We present HI spectral-line imaging of the extremely metal-poor galaxy DDO 68 .
This system has a nebular oxygen abundance of only \sim 3 % Z _ { \odot } , 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 HI spectral-line imaging obtained with the Karl G. Jansky Very Large Array The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities , Inc . , we have discovered a previously unknown companion of DDO 68 .
This low-mass ( M _ { HI } = 2.8 \times 10 ^ { 7 } M _ { \odot } ) , recently star-forming ( SFR _ { FUV } = 1.4 \times 10 ^ { -3 } M _ { \odot } yr ^ { -1 } , SFR _ { H \alpha } < 7 \times 10 ^ { -5 } M _ { \odot } yr ^ { -1 } ) companion has the same systemic velocity as DDO 68 ( V _ { sys } = 506 km s ^ { -1 } ; D = 12.74 \pm 0.27 Mpc ) and is located at a projected distance of \sim 42 kpc .
New HI maps obtained with the 100m Robert C. Byrd Green Bank Telescope provide evidence that DDO 68 and this companion are gravitationally interacting at the present time .
Low surface brightness HI gas forms a bridge between these objects .