We present 65 Sloan Digital Sky Survey (SDSS) spectra of 62 star-forming galaxies with oxygen abundances 12 + logO/H ~ 7.5-8.4. Redshifts of selected galaxies are in the range z~0.36-0.70. This allows us to detect the redshifted MgII 2797,2803 emissi
on lines. Our aim is to use these lines for the magnesium abundance determination. The MgII emission was detected in ~2/3 of the galaxies. We find that the MgII 2797 emission-line intensity follows a trend with the excitation parameter x= O^{2+}/O that is similar to that predicted by CLOUDY photoionised HII region models, suggesting a nebular origin of MgII emission. The Mg/O abundance ratio is lower by a factor ~2 than the solar ratio. This is probably the combined effect of interstellar MgII absorption and depletion of Mg onto dust. However, the effect of dust depletion in selected galaxies, if present, is small, by a factor of ~2 lower than that of iron.
(abridged) Strongly star-forming galaxies of subsolar metallicities are typical of the high-redshift universe. Here we therefore provide accurate data for two low-z analogs, the well-known low-metallicity emission-line galaxies Haro 11 and ESO 338-IG
004. On the basis of Very Large Telescope/X-shooter spectroscopic observations in the wavelength range 3000-24000AA, we use standard direct methods to derive physical conditions and element abundances. Furthermore, we use X-shooter data together with Spitzer observations in the mid-infrared range to attempt to find hidden star formation. We derive interstellar oxygen abundances of 12 + log O/H = 8.33+/-0.01, 8.10+/-0.04, and 7.89+/-0.01 in the two HII regions B and C of Haro 11 and in ESO 338-IG 004, respectively. The observed fluxes of the hydrogen lines correspond to the theoretical recombination values after correction for extinction with a single value of the extinction coefficient C(Hbeta) across the entire wavelength range from the near-ultraviolet to the NIR and mid-infrared for each of the studied HII regions. Therefore there are no emission-line regions contributing to the line emission in the NIR range, which are hidden in the optical range. The agreement between the extinction-corrected and CLOUDY-predicted fluxes implies that a HII region model including only stellar photoionisation is able to account for the observed fluxes, in both the optical and NIR ranges. All observed spectral energy distributions (SEDs) can be reproduced quite well across the whole wavelength range by model SEDs except for Haro 11B, where there is a continuum flux excess at wavelengths >1.6mum. It is possible that one or more red supergiant stars are responsible for the NIR flux excess in Haro 11B. We find evidence of a luminous blue variable (LBV) star in Haro 11C.
(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-me
tallicity 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.
(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 main
ly 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.
We present spectra of a large sample of low-metallicity blue compact dwarf galaxies which exhibit broad components in their strong emission lines, mainly in Hbeta, [O III]4959, 5007 and Halpha. Twenty-three spectra have been obtained with the MMT, 14
of which show broad emission. The remaining 21 spectra with broad emission have been selected from the Data Release 5 of the Sloan Digital Sky Survey. The most plausible origin of broad line emission is the evolution of massive stars and their interaction with the circumstellar and interstellar medium. The broad emission with the lowest H$alpha$ luminosities (10^36 - 10^39 erg/s) is likely produced in circumstellar envelopes around hot Ofp/WN9 and/or LBV stars. The broad emission with the highest Halpha luminosities (10^40 - 10^42 erg/s) probably arises from type IIp or type IIn supernovae (SNe). It can also come from active galactic nuclei (AGN) containing intermediate-mass black holes, although we find no strong evidence for hard non-thermal radiation in our sample galaxies. The oxygen abundance in the host galaxies with SN candidates is low and varies in the range 12 + log O/H = 7.36 - 8.31. However, type IIn SN / AGN candidates are found only in galaxies with 12 + log O/H < 7.99. Spectroscopic monitoring of these type IIn SN / AGN candidates over a time scale of several years is necessary to distinguish between the two possibilities.
