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- Guseva, N. G., et al.
(författare)
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An investigation of the luminosity-metallicity relation for a large sample of low-metallicity emission-line galaxies
- 2009
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 505:1, s. 63-U98
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Tidskriftsartikel (refereegranskat)abstract
- Context. We present 8.2m VLT spectroscopic observations of 28 H II regions in 16 emission-line galaxies and 3.6m ESO telescope spectroscopic observations of 38 H II regions in 28 emission-line galaxies. These emission-line galaxies were selected mainly from the data release 6 (DR6) of the Sloan digital sky survey (SDSS) as metal-deficient galaxy candidates. Aims. We collect photometric and high-quality spectroscopic data for a large uniform sample of star forming galaxies including new observations. Our aim is to study the luminosity-metallicity (L-Z) relation for nearby galaxies, especially at its low-metallicity end and compare it with that for higher-redshift galaxies. Methods. Physical conditions and element abundances in the new sample are derived with the T-e-method, excluding six H II regions from the VLT observations and nearly two third of the H II regions from the 3.6m observations. Element abundances for the latter galaxies were derived with the semiempirical strong-line method. Results. From our new observations we find that the oxygen abundance in 61 out of the 66 H II regions of our sample ranges from 12 + log O/H = 7.05 to 8.22. Our sample includes 27 new galaxies with 12 + log O/H < 7.6 which qualify as extremely metal-poor star-forming galaxies (XBCDs). Among them are 10 H II regions with 12 + log O/H < 7.3. The new sample is combined with a further 93 low-metallicity galaxies with accurate oxygen abundance determinations from our previous studies, yielding in total a high-quality spectroscopic data set of 154 H II regions. 9000 more galaxies with oxygen abundances, based mainly on the T-e-method, are compiled from the SDSS. Photometric data for all galaxies of our combined sample are taken from the SDSS database while distances are from the NED. Our data set spans a range of 8 mag with respect to its absolute magnitude in SDSS g (-12 greater than or similar to M-g greater than or similar to -20) and nearly 2 dex in its oxygen abundance (7.0 less than or similar to 12 + log O/H less than or similar to 8.8), allowing us to probe the L-Z relation in the nearby universe down to the lowest currently studied metallicity level. The L-Z relation established on the basis of the present sample is consistent with previous ones obtained for emission-line galaxies.
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| 2. |
- Guseva, N. G., et al.
(författare)
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VLT spectroscopy of low-metallicity emission-line galaxies : abundance patterns and abundance discrepancies
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 529, s. A149-
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Izotov, Y. I., et al.
(författare)
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Tol 2240-384-a new low-metallicity AGN candidate
- 2010
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 517, s. A90-
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Tidskriftsartikel (refereegranskat)abstract
- Context. Active galactic nuclei (AGNs) have typically been discovered in massive galaxies of high metallicity. Aims. We attempt to increase the number of AGN candidates in low metallicity galaxies. We present VLT/UVES and archival VLT/FORS1 spectroscopic and NTT/SUSI2 photometric observations of the low-metallicity emission-line galaxy Tol 2240-384 and perform a detailed study of its morphology, chemical composition, and emission-line profiles. Methods. The profiles of emission lines in the UVES and FORS1 spectra are decomposed into several components with different kinematical properties by performing multicomponent fitting with Gaussians. We determine abundances of nitrogen, oxygen, neon, sulfur, chlorine, argon, and iron by analyzing the fluxes of narrow components of the emission lines using empirical methods. We verify with a photoionisation model that the physics of the narrow-line component gas is similar to that in common metal-poor galaxies. Results. Image deconvolution reveals two high-surface brightness regions in Tol 2240-384 separated by 2.4 kpc. The brightest southwestern region is surrounded by intense ionised gas emission that strongly affects the observed B - R colour on a spatial scale of similar to 5 kpc. The profiles of the strong emission lines in the UVES spectrum are asymmetric and all these lines apart from H alpha and H beta can be fitted by two Gaussians of FWHM similar to 75-92 km s(-1) separated by similar to 80 km s(-1) implying that there are two regions of ionised gas emitting narrow lines. The oxygen abundances in both regions are equal within the errors and in the range 12 + log O/H = 7.83-7.89. The shapes of the H alpha and H beta lines are more complex. In particular, the H alpha emission line consists of two broad components of FWHM similar to 700 km s(-1) and 2300 km s(-1), in addition to narrow components of two regions revealed from profiles of other lines. This broad emission in H alpha and H beta associated with the high-excitation, brighter southwestern H II region of the galaxy is also present in the archival low-and medium-resolution VLT/FORS1 spectra. The extraordinarily high luminosity of the broad H alpha line of 3x10(41) erg s(-1) cannot be accounted for by massive stars at different stages of their evolution. The broad Ha emission persists over a period of 7 years, which excludes supernovae as a powering mechanism of this emission. This emission most likely arises from an accretion disc around a black hole of mass similar to 10(7) M-circle dot.
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