Discovery of a gas-rich companion to the extremely metal-poor galaxy DDO 68

John M. Cannon, Megan Johnson, Kristen B.W. McQuinn, Erik D. Alfvin, Jeremy Bailin, H. Alyson Ford, Léo Girardi, Alec S. Hirschauer, Steven Janowiecki, John J. Salzer, Angela Van Sistine, Andrew Dolphin, E. C. Elson, Baerbel Koribalski, Paola Marigo, Jessica L. Rosenberg, Philip Rosenfield, Evan D. Skillman, Aparna Venkatesan, Steven R. Warren

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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.

Original languageEnglish (US)
Article numberL1
JournalAstrophysical Journal Letters
Issue number1
StatePublished - May 20 2014


  • galaxies: dwarf
  • galaxies: evolution
  • galaxies: individual (DDO 68, DDO 68 C)
  • galaxies: irregular


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