IZI: INFERRING THE GAS PHASE METALLICITY (Z) AND IONIZATION PARAMETER (q) OF IONIZED NEBULAE USING BAYESIAN STATISTICS

Abstract

We present a new method for inferring the metallicity (Z) and ionization parameter (q) of HII regions and star-forming galaxies using strong nebular emission lines (SEL). We use Bayesian inference to derive the joint and marginalized posterior probability density functions for Z and q given a set of observed line fluxes and an input photo-ionization model. Our approach allows the use of arbitrary sets of SELs and the inclusion of flux upper limits. The method provides a self-consistent way of determining the physical conditions of ionized nebulae that is not tied to the arbitrary choice of a particular SEL diagnostic and uses all the available information. Unlike theoretically calibrated SEL diagnostics the method is flexible and not tied to a particular photo-ionization model. We describe our algorithm, validate it against other methods, and present a tool that implements it called IZI. Using a sample of nearby extra-galactic HII regions we assess the performance of commonly used SEL abundance diagnostics. We also use a sample of 22 local HII regions having both direct and recombination line (RL) oxygen abundance measurements in the literature to study discrepancies in the abundance scale between different methods. We find that oxygen abundances derived through Bayesian inference using currently available photo-ionization models in the literature can be in good (∼30%) agreement with RL abundances, although some models perform significantly better than others. We also confirm that abundances measured using the direct method are typically ∼ 0.2 dex lower than both RL and photo-ionization model based abundances.

Biases in Metallicity Measurements from Global Galaxy Spectra: The Effects of Flux Weighting and Diffuse Ionized Gas Contamination

“…Empirical direct-method calibrations may be biased towards lower metallicities due to the presence of temperature gradients and inhomogeneities within the ionized gas (Stasińska 2005;Bresolin 2007), although this problem primarily affects high-metallicity, low-temperature H ii regions. Direct-method metallicities may indeed have a normalization bias, but have been shown to tightly correlate with metallicities determined from oxygen recombination lines with a slope of unity (Blanc et al 2015) and an offset of ∼ −0.2 dex. For photoionization models, it is difficult to determine the proper combination of input parameters and physical conditions that produce realistic H ii regions because of degeneracies among parameters.…”

“…This method is the most accurate method of metallicity determina-tion that can be applied to reasonably large samples (N > 100) of low-redshift galaxies. The utility of the direct-method has been demonstrated by the observation that direct-method metallicities tightly correlate with metallicities obtained from oxygen recombination lines that more directly measure the oxygen abundance, where the relation has a slope of unity but an offset of ∼ 0.2 dex from a one-to-one relation (Blanc et al 2015). Metal recombination lines are ∼ 10 4 times weaker than strong lines and thus are not a practical metallicity indicator for any large sample.…”

SDSS-IV MaNGA: the impact of diffuse ionized gas on emission-line ratios, interpretation of diagnostic diagrams and gas metallicity measurements

“…The strong-line method of Blanc et al (2015; IZI hereafter) cannot be applied to DIG to get an accurate metallicity because it currently contains only H ii region models which fail to describe the DIG.…”

“…Metallicities derived using N2O2 are optimal because they exhibit the smallest bias and error. Using O3N2, R 23 , N2=[N ii]/Hα, and N2S2Hα (Dopita et al 2016) to derive metallicities introduces bias in the derived metallicity gradients as large as the gradient itself.The strong-line method of Blanc et al (2015; IZI hereafter) cannot be applied to DIG to get an accurate metallicity because it currently contains only H ii region models which fail to describe the DIG. …”

[O ii] nebular emission from Mg ii absorbers: star formation associated with the absorbing gas

“…In a stacking analysis of galaxies between 0.2 < z < 0.6 from the SHELS galaxy redshift survey, Kewley et al (2015) have studied We measure the gas phase metallicity (Z) and ionization parameter (q) of the ionized nebula using the izi (Inferring metallicity and ionization parameters) code described in Blanc et al (2015) and assuming the photoionization model results of Levesque et al (2010). The Z and q for the three z bins are listed in column 5 and 6 of Table 4.…”