paper

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

arXiv:1810.02830 · doi:10.1093/mnras/sty2911

Abstract

We present chemical abundances and radial velocities of six HII 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 (MODS) at the Large Binocular Telescope (LBT). In the three regions where the [O III]4363 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)=7.140.07 in the northernmost region of the galaxy and, although with large uncertainties, an even lower 12+log(O/H)=6.960.09 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 -0.060.03 dex/kpc (or -0.30 dex ) with the direct method, and a steeper gradient of -0.120.03 dex/kpc (or -0.59 dex ) from the indirect method. For the -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), at variance with the suggested existence of a tight plateau at in extremely metal poor dwarfs. The derived helium mass fraction ranges from Y=0.2400.005 to Y=0.250.02, compatible with standard big bang nucleosynthesis. Finally, we measured HII region radial velocities in the range 479522 km/s from the tail to the head of the "comet", consistent with the rotation derived in the HI.

arXiv commentAccepted for publication in MNRAS

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