Theoretical Modeling of Starburst Galaxies

TL;DR

This work interrogates how two stellar population synthesis codes, PEGASE v2.0 and STARBURST99, shape the extreme-ultraviolet (EUV) ionizing fields that drive photoionization in starburst galaxies. By embedding their EUV inputs into the MAPPINGS III dust-aware photoionization framework and comparing to a large sample of infrared-selected starbursts, the study finds PEGASE yields a harder $1-4$ $\mathrm{Ryd}$ continuum than STARBURST99, largely due to Wolf-Rayet atmosphere treatments; continuum metal blanketing is proposed as a potential remedy to reproduce observations. Supernova remnant shocks contribute negligibly to the observed optical line ratios, supporting a photoionization-dominated picture for these systems. The authors derive a theoretical extreme starburst line that better delineates starbursts from AGN on optical diagnostic diagrams and discuss Wolf-Rayet signatures and metallicity effects as key constraints for modeling. Altogether, the paper advances a framework for classifying starbursts and AGN through self-consistent stellar populations, photoionization, and dust physics, with implications for the interpretation of massive-star feedback in galaxies.

Abstract

We have modeled a large sample of infrared starburst galaxies using both the PEGASE v2.0 and STARBURST99 codes to generate the spectral energy distribution of the young star clusters. PEGASE utilizes the Padova group tracks while STARBURST99 uses the Geneva group tracks, allowing comparison between the two. We used our MAPPINGS III code to compute photoionization models which include a self-consistent treatment of dust physics and chemical depletion. We use the standard optical diagnostic diagrams as indicators of the hardness of the EUV radiation field in these galaxies. These diagnostic diagrams are most sensitive to the spectral index of the ionizing radiation field in the 1-4 Rydberg region. We find that warm infrared starburst galaxies contain a relatively hard EUV field in this region. The PEGASE ionizing stellar continuum is harder in the 1-4 Rydberg range than that of STARBURST99. As the spectrum in this regime is dominated by emission from Wolf-Rayet (W-R) stars, this difference is most likely due to the differences in stellar atmosphere models used for the W-R stars. We believe that the stellar atmospheres in STARBURST99 are more applicable to the starburst galaxies in our sample, however they do not produce the hard EUV field in the 1-4 Rydberg region required by our observations. The inclusion of continuum metal blanketing in the models may be one solution. Supernova remnant (SNR) shock modeling shows that the contribution by mechanical energy from SNRs to the photoionization models is << 20%. The models presented here are used to derive a new theoretical classification scheme for starbursts and AGN galaxies based on the optical diagnostic diagrams.