle]) yr super(-1)) companion has the same systemic velocity as DDO 68 (V sub(sys) = 506 km s super(-1); D = 12.74 + or - 0.27 Mpc) and is located at a projected distance of ~42 kpc. New H I maps obtained with the 100 m Robert C. Byrd Green Bank Telescope provide evidence that DDO 68 and this companion are gravitationally interacting at the present time. Low surface brightness H I gas forms a bridge between these objects.