abundances in “green pea” star-forming galaxies

Abundances in “Green Pea” Star-forming GalaxiesAuthor(s): Steven A. HawleySource: Publications of the Astronomical Society of the Pacific, Vol. 124, No. 911 (January2012), pp. 21-35Published by: The University of Chicago Press on behalf of the Astronomical Society of the PacificStable URL: http://www.jstor.org/stable/10.1086/663866 .

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Abundances in “Green Pea” Star-forming Galaxies

STEVEN A. HAWLEY

Department of Physics and Astronomy, 1251 Wescoe Hall Drive, University of Kansas, Lawrence, KS 66045; [email protected].

Received 2011 June 30; accepted 2011 November 17; published 2012 January 19

ABSTRACT. He II λ4686 is identified in the spectra of nine of the original “Green Peas,” a type of compactstar-forming galaxy characterized by low mass; low metallicity; strong [O III] λλ4959, 5007; and redshifts in therange of ∼0:1–0:4. Measured λ4686/Hβ ratios are roughly 1–2%, consistent with photoionization by Wolf-Rayetstars. Emission-line intensities are measured from Sloan Digital Sky Survey spectra for 71 Green Peas and are usedto determine Te-based abundances of O, N, Ne, S, and He. Neon abundances confirm the mass-metallicity relationinferred from O/H. The N/O ratio is roughly constant with O/H, and the average N/O is evidence of a modestnitrogen enhancement compared with other low-metallicity galaxies. Nitrogen enrichment could be due to Wolf-Rayet stars or to intermediate-mass stars during a previous quiescent period. The Te-based abundances allow areevaluation of some of the strong-line methods favored for estimating O/H or N/O in large spectroscopic surveysof star-forming galaxies. Photoionization byWolf-Rayet stars raises questions about the validity of strong-line meth-ods based on [N II]/Hα, [N II]/[O III], or [N II]/[S II], as those line ratios are known to be ionization-sensitive.Analysis of these measurements shows that ionization, low metallicity, and the small variation in important lineratios in the Green Pea spectra all affect the behavior of one or more of the N2, O3N2, N2O2 and N2S2 strong-linemethods. The previous findings for trends in O/H and N/O in the Green Peas can be reproduced and the discre-pancies can be explained. In particular, the reported increase of N/O with O/H appears to be a bias introduced bycombining N2 with N2S2. N2O2 does not give valid results in the Green Peas, while N2 and N2S2 do, although thecalibrations of the N2 and N2S2 methods based on Green Pea abundances are different from the existing calibrationsbased primarily on abundances in extragalactic H II regions and H II galaxies.

1. INTRODUCTION

Large spectroscopic surveys such as the Sloan Digital SkySurvey (SDSS; York et al. 2000) have enabled the investigationof mass-metallicity relations in tens of thousands of star-forminggalaxies. The benchmark study by Tremonti et al. (2004) usedSDSS spectroscopy to analyze stellar mass and gas-phase metal-licities in 53,000 star-forming galaxies at z ∼ 0:1. The value ofthe metallicity and the slope of the mass-metallicity relation andvariations in other key abundance measures, such as N/O, can beuseful constraints in modeling the chemical evolution of star-forming galaxies and assessing the importance of processes con-tributing to oxygen and nitrogen enrichment.

Consequently, an accurate measurement of O/H, the normalproxy for metallicity, is key to the determination of the mass-metallicity relation. The preferred technique is to measure atemperature-sensitive emission line, such as [O III] λ4363 or[N II] λ5755, to determine Te, which in turn is used to calculateionic abundances from the other lines of interest. This Te-basedapproach is also commonly referred to as the direct method. Un-fortunately, the lines necessary for the Te measurement are oftentoo weak to be observed, necessitating the application of othertechniques for estimating O/H or N/O. Tremonti et al. (2004)used stellar evolution synthesis and photoionization models

to derive gas-phase oxygen abundances. In several other studies,strong-line methods have been applied to estimate O/H or N/O.Ratios of certain more easily observable lines, such as [O II],[O III], Hα, Hβ, [N II], and [S II] are sensitive to metallicityin ways that have been calibrated by several investigators eithertheoretically by the use of photoionization models, empiricallyusing observations of objects with known metallicity based onmeasured Te, or semiempirically, where models and Te-basedabundances are used together. Definitions and important fea-tures of some common strong-line methods are listed in Table 1.

