Line intensities of SDSS HII regions

Our final sample consists of 181 SDSS spectra of HII regions in galaxies in the redshift range from ~0.025 to ~0.17. The t_2,O_-t_3,O_ diagram is examined.

In the SDSS HII regions, the electron temperature t_2,O_ is found to have a large scatter at a given value of the electron temperature t_3,O_. The majority of the SDSS HII regions lie below the t_2,O-t3,O_ relation derived for HII regions in nearby galaxies, i.e. the positions of the SDSS HII regions show a systematic shift towards lower t_2,O_ temperatures or/and towards higher t_3,O_ temperatures. The scatter and shift of the SDSS HII regions in the t_2,O_-t_3,O_ diagram can be understood if they are composite nebulae excited by two or more ionizing sources of different temperatures.