Constraints on the production and escape of ionizing radiation from the emission-line spectra of metal-poor star-forming galaxies

Abstract

We explore the production and escape of ionizing photons in young galaxies by investigating the ultraviolet and optical emission-line properties of models of ionizationbounded and density-bounded H regions, active-galactic-nucleus (AGN) narrow-line regions and radiative shocks computed all using the same physically-consistent description of element abundances and depletion on to dust grains down to very low metallicities. We compare these models with a reference sample of metal-poor star-forming galaxies and Lyman-continuum (LyC) leakers at various redshifts, which allows the simultaneous exploration of more spectral diagnostics than typically available at once for individual subsamples. We confirm that current single-and binary-star population synthesis models do not produce hard-enough radiation to account for the high-ionization emission of the most metal-poor galaxies. Introducing either an AGN or radiative-shock component brings models into agreement with observations. A published model including X-ray binaries is an attractive alternative to reproduce the observed rise in He λ4686/Hβ ratio with decreasing oxygen abundance in metal-poor star-forming galaxies, but not the high observed He λ4686/Hβ ratios of galaxies with large EW(Hβ). A source of harder ionizing radiation appears to be required in these extreme objects, such as an AGN or radiative-shock component, perhaps linked to an initial-mass-function bias toward massive stars at low metallicity. This would also account for the surprisingly high [O ]/[O ] ratios of confirmed LyC leakers relative to ionization-bounded models. We find no simple by-eye diagnostic of the nature of ionizing sources and the escape of LyC photon, which require proper simultaneous fits of several lines to be discriminated against. galactic nuclei (AGN), arising from gas accretion onto primordial black holes; radiative shocks, induced by large-scale gas flows; and the leakage of Lyman-continuum (LyC) photons through a porous interstellar medium (ISM), contributing to reionization (see, e.g., the review by Stark 2016). In waiting for advent of the James Webb Space Telescope (JWST), which will enable deep rest-frame ultraviolet and optical emission-line spectroscopy of galaxies into the reionization era at redshifts z ∼ 10-15, more nearby metal-poor galaxies approaching the properties of primeval galaxies offer a useful laboratory in which to test our ability to interpret emission-line spectra.

Exploring chemical homogeneity in dwarf galaxies: a VLT-MUSE study of JKB 18

“…These emissionline diagnostic diagrams or 'BPT' diagrams separate emission-line ratios according to the source of ionizing radiation, with the overlaid 'maximum starburst line' from Kewley et al (2001) separating photoionized gas (below the line) from harder ionizing sources such as shocks and active galactic nuclei (above the line). Since all of the emission-line ratios from JKB 18 lie below the solid line, we can confirm that the gas is dominated by photoionization, however, as demonstrated by Plat et al (2019) and Kewley et al (2013), given the low metallicity of the gas, contributions from shock excitation cannot be ruled out. Both diagnostics maps show an interesting ionization structure -while the central star-forming region (marked as 'X') shows (an expected) high [O III]/Hβ, other regions of similar excitation lie at the edges of some star-forming regions and are offset from the areas of strongest star formation (as indicated by the overlaid Hα contours), suggesting that the ionizing photons are not propagating in a uniform direction from the centre of the ionized gas and instead preferentially move towards the outer boundary of the ionized gas.…”

Reconciling escape fractions and observed line emission in Lyman-continuum-leaking galaxies

“…The two discrepancies might arise from the fact that the models do not take into account a DIG component, which can significantly contribute to the emission of [O i] and [S ii] (Sanders et al 2017(Sanders et al , 2020. Alternatively, these discrepancies could also be attributed to the presence of shocks, which are not included in our models (see, e.g., Plat et al 2019) because of problems with atomic data for sulfur (Izotov et al 2006;Kewley & Dopita 2002;Kewley et al 2019), or others. The sulfur line ratio [S ii]/[S iii] is also sensitive to more subtle effects including stellar winds, outflows, and photoevaporation of photo-dissociation regions (PDRs) in Galactic dense gaseous environments (e.g., Westmoquette et al 2013;McLeod et al 2015).…”

“…He ii, and others) and also of low-ionization lines (e.g., Schmitt 1998;Stasińska et al 2015;Plat et al 2019). The effect of a contribution from shock models with different magnetic field strengths (tuned to reproduce a typical He ii/Hβ ratio) on the O32−O13 diagram is shown in Stasińska et al (2015).…”

“…Furthermore, they conjectured that some leakage might occur because of a nebular covering factor smaller than unity, and that [O i] emission could come from optically thick clumps embedded in a density-bounded medium (Stasińska et al 2015). Related to this, Plat et al (2019) reduced. To solve this discrepancy these latter authors invoke a contribution of hard radiation from active galactic nuclei (AGNs) or radiative shocks.…”

X-ray properties of He ii λ 1640 emitting galaxies in VANDELS

“…As argued by S20, even though it remains unclear whether XRBs are the dominant producers of He II ionising photons or not, binary-star models (Eldridge et al 2017) overall do a better job at producing more He II ionizing photons compared to single star models (see also Steidel et al 2016). Recent modelling of production of ionizing radiation in star-forming galaxies by Plat et al (2019) showed that the highest He II EWs are produced in low-metallicity stellar populations (both single and binary-star models) with highionization parameter values, log U ≥ −2. However, to explain the highest He II EWs observed in the literature, the stellar populations must have very young ages (log age/yr < 7).…”