VLT spectroscopy of low-metallicity emission-line galaxies: abundance patterns and abundance discrepancies
2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 529, A149- p.Article in journal (Refereed) Published
Context. We present deep spectroscopy of a large sample of low-metallicity emission-line galaxies. Aims. The main goal of this study is to derive element abundances in these low-metallicity galaxies. Methods. We analyze 121 VLT spectra of H II regions in 46 low-metallicity emission-line galaxies. Of these spectra 83 are archival VLT/FORS1 + UVES spectra of H II regions in 31 low-metallicity emission-line galaxies that are studied for the first time with standard direct methods to determine the electron temperatures, the electron number densities, and the chemical abundances. Results. The oxygen abundance of the sample lies in the range 12 + log O/H = 7.2-8.4. We confirm previous findings that Ne/O increases with increasing oxygen abundance, likely because of a higher depletion of oxygen in higher-metallicity galaxies. The Fe/O ratio decreases from roughly solar at the lowest metallicities to about one tenth of solar, indicating that the degree of depletion of iron into dust grains depends on metallicity. The N/O ratio in extremely low-metallicity galaxies with 12 + log O/H < 7.5 shows a slight increase with decreasing oxygen abundance, which could be the signature of enhanced production of primary nitrogen by rapidly rotating stars at low metallicity. We present the first empirical relation between the electron temperature derived from [S III]lambda 6312/lambda 9069 or [N II]lambda 5755/lambda 6583 and the one derived from [O III]lambda 4363/lambda(4959 + 5007) in low-metallicity galaxies. We also present an empirical relation between t(e) derived from [O II]lambda 3727/(lambda 7320 + lambda 7330) or [S II]lambda 4068/(lambda 6717 + lambda 6730) and [O III]lambda 4363/lambda(4959 + 5007). The electron number densities N(e)(Cl III) and N(e)(Ar IV) were derived in a number of objects and are found to be higher than N(e)(O II) and N(e)(S II). This has potential implications for the derivation of the pregalactic helium abundance. In a number of objects, the abundances of C(++) and O(++) could be derived from recombination lines. Our study confirms the discrepancy between abundances found from recombination lines (RLs) and collisionally excited lines (CELs) and that C/O increases with O/H.
Place, publisher, year, edition, pages
2011. Vol. 529, A149- p.
galaxies: starburst, methods: observational, galaxies: abundances, galaxies: dwarf, galaxies: ISM
Astronomy, Astrophysics and Cosmology
IdentifiersURN: urn:nbn:se:su:diva-68128DOI: 10.1051/0004-6361/201016291ISI: 000294240700024OAI: oai:DiVA.org:su-68128DiVA: diva2:472169
authorCount :62012-01-032012-01-032012-01-03Bibliographically approved