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 , 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 mass-metallicity and luminosity-metallicity 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 H I 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 ( = 2.8 × 10 ⁷ M ), recently star-forming (SFRFUV = 1.4 × 10^-3 M yr^-1, SFRHα < 7 × 10^-5 M yr^-1) companion has the same systemic velocity as DDO 68 (V sys = 506 km s ^-1; D = 12.74 ± 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.