Modelling the nebular emission from primeval to present-day star-forming galaxies

TL;DR

The paper develops a self-consistent UV/optical nebular-emission framework for star-forming galaxies across wide metallicities, explicitly incorporating dust depletion and the distinction between gas-phase and interstellar metal content. It couples an updated stellar population synthesis with the Cloudy photoionization code, parameterized by $Z$, $Z_{ism}$, $U_S$, $\xi_d$, C/O, $n_H$, and $m_{up}$, and provides a large grid of 21,168 models. The results show UV lines are particularly sensitive to C/O, density, IMF upper mass cutoff, and dust content, while optical lines reveal complex metallicity effects; standard direct-$T_e$ abundance prescriptions are reliable only near solar metallicity, necessitating full self-consistent modeling at non-solar metallicities. The grid reproduces observed optical and UV diagnostics from local to high-redshift galaxies and is publicly available to support robust abundance inferences, including ionization correction factors and interstellar versus gas-phase metal content.

Abstract

We present a new model of the nebular emission from star-forming galaxies in a wide range of chemical compositions, appropriate to interpret observations of galaxies at all cosmic epochs. The model relies on the combination of state-of-the-art stellar population synthesis and photoionization codes to describe the ensemble of HII regions and the diffuse gas ionized by young stars in a galaxy. A main feature of this model is the self-consistent yet versatile treatment of element abundances and depletion onto dust grains, which allows one to relate the observed nebular emission from a galaxy to both gas-phase and dust-phase metal enrichment. We show that this model can account for the rest-frame ultraviolet and optical emission-line properties of galaxies at different redshifts and find that ultraviolet emission lines are more sensitive than optical ones to parameters such as C/O abundance ratio, hydrogen gas density, dust-to-metal mass ratio and upper cutoff of the stellar initial mass function. We also find that, for gas-phase metallicities around solar to slightly sub-solar, widely used formulae to constrain oxygen ionic fractions and the C/O ratio from ultraviolet and optical emission-line luminosities are reasonable faithful. However, the recipes break down at non-solar metallicities, making them inappropriate to study chemically young galaxies. In such cases, a fully self-consistent model of the kind presented in this paper is required to interpret the observed nebular emission.

Figures (7)

