Interrogating Seyferts with NebulaBayes: Spatially Probing the Narrow-line Region Radiation Fields and Chemical Abundances

Abstract

NebulaBayes is a new Bayesian code that implements a general method of comparing observed emission-line fluxes to photoionization model grids. The code enables us to extract robust, spatially resolved measurements of abundances in the extended narrow line regions (ENLRs) produced by Active Galactic Nuclei (AGN). We observe near-constant ionization parameters but steeply radially-declining pressures, which together imply that radiation pressure regulates the ENLR density structure on large scales. Our sample includes four 'pure Seyfert' galaxies from the S7 survey that have extensive ENLRs. NGC 2992 shows steep metallicity gradients from the nucleus into the ionization cones. An inverse metallicity gradient is observed in ESO 138-G01, which we attribute to a recent gas inflow or minor merger. A uniformly high metallicity and hard ionizing continuum are inferred across the ENLR of Mrk 573. Our analysis of IC 5063 is likely affected by contamination from shock excitation, which appears to soften the inferred ionizing spectrum. The peak of the ionizing continuum E peak is determined by the nuclear spectrum and the absorbing column between the nucleus and the ionized nebula. We cannot separate variation in this intrinsic E peak from the effects of shock or H II region contamination, but E peak measurements nevertheless give insights into ENLR excitation. We demonstrate the general applicability of NebulaBayes by analyzing a nuclear spectrum from the non-active galaxy NGC 4691 using a H II region grid. The NLR and H II region model grids are provided with NebulaBayes for use by the astronomical community.

GASP – XIX. AGN and their outflows at the centre of jellyfish galaxies

“…In those cases where two components were required for the fit, we considered the summed fluxes for the comparison with models. We proceed as follows: 1. we selected only spaxels classified in P17b as either AGN, composite or LINERs; 2. for each spaxel we run NebulaBayes (Thomas et al 2018), a python code that adopts a Bayesian approach to select the model optimally fitting the target emission line fluxes.…”

“…For all galaxies, 1 arcsec is ∼ 1 kpc (see Table 1). the assumptions and parameters of these models is given in Thomas et al (2018), we summarize here the main aspects. For HII regions, the ionizing continuum is defined by the SLUG2 (Krumholz et al 2015) stellar population synthesis code, with five metallicities (Z = 0.0004, 0.004, 0.008, 0.02, 0.05).…”

The Mass–Metallicity Relation of Local Active Galaxies

“…The fluxes are normalized to Hβ for comparison with the models in the calculation of the likelihood, but we know that other line ratios may effectively constrain parameters of interest. Hence, we use the NebulaBayes 'line ratio prior' feature (described in the appendix of Thomas et al 2018) to apply priors using two diagnostic ratios. These are [N II]/[O II] (a sensitive metallicity diagnostic for metallicites above half Solar; Kewley & Dopita 2002), which slightly reduces the scatter in the inferred O/H, and [S II] λ6731 / [S II] λ6716, an electron density diagnostic (e.g.…”

“…NebulaBayes allows the inclusion of a relative error on the model grid fluxes, for which we used a value of 0.35 (as a proportion of each model flux; see Section 2.2 of Thomas et al 2018). This 'grid error' is used in both likelihood and prior calculations.…”

Rest-frame UV and optical emission line diagnostics of ionized gas properties: a test case in a star-forming knot of a lensed galaxy at z ∼ 1.7

“…In addition to strong emission line diagnostics (SEL), Bayesian techniques are also becoming increasingly important in inferring ionised gas properties due to their ability to probe asymmetry and non-trivial topography in the probability distributions of the properties. Recent Bayesian estimation tools like IZI (Blanc et al 2015), BOND (Vale Asari et al 2016), HII-CHI-mistry (Pérez-Montero 2015) and Neb-ulaBayes (Thomas et al 2018) have proven useful in inferring nebular gas properties. However, in light of the recent development of SEL diagnostics, particularly the rest-frame UV diagnostics, it is necessary to test the agreement between the Bayesian and SEL techniques.…”