Mon. Not. R. Astron. Soc., 482, 3892-3914 (2019/January-3)
Chemical abundances and radial velocities in the extremely metal-poor galaxy DDO 68.
ANNIBALI F., LA TORRE V., TOSI M., NIPOTI C., CUSANO F., ALOISI A., BELLAZZINI M., CIOTTI L., MARCHETTI A., MIGNOLI M., ROMANO D. and SACCHI E.
Abstract (from CDS):
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–1 (or -0.30 dex R25–1) with the direct method, and a steeper gradient of -0.12 ± 0.03 dex kpc–1 (or -0.59 dex R25–1) 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–1 from the tail to the head of the 'comet', consistent with the rotation derived in the H I.
© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
(ISM): H ii regions - galaxies: abundances - galaxies: dwarf - galaxies: individual: (DDO 68) - galaxies: interactions
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