• Figure 1: $\log\hbox{(N/O)$_\mathrm{gas}$}$ as a function of $12+\log\hbox{(O/H)$_\mathrm{gas}$}$, for models with interstellar metallicity $\hbox{$Z_\textnormal{\tiny ism}$}=0.0001$, 0.0002, 0.0005, 0.001, 0.002, 0.004, 0.006, 0.008, 0.010, 0.014, 0.017, 0.020, 0.030 and 0.040 (colour-coded as indicated), carbon-to-oxygen abundance ratio $\hbox{C/O}=0.1$ (triangle), 1.0 (star) and 1.4 (circle) times (C/O)$_\odot$, and dust-to-metal mass ratio $\hbox{$\xi_{\rm{d}}$}=0.1$, 0.3 and 0.5 (in order of increasing symbol size). The data (identical to those in fig. 2 of groves04b) are abundance datasets compiled from emission-line studies of individual H ii regions in the giant spiral galaxy M101 kennicutt03 and different types of starburst galaxies (H ii [crosses] and starburst-nucleus [dots]; see also mouhcine02).
• Figure 2: Luminosity ratios of prominent optical emission lines predicted by the photoionization models described in Section \ref{['sec:modelling']}: (a) [O iii]$\lambda5007$/H$\beta$ against [N ii]/H$\alpha$; (b) [O iii]$\lambda5007$/H$\beta$ against [S ii]$\lambda\lambda6717,6731$/H$\alpha$; (c) [O iii]$\lambda5007$/H$\beta$ against [N ii]$\lambda6584$/[O ii]$\lambda3727$; and (d) [O iii]$\lambda5007$/H$\beta$ against [O ii]$\lambda3727$/[O iii]$\lambda5007$. The models assume constant star formation over the past 100 Myr and fixed hydrogen density, $\hbox{$n_{\mathrm{H}}$}=100\,{\rm cm}^{-3}$, carbon-to-oxygen ratio, (C/O)$_\odot$=0.44, and IMF upper mass cutoff, $\hbox{$m_{\rm{up}}$}=100\,\hbox{$\mathrm{M}_{\sun}$}$. They are shown for 14 interstellar metallicities in the range $0.0001\leq\hbox{$Z_\textnormal{\tiny ism}$}\leq0.040$ (colour-coded as in Fig. \ref{['fig:NO_OH']}; see Table \ref{['tab:12logOH']}), 7 zero-age ionization parameters in the range $-4\leq\log\hbox{$U_{\rm{S}}$}\leq-1$, in bins of 0.5 dex (in order of increasing [O iii]/H$\beta$ ratio at fixed metallicity), and 3 dust-to-metal mass ratios $\hbox{$\xi_{\rm{d}}$}=0.1$, 0.3 and 0.5 (in order of increasing symbol size). In each panel, a line links models with $\log\hbox{$U_{\rm{S}}$}=-3.0$ at all metallicities, while the black cross shows the 'standard' model defined in Section \ref{['sec:obs_SDSS']}. The grey dots show high-quality observations of star-forming galaxies from the SDSS DR7, corrected for attenuation by dust as described in brinchmann04.
• Figure 7: Luminosity ratios of prominent ultraviolet emission lines predicted by the photoionization models described in Section \ref{['sec:modelling']}: (a) C iii]$\lambda1908$/O iii]$\lambda1666$ against Si iii]$\lambda1888$/C iii]$\lambda1908$; (b) He ii$\lambda1640$/O iii]$\lambda1666$ against C iv$\lambda1550$/C iii]$\lambda1908$; (c) N v$\lambda1240$/He ii$\lambda1640$ against C iii]$\lambda1908$/He ii$\lambda1640$; and (d) C iv$\lambda1550$/C iii]$\lambda1908$ against C iv$\lambda1550$/He ii$\lambda1640$. In each panel, the models and solid line are the same as in Fig. \ref{['fig:entire_grid']}. In panel (a), the grey and red crosses refer to observations (including error bars and upper limits) of, respectively, six giant extragalactic H ii regions in nearby low-luminosity, metal-poor, dwarf irregular galaxies observed with HST/FOS by garnett95 and four low-mass, gravitationally-lensed dwarf galaxies at redshift in the range $2\lesssim z\lesssim3$ observed with Keck/LRIS and VLT/FORS2 by stark14.
• Figure 12: C$^{+2}$/O$^{+2}$ ionic abundance ratio estimated from emission-line luminosities via standard formulae involving the direct-$T_{\rm{e}}$ method plotted against true C$^{+2}$/O$^{+2}$ ratio, for the same models as in Fig. \ref{['fig:entire_grid']}: (a) according the prescription of garnett95icf; (b) according the prescription of izotov99; (c) according the prescription of erb10. (d) Model electronic temperature in the O$^+2$ plotted against gas-phase oxygen abundance for the same models as in panels (a)--(c). Dashed horizontal lines brackett the approximate $T_{\rm{e}}$ range over which equations \ref{['eq:eq_c3o3_garnett']}--\ref{['eq:eq_c3o3_erb']} were calibrated (see text for details).
• Figure 13: (a) Ionization correction factor entering equation \ref{['eq:co_from_icfr']}, $[X(\rm{C}^{+2})/X(\rm{O}^{+2})]^{-1}$, plotted against volume-averaged fraction of doubly-ionized oxygen, $X(\rm{O}^{+2})$, for the same models as in Fig. \ref{['fig:entire_grid']}. The circles, squares and triangles show the results of early calculations by garnett95 for OB stellar associations of metallicity 0.001, 0.004 and 0.008, respectively, and ages 0 Myr (filled symbols) and 2 Myr (open symbols; see text for details). (b) $X(\rm{O}^{+2})$ estimated from emission-line luminosities via standard formulae involving the direct-$T_{\rm{e}}$ method (equations \ref{['xo']}--\ref{['oiii']}) plotted against true $X(\rm{O}^{+2})$, for the same models as in (a).