We have re-evaluated empirical expressions for the abundance determination of N, O, Ne, S, Cl, Ar and Fe taking into account the latest atomic data and constructing an appropriate grid of photoionization models with state-of-the art model atmospheres. Using these expressions we have derived heavy element abundances in the ∼310 emission-line galaxies from the Data Release 3 of the Sloan Digital Sky Survey (SDSS) with an observed Hβ flux F(Hβ) > 10 −14 erg s −1 cm −2 and for which the [O iii] λ4363 emission line was detected at least at a 2σ level, allowing abundance determination by direct methods. The oxygen abundance 12 + log O/H of the SDSS galaxies lies in the range from ∼7.1 (Z /30) to ∼8.5 (0.7 Z ). The SDSS sample is merged with a sample of 109 blue compact dwarf (BCD) galaxies with high quality spectra, which contains extremely low-metallicity objects. We use the merged sample to study the abundance patterns of low-metallicity emission-line galaxies. We find that extremely metal-poor galaxies (12 + log O/H < 7.6, i.e. Z < Z /12) are rare in the SDSS sample. The α element-to-oxygen abundance ratios do not show any significant trends with oxygen abundance, in agreement with previous studies, except for a slight increase of Ne/O with increasing metallicity, which we interpret as due to a moderate depletion of O onto grains in the most metal-rich galaxies. The Fe/O abundance ratio is smaller than the solar value, by up to 1 dex at the high metallicity end. We also find that Fe/O increases with decreasing Hβ equivalent width EW(Hβ). We interpret this as a sign of strong depletion onto dust grains, and gradual destruction of those grains on a time scale of a few Myr. All the galaxies are found to have log N/O > -1.6, implying that they have a different nature than the subsample of high-redshift damped Lyα systems with log N/O of ∼-2.3 and that their ages are larger than 100-300 Myr. We confirm the apparent increase in N/O with decreasing EW(Hβ), already shown in previous studies, and explain it as the signature of gradual nitrogen ejection by massive stars from the most recent starburst.

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We present high-quality ground-based spectroscopic observations of 54 supergiant H II regions in 50 low-metallicity blue compact galaxies with oxygen abundances 12 ] log O/H between 7.1 and 8.3. We use the data to determine abundances for the elements N, O, Ne, S, Ar, and Fe. We also analyze Hubble Space T elescope (HST ) Faint Object Spectrograph archival spectra of 10 supergiant H II regions to derive C and Si abundances in a subsample of seven BCGs. The main result of the present study is that none of the heavy elementÈtoÈoxygen abundance ratios studied here ( This constancy (Z ¹ Z _ /20). implies that all of these heavy elements have a primary origin and are produced by the same massive (M º 10 stars responsible for O production. The dispersion of the ratios C/O and N/O in these M _ ) galaxies is found to be remarkably small, being only^0.03 and^0.02 dex, respectively. This very small dispersion is strong evidence against any time-delayed production of C and primary N in the lowest metallicity BCGs (secondary N production is negligible at these low metallicities). The absence of a timedelayed production of C and N is consistent with the scenario that galaxies with 12 ] log O/H ¹ 7.6 are now undergoing their Ðrst burst of star formation, and that they are therefore young, with ages not exceeding 40 Myr. If very low metallicity BCGs are indeed young, this would argue against the commonly held belief that C and N are produced by intermediate-mass (3 stars at very M _ ¹ M ¹ 9 M _ ) low metallicities, as these stars would not have yet completed their evolution in these lowest metallicity galaxies. In higher metallicity BCGs (7.6 \ 12 ] log O/H \ 8.2), the abundance ratios Ne/O, Si/O, S/O, Ar/O, and Fe/O retain the same constant value they had at lower metallicities. By contrast, there is an increase of C/O and N/O along with their dispersions at a given O. We interpret this increase as due to the additional contribution of C and primary N production in intermediate-mass stars, on top of that by high-mass stars. The above results lead to the following timeline for galaxy evolution : (1) all objects with 12 ] log O/H ¹ 7.6 began to form stars less than 40 Myr ago ; (2) after 40 Myr, all galaxies have evolved so that 12 ] log O/H [ 7.6 ; (3) by the time intermediate-mass stars have evolved and released their nucleosynthetic products (100È500 Myr), all galaxies have become enriched to 7.6 \ 12 ] log O/H \ 8.2. The delayed release of primary N at these metallicities greatly increases the scatter in N/O ; (4) later, when galaxies get enriched to 12 ] log O/H [ 8.2, secondary N production becomes important. BCGs show the same O/Fe overabundance with respect to the Sun (D0.4 dex) as Galactic halo stars, suggesting the same chemical enrichment history. We compare heavy elements yields derived from the observed abundance ratios with theoretical yields for massive stars and Ðnd general good agreement. However, the theoretical models are unable to reproduce the observed N/O and Fe/O. Further theoretical developments ...

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We present observations with the Cosmic Origins Spectrograph onboard the Hubble Space Telescope of five star-forming galaxies at redshifts z in the range 0.2993 -0.4317 and with high emission-line flux ratios O 32 = [O iii]λ5007/[O ii]λ3727 ∼ 8 -27 aiming to detect the Lyman continuum (LyC) emission. We detect LyC emission in all galaxies with the escape fractions f esc (LyC) in a range of 2 -72 per cent. A narrow Lyα emission line with two peaks in four galaxies and with three peaks in one object is seen in medium-resolution COS spectra with a velocity separation between the peaks V sep varying from ∼153 km s −1 to ∼ 345 km s −1 . We find a general increase of the LyC escape fraction with increasing O 32 and decreasing stellar mass M ⋆ , but with a large scatter of f esc (LyC). A tight anti-correlation is found between f esc (LyC) and V sep making V sep a good parameter for the indirect determination of the LyC escape fraction. We argue that one possible source driving the escape of ionizing radiation is stellar winds and radiation from hot massive stars.

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