Liang et al. (2006) used a sample of approximately 39,000SDSS star-forming galaxies to calibrate several strong-linemethods against the R23 index (R23 ¼ f½O III� þ ½O II�g=Hβ)for logO=Hþ 12 in the range of 8.4–9.3. They concludedthat N2O2 was the best method to use for metal-rich galaxies.Kewley & Ellison (2008) investigated 10 metallicity calibrationson a sample of more than 27,000 star-forming galaxies fromSDSS. They concluded that there can be considerable variationin the mass-metallicity relations derived from the differentstrong-line methods, although they find conversion relationshipsamong most of their calibrations. They recommend the N2O2method calibrated by Kewley &Dopita (2002) and the N2 meth-od calibrated by Pettini & Pagel (2004, hereafter PP04) for pro-viding more accurate relative metallicities.

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PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC, 124:21–35, 2012 January© 2012. The Astronomical Society of the Pacific. All rights reserved. Printed in U.S.A.



These and other investigations have shown that, despite theexistence of systematic errors, common strong-line methodsgive reliable relative O/H abundances in star-forming galaxiesover a range in ionization and oxygen abundance, at least forlogO=Hþ 12 > 8:4. However, three recent studies of a poten-tially distinct subset of star-forming galaxies, known as “GreenPeas,” may suggest caution. Green Peas (hereafter, GPs) is thename given to a class of objects initially discovered as part ofthe Galaxy Zoo project (Lintott et al. 2008) and subsequentlycharacterized by Cardamone et al. (2009, hereafter C09) as low-mass galaxies, exhibiting strong [O III] emission, with redshiftsin the range of 0:11 ≤ z ≤ 0:36, and images that are unresolvedon the scale of the SDSS. The GPs were identified by C09 pri-marily from large r-band intensities compared with g- andi-band intensities, a result of the combination of redshift andstrong [O III]. Metallicities were estimated from the N2O2 meth-od to be logO=Hþ 12 ∼ 8:7 independent of galaxy mass.

Amorìn et al. (2010, hereafter A10) also determined O/H inthe GPs by measuring [O III] λ4363 in the spectra of ∼70% ofthe C09 sample. The resulting Te-based abundances were com-bined with estimates of O/H and N/O from application of the N2and N2S2 methods, respectively. A10 find the average O/H tobe lower by ∼0:65 dex compared with C09. Whereas C09 de-termined the O/H ratio to be independent of stellar mass, A10find that O/H varies with mass in a manner similar to what theyfind in other star-forming galaxies from the SDSS, although off-set by ≥0:3 dex to lower metallicities. C09 did not determine

N/O ratios, but A10 find that the N/O ratio in the GPs increaseswith O/H and that N/O is systematically larger at a givenmetallicity than in other SDSS star-forming galaxies (SFGs).A10 conclude that these GP properties can be explained byinteraction-induced inflow of gas coupled with a supernova-driven metal-rich gas loss.

More recently, Izotov et al. (2011, hereafter I11) found thatthe original GPs from C09 constitute a subset of a much larger(by an order of magnitude) family of luminous compact galaxies(LCGs) that they identify from the SDSS by specifically select-ing objects based on Hβ equivalent width, rather than strong[O III]. I11 measure [O III] temperatures and are able to deter-mine abundances for O, N, Ne, S, Fe, and Ar in the sample ofLCGs, including O/H values for 66 of the original GPs. The I11results confirm a trend of increasing O/H with galaxy mass inthe GPs. However, they find a shallower slope to the mass-metallicity relation than A10, which they conclude results fromdifferences in derived masses, as well as differences betweenTe-based oxygen abundances and estimates from strong-linemethods. They also find that N/O is constant with respect toO/H for the entire LCG sample that includes the 66 GPs. Theynote that this is in contrast with the finding of A10 for the GPs,although they do not resolve the discrepancy.

The behavior of N2 and N2O2 in the GPs as found by A10and C09, respectively, differs from the finding of Kewley &Ellison (2008) that N2 and N2O2 are reliable at least for relativeabundances of O/H in their sample of star-forming galaxies.

