Discovery of a Gas-rich Companion to the Extremely Metal-poor Galaxy DDO 68
Bailin, Jeremy; McQuinn, Kristen B. W.; Cannon, John M.; Skillman, Evan D.; Salzer, John J.; Hirschauer, Alec S.; Dolphin, Andrew; Elson, E. C.; Warren, Steven R.; Janowiecki, Steven; Van Sistine, Angela; Girardi, Léo; Marigo, Paola; Rosenfield, Philip; Johnson, Megan; Rosenberg, Jessica L.; Ford, H. Alyson; Alfvin, Erik D.; Koribalski, Baerbel; Venkatesan, Aparna
United States, Australia, Italy, South Africa
Abstract
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 (M_{H\,\scriptsize{I}} = 2.8 × 107 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.