Maryam Shirazi, Jarle Brinchmann
We present a sample of 2865 emission line galaxies with strong nebular He II
{\lambda}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 {\lambda}4686/H{\beta} versus [N
II] {\lambda}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 {\lambda}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 {\lambda}4686 emission. We
confirm previous findings that the models can predict He II {\lambda}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 {\lambda}4686 emission. We
discuss possible reasons for this and possible mechanisms for the He II
{\lambda}4686 emission in these galaxies.
View original:
http://arxiv.org/abs/1201.1290
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