TABLE 1

COMMON STRONG-LINE METHODS

Strong-line method (Output) Definition and applicability Important characteristics

N2 (O/H) . . . . . . . . . . . . . . . . . . log ([N II] λ6583/Hα)7:2 < logO=Hþ 12 < 9:1 (D02)7:5 < logO=Hþ 12 < 8:7 (PP04)7:2 < logO=Hþ 12 < 8:9 (Y07)

Insensitive to flux calibration or reddening correction;useful with restricted wavelength coverage;ionization-sensitive (KD02, Y07); calibrated withextragalactic H II regions and H II galaxies(D02, PP04, Y07, PMC09); estimated systematicerror ∼0:3 dex (PP04, PMC09), rms error 0.159 dex (Y07)

N2O2 (O/H) . . . . . . . . . . . . . . . log ([N II] λ6583/[O II] λ3727)logO=Hþ 12 > 8:4 (KE08)

Requires extensive wavelength coverage;sensitive to flux calibration or reddening correction;not ionization-sensitive (KD02); calibrated withphotoionization models (KD02); estimatedaccuracy ∼0:1 dex (KE08)

O3N2 (O/H) . . . . . . . . . . . . . . . logfð½O III�λ5007=HβÞ=ð½N II�λ6583=HαÞglogO=Hþ 12 > ∼8:0 (PP04, PMC09)7:2 < logO=Hþ 12 < 8:44 (Y07)

Insensitive to flux calibration or reddening correction;requires moderate wavelength coverage;ionization-sensitive (KD02); calibrated withextragalactic H II regions and H II galaxies(PP04, Y07, PMC09); estimated systematicerror ∼0:25 dex (PP04); rms error 0.199 dex (Y07)

N2S2 (N/O) . . . . . . . . . . . . . . . . logf½N II�λ6583=½S II�ðλ6717þ λ6731Þgvalid at all ranges ofmetallicity (PMC09)

Relatively insensitive to flux calibration orreddening correction; ionization-sensitive (KD02);calibrated by extragalactic H II regions andH II galaxies (PMC09); estimatedaccuracy ∼0:3 dex (PMC09)

REFERENCES.—(D02) Denicoló et al. 2002; (KD02) Kewley & Dopita 2002; (KE08) Kewley & Ellison 2008; (PP04) Pettini & Pagel 2004;(PMC09) Perez-Montero & Contini 2009; (Y07) Yin et al. 2007.

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2012 PASP, 124:21–35



N2O2 commonly overestimates the O/H abundances comparedwith other strong-line methods and direct abundance deter-minations, although it yields O/H values that are consistentin a relative sense with other strong-line methods and direct de-terminations over a wide range of physical conditions (e.g.,Kewley & Ellison 2008; Bresolin et al. 2009; Hawley 2011).

Additionally, it is important to resolve the discrepancy be-tween the results of A10 and I11 regarding nitrogen enrichment.Variations in N/O with O/H are used to assess the productionsites for nitrogen (Izotov & Thuan 1999, Henry et al. 2000).Henry et al. (2000) compiled abundances from the literaturefor numerous Galactic and extragalactic H II regions and mod-eled the buildup of nitrogen and oxygen over time. The ob-served behavior is characterized as bimodal with constant N/Oat logO=Hþ 12 < 8 and a steep rise in N/O at logO=Hþ12 > 8. They concluded that this behavior can be explained be-cause at low-metallicities nitrogen and oxygen increase inlockstep until roughly logO=Hþ 12 ¼ 8:3, where secondarynitrogen production becomes significant. Consequently, at thevalues of logO=Hþ 12 in the GPs as determined by A10and I11, nitrogen production would not be expected to be afunction of metallicity, although some mechanisms to explainextra production of primary nitrogen were proposed by A10.Liang et al. (2006) found that secondary nitrogen contributesto the N/O ratios in their sample of 38,000 galaxies and beginsto dominate at higher metallicities. However, their sample is re-stricted to galaxies with logO=Hþ 12 > 8:4.

