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 ] \lambda 4363 emission line was detected , allowing abundance determination by direct methods .
We found no extremely metal-deficient galaxy ( Z \la Z _ { \odot } /12 ) 12+log ( O/H ) _ { \odot } =8.69 \pm 0.05 ( Lodders [ 2003 ] ) . , 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 \sim 7.6 ( Z _ { \odot } /12 ) to \sim 8.4 ( Z _ { \odot } /2 ) .
This sample is merged with a sample of \sim 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 \alpha 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 \la –1.6 was discovered implies that local low-metallicity emission-line galaxies are of a different nature than high-redshift damped Ly \alpha systems with log N/O of \sim –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 .