Abundance patterns in the low-metallicity emission-line galaxies from the Early Data Release of the Sloan Digital Sky Survey

We have derived element abundances in 310 emission-line galaxies from the Early Data Release of the Sloan Digital Sky Survey (SDSS) for which the [O iii] λ4363 emission line was detected, allowing abundance determination by direct methods. We found no extremely metal-deficient galaxy (Z ∼ Z⊙/12) ⋆, probably as a consequence of selection effects in the SDSS sample. The oxygen abundance 12 + log O/H of the SDSS galaxies sample lies in the range from ∼ 7.6 (Z⊙/12) to ∼ 8.4 (Z⊙/2). This sample is merged with a sample of ∼100 blue compact dwarf galaxies with high quality spectra containing some very low-metallicity objects to study the abundance patterns of low-metallicity emission-line galaxies. We find that the α element-to-oxygen abundance ratios do not show any significant trends with the oxygen abundance, in agreement with previous studies. The Fe/O abundance ratio is smaller than the solar value, which we interpret as an indication that type Ia supernovae have not yet appeared in these galaxies, implying an age of less than 1 – 2 Gyr. However, a slight decrease of the Fe/O abundance ratio with increasing metallicity suggests some depletion of iron onto dust in the galaxies with higher metallicities. The N/O abundance ratio ranges from log N/O= –1.6 to –0.8. The fact that no galaxy with log N/O ∼ –1.6 was discovered implies that local low-metallicity emission-line galaxies are of a different nature than high-redshift damped Lyα systems with log N/O of ∼ –2.3 and that their ages are probably larger than 100 – 300 Myr. Our data indicate the existence of a gradual nitrogen enrichment on a time-scale of a few Myr.