It is important to have confidence in the techniques used formetallicity estimates, and the discrepancies noted previouslymotivated this study to independently determine O/H and N/Oin the GPs in an effort to reproduce and explain the existingconflicting findings. It is also of interest to obtain abundancesof other elements in the original GPs to further investigate theirindividual properties and the extent to which they may be dis-tinct from other star-forming galaxies. Examination of the GPspectra available from SDSS Data Release 7 (Abazajian et al.2009) shows measurable emission lines of several importantelements; specifically, neon, sulfur, and helium. Neon wouldbe expected to scale with oxygen and therefore serves as con-firmation of any trend in O/H with mass. Sulfur abundances canbe estimated from the subset of objects with measurable [S III]λ6312 and could serve as an additional indicator of any trendin the α-elements with stellar mass in the GPs. Furthermore,He II λ4686 is present in nine of the GPs, which, with He I

λ5876, allows a determination of the helium abundance.The detection of He II λ4686 has not previously been re-

ported in the GPs. Some H II regions in low-metallicity H II gal-axies and blue compact dwarf (BCD) galaxies are known toshow λ4686 emission (Schaerer 1998) with He II λ4686 emis-sion typically 1–2 % of Hβ. He II λ4686/Hβ ratios in theGPs range from 0.66–2.13%, consistent with models wherethe He II emission is due to ionization by Wolf-Rayet stars(Schaerer 1998).

The presence of He II λ4686 and strong [O III] makes the GPspectra more similar to planetary nebulae than H II regions, rais-ing questions about the applicability of the strong-line methodscommonly used on large surveys of star-forming galaxies. The[N II]/Hα and [N II]/[O III] ratios are ionization-sensitive, basedon both theory and observation (Kewley & Dopita 2002; Relañoet al. 2010). Yin et al. (2007) studied the N2 method in 695 low-metallicity galaxies and H II regions and found that the calibra-tion has some dependence on ionization. If the GPs are ionizedby spectral energy distributions (SEDs) that are sufficiently dif-ferent from those in the objects on which the calibrations of thestrong-line methods are based, the existing calibrations may notbe appropriate and, potentially, some methods may not be valid.

The article is organized as follows. Section 2 discusses thedata used in the subsequent analysis, § 3 presents the results,with the discussion in § 4, followed by a brief summary in § 5.

2. THE DATA

The objects for this study were taken from Table 4 in C09,which includes 80 objects identified as GPs, along with O/Habundances from the N2O2 method and an estimate of the stel-lar mass. For each GP the one-dimensional calibrated spectrumwas obtained from SDSS Data Release 7 (Abazajian et al.2009). The spectra cover a wavelength range of roughly λ3800to λ9200, which allows the measurement of emission lines ofinterest between [O II] λ3727 and [S II] λ6731 in the rest frame.

Emission-line fluxes were obtained using Specview.1 In gen-eral, the GP spectra show strong emission lines and little con-tinuum. However, in two cases the galaxy contribution wasjudged to be more significant than in the other GPs and, ratherthan attempt to correct the emission-line measurements for anyunderlying galaxy component, those two objects were excludedfrom the analysis. An additional seven spectra had Hα/Hβ ratiosthat were significantly less than the expectation under the as-sumption of no reddening and pure case B recombination(Osterbrock & Ferland 2006), and those objects were excludedas well, although the few with good signal-to-noise ratios (S/N)could be utilized for O/H estimates from N2 and O3N2 sincethose strong-line methods are insensitive to flux calibrationor reddening correction.

Reddening corrections were applied to the measured line in-tensities assuming pure case B recombination and utilizing thereddening curve as tabulated by Osterbrock & Ferland (2006)for RV ¼ 3:1. The EðB� V Þ values determined for the GPsrange from ∼0 to ∼0:3, in agreement with the values foundby C09. [O III] λ4363 was measured in 57 of the GP spectra.[S III] λ6312 was measured in 15 objects, and He II λ4686 wasmeasured in nine objects. In general, the measurement errors forthe stronger lines are on the order of a few percent. In the case of

1 Specview is a product of the Space Telescope Science Institute, which isoperated by AURA for NASA.

ABUNDANCES IN “GREEN PEA” STAR-FORMING GALAXIES 23

2012 PASP, 124:21–35



[O III] λ4363 the measurement errors ranged from ∼3–10%. Thereddening-corrected relative line intensities used in the subse-quent analysis, normalized to Hβ ¼ 100, along with each ob-ject’s SDSS ID and c, the logarithmic extinction at Hβ, areprovided in Tables 2 and 3.

