Cloudy-Maraston: Integrating nebular continuum and line emission with the Maraston stellar population synthesis models

TL;DR

This paper presents Cloudy-augmented Maraston M24 stellar population models that include nebular continuum and line emission, using updated Geneva tracks with stellar rotation. By coupling the M24 ionizing spectra to Cloudy through Synthesizer grids, the authors systematically explore the dependence of nebular emission on age, metallicity, ionization parameter $U$, and gas density $n_{ m H}$, and compare predictions to JWST and SDSS emission-line diagnostics. They find strong agreement with Hβ, Hα, [N II] 6583, and [S II] 6713 across models, but substantial differences in high-ionization lines like [O III] 5007 and He II 1640 due to variations in hard ionizing photon production, driven by rotation and WR-phase temperature treatments; the difference in $\\Delta \hat{Q}_{\rm [O III]\\lambda 5007}$ can reach up to $6 \times 10^9 \\mathrm{s}^{-1} \\mathrm{M}_{\odot}^{-1}$. The work shows that young, high-$U$, high-metallicity models best match JWST data, while the choice of SPS model substantially affects predictions used in SED fitting and cosmological simulations. Overall, the study highlights the importance of including nebular emission and carefully selecting stellar input physics when interpreting high-redshift galaxy observations and their theoretical counterparts.

Abstract

The James Webb Space Telescope has ushered in an era of abundant high-redshift observations of young stellar populations characterized by strong emission lines, motivating us to integrate nebular emission into the new Maraston stellar population model which incorporates the latest Geneva stellar evolutionary tracks for massive stars with rotation. We use the photoionization code Cloudy to obtain the emergent nebular continuum and line emission for a range of modelling parameters, then compare our results to observations on various emission line diagnostic diagrams. We carry out a detailed comparison with several other models in the literature assuming different input physics, including modified prescriptions for stellar evolution and the inclusion of binary stars, and find close agreement in the H$\rm β$, H$\rm α$, [N II]$λ6583$, and [S II]$λ6731$ luminosities between the models. However, we find significant differences in lines with high ionization energies, such as He II$λ$1640 and [O III]$λ5007$, due to large variations in the hard ionizing photon production rates. The models differ by a maximum of $\hat{Q}_{\rm [O III]λ5007} = \rm 6 \times 10^9 \; s^{-1} \, M_{\odot}^{-1}$, where these differences are mostly caused by the assumed stellar rotation and effective temperatures for the Wolf Rayet phase. Interestingly, rotation and uncorrected effective temperatures in our single star population models alone generate [O III] ionizing photon production rates higher than models including binary stars with ages between 1 to 8 Myr. These differences highlight the dependence of derived properties from SED fitting on the assumed model, as well as the sensitivity of predictions from cosmological simulations.

Figures (19)

• Figure 1: Our chosen M24 ionizing spectra for solar metallicity with ages varying from $10^6$ to $10^{8}$ years, shown with dashed lines at the wavelengths associated with the ionization energies of H I, He I, and [O III].
• Figure 2: The specific hydrogen ionizing photon luminosity of the M24 simple stellar population model as a function of age with varying metallicity values ($Z = 0.003-0.02$) and a Salpeter or Kroupa IMF.
• Figure 3: The specific ionizing photon luminosity of the M24 model as a function of age for solar metallicity ($Z = 0.014$) for different rotations.
• Figure 4: The components of a M24 spectrum with an age of 1 Myr, solar metallicity and a reference ionization parameter of $U = 10^{-2}$. Here the incident component is the spectra that ionizes the cloud in our photoionization modelling, transmitted is the incident spectra that is transmitted through the cloud, nebular is the sum of the nebular continuum and line emission, and the total is the sum of the transmitted and nebular emission. Labelled are the positions of visible emission lines from optical to near-infrared wavelengths.
• Figure 5: The total emission for the M24 solar metallicity models with a reference ionization parameter of $U = 10^{-2}$ and ages varying between $10^5$ and $10^7$ years. The positions of common emission lines, as well as a CO molecular transition, from optical to mid-infrared are indicated with dashed lines.