The electron temperatures of SDSS high-metallicity giant extragalactic HII regions

TL;DR

This study analyzes SDSS high-metallicity HII regions, revealing significant temperature variations and shifts in electron temperature relations, suggesting these regions are composite nebulae excited by multiple ionizing sources.

Contribution

It provides the first comprehensive analysis of electron temperatures in SDSS high-metallicity HII regions, highlighting their complex excitation conditions and deviations from nearby galaxy relations.

Findings

Large scatter in electron temperature t_2,O at given t_3,O.

Systematic shift of SDSS HII regions towards lower t_2,O or higher t_3,O.

Evidence that these regions are composite nebulae with multiple ionizing sources.

Abstract

Spectra of high-metallicity (12+log(O/H) > 8.2) HII regions where oxygen auroral lines are measurable in both the O+ and O++ zones, have been extracted from the Data Release 6 of the Sloan Digital Sky Survey (SDSS). 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-t_3,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.