Using Strong Lines to Estimate Abundances in Extragalactic H II Regions and Starburst Galaxies

By Lisa J. Kewley, M. W. D. O. Astronomy, A. Anu, H. CfA. 2002

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We have used a combination of stellar population synthesis and photoionization models to develop a set of ionization parameter and abundance diagnostics based only on the use of the strong optical emission lines. These models are applicable to both extragalactic H II regions and star-forming galaxies. We show that, because our techniques solve explicitly for both the ionization parameter and the chemical abundance, the diagnostics presented here are an improvement on earlier techniques based on strong emission-line ratios. Our techniques are applicable at all metallicities. In particular, for metallicities above half solar, the ratio [N II]/[O II] provides a very reliable diagnostic since it is ionization parameter independent and does not have a local maximum. This ratio has not previously been used historically because of worries about reliable calibration over such a long baseline, and reddening correction concerns. However, we show that the use of classical reddening curves and standard calibration are quite sufficient to allow this [N II]/[O II] diagnostic to be used with confidence as a reliable abundance indicator. As we had shown, the commonly used abundance diagnostic R23 depends strongly on the ionization parameter, while the commonly used ionization parameter diagnostic [O III]/[O II] depends strongly on abundance. The iterative method of solution presented here allows both of these parameters to be obtained without recourse to the use of temperature-sensitive line ratios involving faint emission lines. We compare three commonly used abundance diagnostic techniques and show that individually, they contain systematic and random errors. This is a problem affecting many abundance diagnostics, and the errors generally have not been properly studied or understood due to the lack of a reliable comparison abundance, except for very low metallicities, where the [O III] λ4363 auroral line is used. Here we show that the average of these techniques provides a fairly reliable comparison abundance indicator against which to test new diagnostic methods. The cause of the systematic effects are discussed, and we present a new "optimal" abundance diagnostic method based on the use of line ratios involving [N II], [O II], [O III], [S II], and the Balmer lines. This combined diagnostic appears to suffer no apparent systematic errors, can be used over the entire abundance range and significantly reduces the random error inherent in previous techniques. Finally, we give a recommended procedure for the derivation of abundances in the case that only spectra of limited wavelength coverage are available so that the optimal method can no longer be used.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/cde470d2151c7e2aefdcbf37e37bfb481a2bc78e

