Line Emission From Radiation-Pressurized H Ii Regions. I. Internal Structure and Line Ratios

Abstract

The emission line ratios [ O III ] λ5007/Hβ and [ N II ] λ6584/Hα have been adopted as an empirical way to distinguish between the fundamentally different mechanisms of ionization in emission-line galaxies. However, detailed interpretation of these diagnostics requires calculations of the internal structure of the emitting H II regions, and these calculations depend on the assumptions one makes about the relative importance of radiation pressure and stellar winds. In this paper we construct a grid of quasi-static H II region models to explore how choices about these parameters alter H II regions' emission line ratios. We find that, when radiation pressure is included in our models, H II regions reach a saturation point beyond which further increases in the luminosity of the driving stars does not produce any further increase in effective ionization parameter, and thus does not yield any further alteration in an H II region's line ratio. We also show that, if stellar winds are assumed to be strong, the maximum possible ionization parameter is quite low. As a result of this effect, it is inconsistent to simultaneously assume that H II regions are wind-blown bubbles and that they have high ionization parameters; some popular H II region models suffer from this inconsistency. Our work in this paper provides a foundation for a companion paper in which we embed the model grids we compute here within a population synthesis code that enables us to compute the integrated line emission from galactic populations of H II regions.

Synthetic observations of star formation and the interstellar medium

“…Ercolano et al (2012) show that while the BPT diagram correctly identifies some components of the emission as being photoionisation-dominated, it incorrectly finds that others are shock-dominated, even though the original calculation did not include the effects of stellar winds (see Figure 9). Yeh et al (2013) similarly illustrated the sensitivity of BPT diagrams to substructure in resolved H ii regions. McLeod et al (2015) used mocassin synthetic observations similar to those discussed in the previous paragraph to help interpret new MUSE data towards the famous pillars of creation.…”

A Universal Density Structure for Circumgalactic Gas

“…RPC conditions have been shown to apply in at least some star forming regions (Draine 2011;Yeh & Matzner 2012;Yeh et al 2013;Verdolini et al 2013), in AGN emission line regions (Dopita et al 2002;Baskin et al 2014a;Stern et al 2014aStern et al , 2016, in AGN absorption line regions Baskin et al 2014b), and possibly in the CGM of quasar hosts, in the part exposed to the quasar radiation (Arrigoni Battaia et al 2016). Assuming that dust grains are embedded in the CGM gas, as suggested by the results of Ménard et al (2010), then the dominant contribution to the integral in eqn.…”

SLUG – stochastically lighting up galaxies – III. A suite of tools for simulated photometry, spectroscopy, and Bayesian inference with stochastic stellar populations

“…The major limitations of the built-in approach are: (1) because slug's built-in calculation relies on pre-tabulated values for the metal line emission and assumes either a constant or a similarly pre-tabulated temperature (which significantly affects the continuum emission), it misses the variation in emission caused by the fact that H ii regions exist at a range of densities and ionization parameters, which in turn induces substantial variations in their nebular emission (e.g. Yeh et al 2013;Verdolini et al 2013);…”

“…(2) slug's built-in calculation assumes a uniform density H ii region, an assumption that will fail at high ionizing luminosities and densities due to the effects of radiation pressure (Dopita et al 2002;Draine 2011;Yeh & Matzner 2012;Yeh et al 2013); (3) slug's calculation correctly captures the effects of stochastic variation in the total ionizing luminosity, but it does not capture the effects of variation in the spectral shape of the ionizing continuum, which preliminary testing suggests can cause variations in line luminosities at the few tenths of a dex level even for fixed total ionizing flux. The reason for accepting these limitations is that the assumptions that cause them also make it possible to express the nebular emission in terms of a few simple parameter that can be pre-tabulated, reducing the computational cost of evaluating the nebular emission by orders of magnitude compared to a fully accurate calculation with cloudy slug.…”