Strongly star forming galaxies in the local Universe with nebular He II 4686 emission

TL;DR

This study analyzes a large sample of local star-forming galaxies with nebular He II 4686 emission, exploring their physical properties, classification, and the origins of the emission, challenging previous models and suggesting alternative mechanisms.

Contribution

It provides a new empirical classification scheme for separating star-forming and AGN galaxies based on emission line diagnostics and investigates the origins of He II 4686 emission beyond Wolf-Rayet star models.

Findings

Star-forming and AGN galaxies form distinct branches in diagnostic diagrams.

40% of star-forming galaxies with He II 4686 lack Wolf-Rayet features.

Models predict He II 4686 emission only for specific stellar ages and metallicities.

Abstract

We present a sample of 2865 emission line galaxies with strong nebular He II 4686 emissions in Sloan Digital Sky Survey Data Release 7 and use this sample to investigate the origin of this line in star-forming galaxies. We show that star-forming galaxies and galaxies dominated by an active galactic nucleus form clearly separated branches in the He II 4686/H{\beta} versus [N II] 6584/H{\alpha} diagnostic diagram and derive an empirical classification scheme which separates the two classes. We also present an analysis of the physical properties of 189 star forming galaxies with strong He II 4686 emissions. These star-forming galaxies provide constraints on the hard ionizing continuum of massive stars. To make a quantitative comparison with observation we use photoionization models and examine how different stellar population models affect the predicted He II 4686 emission. We confirm previous findings that the models can predict He II 4686 emission only for instantaneous bursts of 20% solar metallicity or higher, and only for ages of ~ 4 - 5 Myr, the period when the extreme-ultraviolet continuum is dominated by emission from Wolf-Rayet stars. We find however that 83 of the star-forming galaxies (40%) in our sample do not have Wolf-Rayet features in their spectra despite showing strong nebular He II 4686 emission. We discuss possible reasons for this and possible mechanisms for the He II 4686 emission in these galaxies.