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Citations in the corpus

New fully empirical calibrations of strong-line metallicity indicators in star forming galaxies (2016, methodology): “…et al. 2013; Pilyugin & Grebel 2016), or theoretically, in which oxygen abundance have been inferred via photoionization models (e.g. McGaugh 1991; Zaritsky et al. 1994; Kewley & Dopita 2002; Kobulnicky & Kewley 2004; Tremonti et al. 2004; Dopita et al. 2013, 2016), or be a combination of the two.”
STRONG NEBULAR LINE RATIOS IN THE SPECTRA of z ∼ 2–3 STAR FORMING GALAXIES: FIRST RESULTS FROM KBSS-MOSFIRE (2014, neutral): “…(e.g., [O III]λλ4960,5008/[O II]λλ3727,3729) as well as the so-called “strong-line” metallicity indicators, e.g. those based on ([OIII] + [OII])/Hβ (“R23”; Pagel et al. 1979; Kewley & Dopita 2002), [NII]λ6585/Hα (“N2”) and ([OIII]λ5008/Hβ)/([NII]λ6585/Hα) (“O3N2”; Pettini & Pagel (2004) [PP04]).”
REGULARITY UNDERLYING COMPLEXITY: A REDSHIFT-INDEPENDENT DESCRIPTION OF THE CONTINUOUS VARIATION OF GALAXY-SCALE MOLECULAR GAS PROPERTIES IN THE MASS-STAR FORMATION RATE PLANE (2013, neutral): “As an example, a log-linear relation between αCO and metallicity derived using the R23 indicator (e.g., Pilyugin 2001, Kewley & Dopita 2002) may become a convex or concave function of metallicity when converted directly to a system based on the N2 diagnostic (e.g., Denicoló et al. 2002, Pettini &…”
Chemical abundances in high-redshift galaxies: a powerful new emission line diagnostic (2016, background): “Generally speaking, the strategy here has been to search for a line ratio which depends principally on abundance, and a second which is sensitive to the excitation, for example McGaugh (1991); Kewley & Dopita (2002); Kobulnicky & Kewley (2004).”
Hα Tail, Intracluster H II Regions, and Star Formation: ESO 137-001 in Abell 3627 (2007, methodology): “The ionization parameter can be estimated from the [OIII]λ5007 / [OII] ratio, 0.041, which implies an ionization parameter (q) of ∼ 107 cm/s for gas metallicity of 2.0 solar (Kewley & Dopita 2002).”
THE SPECTRAL EVOLUTION OF THE FIRST GALAXIES. I. JAMES WEBB SPACE TELESCOPE DETECTION LIMITS AND COLOR CRITERIA FOR POPULATION III GALAXIES (2011, methodology): “This gives an effective mean ionization parameter in the same range as observed in local HII regions (Kewley & Dopita 2002), if ffill = 0.01 and n(H) = 100 cm−3.”
Deriving model-based Te-consistent chemical abundances in ionized gaseous nebulae (2014, background): “These ratios also have the advantage that they do not depend on the excitation of the gas (Kewley & Dopita 2002).”
Nebular Emission Line Ratios in z ≃ 2–3 Star-forming Galaxies with KBSS-MOSFIRE: Exploring the Impact of Ionization, Excitation, and Nitrogen-to-Oxygen Ratio (2016, methodology): “R23 is used 9 http://purl.org/mike/mpfitexy to estimate oxygen abundance (e.g., Pagel et al. 1979; McGaugh 1991; Kewley & Dopita 2002), although because of its double-valued nature, an additional parameter (like N2 or O3N2) must be used to break the degeneracy.”
Modelling the nebular emission from primeval to present-day star-forming galaxies (2016, methodology): “…of characterising the dependence of carbon production on metallicity has resulted in the secondary component to be either ignored (e.g., CL01, Kewley & Dopita 2002) or assigned a specific dependence on the O/H ratio (e.g., Dopita et al. 2013) in previous models of nebular emission from H ii…”
THE MOSDEF SURVEY: EXCITATION PROPERTIES OF z ∼ 2.3 STAR-FORMING GALAXIES (2014, methodology): “However, in both Kewley & Dopita (2002) and Nakajima et al. (2013), the combination of O32 and R23 is translated to a combination of metallicity and ionization parameter using the results of photoionization models.”
METALLICITY GRADIENTS AND GAS FLOWS IN GALAXY PAIRS (2010, methodology): “Galaxy pairs that lie close to the field galaxy LZ relation in Figure 1 may have had high central metallicities [log(O/H) + 12> 9.1 in the Kewley & Dopita (2002) [N II]/[O II] scale] prior to the interaction.”
Clues to the Origin of the Mass-Metallicity Relation: Dependence on Star Formation Rate and Galaxy Size (2007, methodology): “…according to the line diagnostic criteria given in Kewley et al. (2001).5 The galaxies must also have available metallicities from the Kewley & Dopita (2002) “recommended” method, masses determined from spectral synthesis modeling (Kauffmann et al. 2003b), aperture-corrected SFRs…”
The SINS/zC-SINF Survey of z ∼ 2 Galaxy Kinematics: SINFONI Adaptive Optics–assisted Data and Kiloparsec-scale Emission-line Properties (2018, methodology): “…AGN and shock excitation, ISM conditions, and possible N/O abundance variations on the [N II]/Hα ratio, as amply discussed in the literature 5 (e.g., Kewley & Dopita 2002; Kewley et al. 2013; Pérez-Montero & Contini 2009; Yuan et al. 2012; Andrews & Martini 2013; Steidel et al. 2014; Shapley et…”
A fundamental plane for field star-forming galaxies (2010, methodology): “The Erb et al. (2006) metallicity estimates, using the N2 method and the calibration of Pettini & Pagel (2004), and the Maiolino et al. (2008), usingthe calibrations of Kewley & Dopita (2002), were recalibrated to the R23 method using the conversions of Kewley & Ellison (2008).”
The Composition Gradient in M101 Revisited. II. Electron Temperatures and Implications for the Nebular Abundance Scale (2003, methodology): “…the oxygen abundance, without any direct electron temperature measurement (e.g., Pagel et al. 1979; Alloin et al. 1979; Edmunds & Pagel 1984; Dopita & Evans 1986; McGaugh 1991; Vı́lchez & Esteban 1996; van Zee et al. 1998; Dutil & Roy 1999; Dı́az & PérezMontero 2000; Kewley & Dopita 2002).”
GALAXY MERGERS AND THE MASS–METALLICITY RELATION: EVIDENCE FOR NUCLEAR METAL DILUTION AND FLATTENED GRADIENTS FROM NUMERICAL SIMULATIONS (2010, methodology): “A study of weakly interacting spirals found no difference in metallicity gradient compared to isolated spirals (Márquez et al. 2003), but the [N II]/Hα metallicity diagnostics used in this study have significant uncertainties (Kewley & Dopita 2002).”
Chemical Abundances of DEEP2 Star-forming Galaxies at z~1.0-1.5 (2005, methodology): “Both detailed photo-ionization models and empirical calibrations have been used to relate the ratios of strong emission lines to chemical abundances (Pagel et al. 1979; Evans & Dopita 1985; Kewley & Dopita 2002; Pettini & Pagel 2004).”
Galaxies with Wolf-Rayet signatures in the low-redshift Universe. A survey using the Sloan Digital Sky Survey (2008, methodology): “We have also verified that the results below do not change significantly if the PP O3N2 estimator, the [N ii]/[O ii] estimator advocated by Kewley & Dopita (2002) or the P-method devised by Pilyugin (2000) are used instead, although slight changes do occur.”