3. ABUNDANCES

3.1. Te-based Abundances (Direct Method)

Ionic abundances were calculated using a two-zone model,where the electron temperature determined from [O III] is ap-plicable to the higher-ionization species (Oþþ, Neþþ, andSþþ), and the electron temperature from [N II] or [O II] is ap-plicable to the lower-ionization species (Oþ, Nþ, Sþ). Given thehigh ionization, a direct measurement of the electron tempera-ture from [N II] or [O II] is not possible. However, a number ofcalibrated relationships exist for estimating Te [N II] or Te [O II]from Te [O III] and other parameters. A10 use a relationship forTe [O II] from Perez-Montero & Contini (2009, hereafterPMC09) that depends on Te [O III] and the measured valuesof the electron density, in turn depending on measurementsof the [S II] λ6717/λ6731 ratio. In many of the GP spectra,the red end has lower S/N and the [S II] lines are more difficultto measure accurately. Alternatively, Pilyugin (2007) proposes arelationship for Te [O II] that depends on Te [O III] and anexcitation parameter, P ≡ ½O III�=ð½O II� þ ½O III�Þ. Values forTe [O II] were calculated for every GP with a determinationof Te [O III] using that relationship. For comparison, a valuefor Te [N II] was also calculated using a relationship fromPMC09. In general, the differences between the derivedTe [O II] and Te [N II] values are small. For the subsequentabundance analysis the Te [O II] from the Pilyugin (2007) re-lationship was adopted for the low-ionization species.

Electron temperatures, densities, and ionic abundances weredetermined from the line intensities using the nebular package(Shaw & Dufour 1995). Total abundances were calculated fromthe ionic abundances by use of the traditional ionization correc-tion factors (ICFs) (Peimbert & Costero 1969; Peimbert &Torres-Peimbert 1977).

The resulting O/H versus mass relation for the GPs is shownin Figure 1. A linear least-squares fit gives

log O=Hþ 12 ¼ ð0:091� 0:068Þ logMþ ð7:26� 0:65Þ;r ¼ 0:33: (1)

The error bars in logO=Hþ 12 derive from the uncertainties inthe line-intensity measurements propagated through the electrontemperature determination, the ionic abundances, and the ICFs.However, the spread in O/H at a given mass is larger. IntrinsicO/H variations at a given mass are likely. Additionally, systema-tic errors in theTe-basedmethod using forbidden lines, includingthe possibility of temperature fluctuations and inadequacies inthe ICFs, generally result in uncertainties comparable with or

larger than the errors resulting from the uncertainties in thestrength of λ4363. Temperature variations in gaseous nebulaecan bias the abundance determinations by 0.1–0.5 dex (Peimbert& Peimbert 2010). Yin et al. (2007) and López-Sánchez &Esteban (2010) propose that temperature fluctuations are a po-tentially important source of systematic error in Te-based abun-dances in star-forming galaxies. In the few cases whereabundances in Galactic or extragalactic H II regions can be de-termined from the much fainter recombination lines, the tradi-tional Te-based method gives systematically lower values by∼0:2–0:3 dex (Esteban et al. 2009). This is often cited as the ex-planation for photoionization model-based strong-line methods,such as N2O2, giving higher absolute values of O/H comparedwith Te-based strong-line methods (e.g., López-Sánchez &Esteban 2010).

The mass-metallicity relation (eq. [1]) agrees within the er-rors with the results of I11 (see Table 4) and is also consistentwith a linear fit to the results of A10, as estimated from theirFigure 3, although their choice of a polynomial fit gives a some-what different relation. It should be noted that I11 reestimatedthe galaxy masses for their subset of the C09 sample using adifferent treatment of the continuum. However, these data showno significant difference in the trend of O/H based on the choiceof GP mass, so for the rest of this article, the C09 masses will beadopted to better enable comparisons with results from A10.

The average logO=Hþ 12 of 8:11� 0:13 is consistent withthe findings of A10, 8:05� 0:14, noting that their average O/Hincludes GPs where the oxygen abundances were estimatedfrom the N2 method. The average of the O/H values for theGP sample quoted by I11 is ∼8:11.

