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Register a new userAn investigation of the luminosity-metallicity relation for a large sample of low-metallicity emission-line galaxies
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(abridged) We present 8.2m VLT spectroscopic observations of 28 HII regions in 16 emission-line galaxies and 3.6m ESO telescope spectroscopic observations of 38 HII 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. 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. From our new observations we find that the oxygen abundance in 61 out of the 66 HII regions of our sample ranges from 12+logO/H=7.05 to 8.22. Our sample includes 27 new galaxies with 12+logO/H<7.6 which qualify as extremely metal-poor star-forming galaxies (XBCDs). Among them are 10 HII regions with 12+logO/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 HII regions. 9000 more galaxies with oxygen abundances, based mainly on the Te-method, are compiled from the SDSS. Our data set spans a range of 8 mag with respect to its absolute magnitude in SDSS g (-12>Mg>-20) and nearly 2 dex in its oxygen abundance (7.0<12+logO/H<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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The emission line survey within the Calar Alto Deep Imaging Survey (CADIS) detects galaxies with very low continuum brightness by using an imaging Fabry-Perot interferometer. With spectroscopic follow-up observations of MB>~-19 CADIS galaxies using FORS2 at the VLT and DOLORES at TNG we obtained oxygen abundances of 5 galaxies at z~0.4 and 10 galaxies at z~0.64. Combining these measurements with published oxygen abundances of galaxies with MB<~-19 we find evidence that a metallicity-luminosity relation exists at medium redshift, but it is displaced to lower abundances and higher luminosities compared to the metallicity-luminosity relation in the local universe. Comparing the observed metallicities and luminosities of galaxies at z<3 with Pegase2 chemical evolution models we have found a favoured scenario in which the metallicity of galaxies increases by a factor of ~2 between z~0.7 and today, and their luminosity decreases by ~0.5-0.9mag.
(abridged) We present deep spectroscopy of a large sample of low-metallicity emission-line galaxies. The main goal of this study is to derive element abundances in these low-metallicity galaxies. We analyze 121 VLT spectra of HII regions in 46 low-metallicity emission-line galaxies. 83 of these spectra are archival VLT/FORS1+UVES spectra of HII 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. The oxygen abundance of the sample lies in the range 12 + log O/H = 7.2-8.4. The Ne/O ratio increases with increasing oxygen abundance. 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+logO/H<7.5 shows a slight increase with decreasing oxygen abundance. We present the first empirical relation between the electron temperature derived from [SIII]6312/9069 or [NII]5755/6583 and the one derived from [OIII]4363/(4959+5007) in low-metallicity galaxies. 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.
A large sample of MgII emitting star-forming galaxies with low metallicity [O/H] = log(O/H)-log(O/H)sun between -0.2 and -1.2 dex is constructed from Data Release 14 of the Sloan Digital Sky Survey. We selected 4189 galaxies with MgII 2797, 2803 emission lines in the redshift range z~0.3-1.0 or 35% of the total Sloan Digital Sky Survey star-forming sample with redshift z>0.3. We study the dependence of the magnesium-to-oxygen and magnesium-to-neon abundance ratios on metallicity. Extrapolating this dependence to [Mg/Ne]=0 and to solar metallicity we derive a magnesium depletion of [Mg/Ne]~-0.4 (at solar metallicity). We prefer neon instead of oxygen to evaluate the magnesium depletion in the interstellar medium because neon is a noble gas and is not incorporated into dust, contrary to oxygen. Thus, we find that more massive and more metal abundant galaxies have higher magnesium depletion. The global parameters of our sample, such as the mass of the stellar population and star formation rate, are compared with previously obtained results from the literature. These results confirm that MgII emission has a nebular origin. Our data for interstellar magnesium-to-oxygen abundance ratios relative to the solar value are in good agreement with similar measurements made for Galactic stars, for giant stars in the Milky Way satellite dwarf galaxies, and with low-metallicity damped Lyman-alpha systems.
Current emission-line based estimates of the metallicity of active galactic nuclei (AGN) at both high and low redshifts indicate that AGN have predominantly solar to supersolar metallicities. This leads to the question: do low metallicity AGN exist? In this paper we use photoionization models to examine the effects of metallicity variations on the narrow emission lines from an AGN. We explore a variety of emission-line diagnostics that are useful for identifying AGN with low metallicity gas. We find that line ratios involving [NII] are the most robust metallicity indicators in galaxies where the primary source of ionization is from the active nucleus. Ratios involving [SII] and [OI] are strongly affected by uncertainties in modelling the density structure of the narrow line clouds. To test our diagnostics, we turn to an analysis of AGN in the Sloan Digital Sky Survey (SDSS). We find a clear trend in the relative strength of [NII] with the mass of the AGN host galaxy. The metallicity of the ISM is known to be correlated with stellar mass in star-forming galaxies; our results indicate that a similar trend exists for AGN. We also find that the best-fit models for typical Seyfert narrow line regions have supersolar abundances. Although there is a mass-dependent range of a factor of 2-3 in the NLR metallicities of the AGN in our sample, AGN with sub-solar metallicities are very rare in the SDSS. Out of a sample of ~23000 Seyfert 2 galaxies we find only ~40 clear candidates for AGN with NLR abundances that are below solar.
(abridged) The present work is a first step to collect homogeneous abundances and near-infrared (NIR) luminosities for a sample of dwarf irregular (dIrr) galaxies, located in nearby groups. The use of NIR luminosities is intended to provide a better proxy to mass than the blue luminosities commonly used in the literature; in addition, selecting group members reduces the impact of uncertain distances. Accurate abundances are derived to assess the galaxy metallicity. Optical spectra are collected for Hii regions in the dIrrs, allowing the determination of oxygen abundances by means of the temperature-sensitive method. For each dIrr galaxy H-band imaging is performed and the total magnitudes are measured via surface photometry. This high-quality database allows us to build a well-defined luminosity-metallicity relation in the range -13 >= M(H) >= -20. The scatter around its linear fit is reduced to 0.11 dex, the lowest of all relations currently available. There might exist a difference between the relation for dIrrs and the relation for giant galaxies, although a firm conclusion should await direct abundance determinations for a significant sample of massive galaxies. This new dataset provides an improved luminosity-metallicity relation, based on a standard NIR band, for dwarf star-forming galaxies. The relation can now be compared with some confidence to the predictions of models of galaxy evolution. Exciting follow-ups of this work are (a) exploring groups with higher densities, (b) exploring nearby galaxy clusters to probe environmental effects on the luminosity-metallicity relation, and (c) deriving direct oxygen abundances in the central regions of star-forming giant galaxies, to settle the question of a possible dichotomy between the chemical evolution of dwarfs and that of massive galaxies.
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