Chemical abundances and radial velocities in the extremely metal-poor galaxy DDO 68
| Autor: | Felice Cusano, Donatella Romano, V. La Torre, Carlo Nipoti, Elena Sacchi, Alessandra Aloisi, Luca Ciotti, M. Mignoli, A. Marchetti, Michele Bellazzini, Monica Tosi, Francesca Annibali |
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| Přispěvatelé: | ITA, USA, Annibali, F, La , Torre, V, Tosi, M, Nipoti, C, Cusano, F, Aloisi, A, Bellazzini, M, Ciotti, L, Marchetti, A, Mignoli, M, Romano, D, Sacchi, E |
| Rok vydání: | 2018 |
| Předmět: |
(ISM): H II regions
galaxies: abundances galaxies: dwarf galaxies: individual (DDO 68) galaxies: interactions Physics 010308 nuclear & particles physics Metallicity Doubly ionized oxygen FOS: Physical sciences Astronomy and Astrophysics Astrophysics Astrophysics - Astrophysics of Galaxies 01 natural sciences Galaxy Spectral line Luminosity Space and Planetary Science Astrophysics of Galaxies (astro-ph.GA) 0103 physical sciences 010303 astronomy & astrophysics Mass fraction Dwarf galaxy Line (formation) |
| Zdroj: | Monthly Notices of the Royal Astronomical Society. 482:3892-3914 |
| ISSN: | 1365-2966 0035-8711 |
| DOI: | 10.1093/mnras/sty2911 |
| Popis: | 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 {\AA}, acquired with the Multi Object Double Spectrograph (MODS) at the Large Binocular Telescope (LBT). In the three regions where the [O III]$\lambda$4363 {\AA} line was detected, we inferred the abundance of He, N, O, Ne, Ar, and S through the "direct" method. We also derived the oxygen abundances of all the six regions adopting indirect method calibrations. We confirm that DDO 68 is an extremely metal-poor galaxy, and a strong outlier in the luminosity - metallicity relation defined by star-forming galaxies. With the direct-method we find indeed an oxygen abundance of 12+log(O/H)=7.14$\pm$0.07 in the northernmost region of the galaxy and, although with large uncertainties, an even lower 12+log(O/H)=6.96$\pm$0.09 in the "tail". This is, at face value, the most metal-poor direct abundance detection of any galaxy known. We derive a radial oxygen gradient of -0.06$\pm$0.03 dex/kpc (or -0.30 dex $R_{25}^{-1}$) with the direct method, and a steeper gradient of -0.12$\pm$0.03 dex/kpc (or -0.59 dex $R_{25}^{-1}$) from the indirect method. For the $\alpha$-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)$\sim-1.4$, 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$\pm$0.005 to Y=0.25$\pm$0.02, compatible with standard big bang nucleosynthesis. Finally, we measured HII region radial velocities in the range 479$-$522 km/s from the tail to the head of the "comet", consistent with the rotation derived in the HI. Comment: Accepted for publication in MNRAS |
| Databáze: | OpenAIRE |
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