Chemical abundances and radial velocities in the extremely metal-poor galaxy DDO 68

Abstract

We present chemical abundances and radial velocities of six H II regions in the extremely metal-poor star-forming dwarf galaxy DDO 68. They are derived from deep spectra in the wavelength range 3500-10 000 Å, acquired with the Multi-Object Double Spectrograph at the Large Binocular Telescope (LBT). In the three regions where the [O III] λ4363 Å line was detected, we inferred the abundance of He, N, O, Ne, Ar, and S through the `direct' method. We also derived the oxygen abundances of all the six regions adopting indirect-method calibrations. We confirm that DDO 68 is an extremely metal-poor galaxy, and a strong outlier in the luminosity-metallicity relation defined by star-forming galaxies. With the direct method, we find indeed an oxygen abundance of 12+log(O/H) = 7.14 ± 0.07 in the northernmost region of the galaxy and, although with large uncertainties, an even lower 12 + log(O/H) = 6.96 ± 0.09 in the `tail'. This is, at face value, the most metal-poor direct abundance detection of any galaxy known. We derive a radial oxygen gradient of -0.06 ± 0.03 dex kpc<SUP>-1</SUP> (or -0.30 dex R_{25}^{-1}) with the direct method, and a steeper gradient of -0.12 ± 0.03 dex kpc<SUP>-1</SUP> (or -0.59 dex R_{25}^{-1}) from the indirect method. For the α-element to oxygen ratios, we obtain values in agreement with those found in other metal-poor star-forming dwarfs. For nitrogen, instead, we infer much higher values, leading to log(N/O)∼- 1.4, at variance with the suggested existence of a tight plateau at -1.6 in extremely metal-poor dwarfs. The derived helium mass fraction ranges from Y = 0.240 ± 0.005 to Y = 0.25 ± 0.02, compatible with standard big bang nucleosynthesis. Finally, we measured H II region radial velocities in the range 479-522 km s<SUP>-1</SUP> from the tail to the head of the `comet', consistent with the rotation derived in the H I.