Radiation fields by intermediate-age stellar populations with binaries as ionizing sources of H II regions

TL;DR

This study explores how intermediate-age stellar populations with binary systems can serve as significant ionizing sources in H II regions, affecting emission line strengths and diagnostic classifications.

Contribution

It demonstrates that radiation fields from intermediate-age populations with binaries can explain certain observed spectral features of H II regions and high-redshift galaxies.

Findings

Radiation fields of IA SPs with binaries can produce strong emission lines.

Diagnostic diagrams shift, blurring the line between star-forming galaxies and AGNs.

He II 1640 line can be generated by IA SPs with binaries.

Abstract

Radiation fields emitted by O, B-type stars or young stellar populations (SPs) are generally considered as significant central ionizing sources (CISs) of classic H II regions. In our previous studies, we show that the inclusion of binary interactions in stellar population synthesis models can significantly increase the ultraviolet spectrum hardness and the number of ionizing photons of intermediate-age (IA, 7<~log(t/yr)<~8) SPs. In this work, we present photoionization models of H II regions ionized by radiation fields emitted by IA SPs, and show that radiation fields of IA SPs, including binary systems, are in theory possible candidates of significant CISs of classic H II regions. When radiation fields of IA SPs comprising binary systems are used as the CISs of classic H II regions, the theoretical strengths of a number of lines (such as [O III]4959', [S II]6716', etc.), weaker than observations, are raised; the border /selection-criterion lines, between star-forming galaxies and AGNs in the diagnostic diagrams (for example, [N II]6583/Ha~[O III]5007/Hb), move into the region occupied originally by AGNs; and He II 1640 line, observed in Lyman break and high-redshift gravitationally lensed galaxies, also can be produced.