Any metallicity trend can be evaluated further by looking atNe/H, which also shows an increase with mass (Fig. 2) and anaverage value of logNe=Hþ 12 ¼ 7:43� 0:18. A simple linearleast-squares fit yields

logNe=Hþ 12 ¼ ð0:128� 0:091Þ logMþ ð6:22� 0:86Þ;r ¼ 0:35; (2)

in agreement with the O/H trend within the errors. The trend ofNe/O with mass is shown in Figure 3 and is roughly constantwith a formal fit:

logNe=O ¼ ð0:039� 0:033Þ logM� ð1:05� 0:31Þ;r ¼ 0:30: (3)

The average logNe=O ¼ �0:68� 0:06 is typical of H II regionsand blue compact galaxies with logO=Hþ 12 < 8:5 (Milingoet al. 2010). The average log Ne/O value found by I11 for theirsample of LCGs can be estimated from their Figure 7 as roughly�0:70. I11 find that Ne/O increases with increasing O/H in theirLCGs, which they explain as resulting from oxygen being in-corporated in grains. These Ne/O values also suggest a smalltrend with O/H.

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TABLE 2

REDDENING-CORRECTED LINE INTENSITIES RELATIVE TO Hβ λλ3727–5007

SSDS ID[O II] [Ne III] Hδ Hγ [O III] He I He II Hβ [O III] [O III]λ3727 λ3868 λ4101 λ4340 λ4363 λ4471 λ4686 λ4861 λ4959 λ5007

587725073921409255 . . . . . 243 33.1 26.9 49.7 … 2.95 … 100 128 377588848899919446344 . . . . . 145 46.9 26.6 48.3 5.44 … … 100 185 588587725576962244831 . . . . . 193 48.0 33.4 54.9 7.84 … … 100 186 543587731187273892048 . . . . . 272 43.2 26.8 50.2 2.74 … … 100 145 428587731513693503653 . . . . . 246 37.5 20.8 55.6 … … … 100 119 386587724233716596882 . . . . . 234 32.4 21.4 44.3 … … … 100 124 369587727179006148758 . . . . . 86.8 40.2 22.6 43.1 11.4 3.76 0.66 100 201 617587724241767825591 . . . . . 195 42.2 25.1 46.6 4.9 3.33 0.86 100 179 532587724240158589061 . . . . . 225 38.5 23.7 45.1 2.85 3.7 … 100 138 412587726032778559604 . . . . . 193 38.7 23.7 47.3 4.5 … … 100 156 465587726032253419628 . . . . . 175 50.7 26.3 47.3 7.52 4.54 0.96 100 197 600588010360138367359 . . . . . 207 53.1 25.6 49.3 6.45 … … 100 188 565587726102030451047 . . . . . 163 45.3 23.5 45.5 5.5 … … 100 177 552587729155743875234 . . . . . 152 47.8 23..2 47.6 9.83 … … 100 186 571587728919520608387 . . . . . 181 43.2 26.3 53.0 6.2 … … 100 171 502587729229297090692 . . . . . 195 50.6 25.3 51.2 8.85 3.19 … 100 199 576587725818034913419 . . . . . 288 36.2 19.3 42.2 … … … 100 114 339587730774416883967 . . . . . 249 44.5 25.0 48.0 5.8 2.42 1.31 100 161 474587730774965354630 . . . . . 118 37.7 23.4 39.2 7.47 … … 100 177 511587728906099687546 . . . . . 284 35.5 22.2 47.7 2.9 … … 100 124 366587725550133444775 . . . . . … … … … … … … … … …588009371762098262 . . . . . 164 54.2 28.5 50.1 5.6 … … 100 187 577588011122502336742 . . . . . 210 57.8 31.4 59.5 … … … 100 176 573588011103712706632 . . . . . 240 27.6 23.9 47.9 … … … 100 116 348588013384341913605 . . . . . 172 49.8 24.9 46.0 8.74 3.44 … 100 176 541587732134315425958 . . . . . 166 45.2 22.2 50.8 5.94 … … 100 179 542587729777439801619 . . . . . 207 41.7 16.3 43.9 … … … 100 167 495587729777446945029 . . . . . 104 56.7 25.4 49.7 12.0 … … 100 219 652587732152555864324 . . . . . 132 59.1 23.4 46.5 12.8 … … 100 220 692587732578845786234 . . . . . 193 30.2 26.6 49.7 6.03 2.44 … 100 191 564587733080270569500 . . . . . 186 37.6 23.1 44.8 4.48 3.6 1.19 100 150 456588297864714387604 . . . . . 254 35.1 24.3 47.3 2.74 … … 100 131 393587735695911747673 . . . . . … … … … … … … … … …587735696987717870 . . . . . 187 48.6 27.0 52.2 8.57 … … 100 175 507587733441055359356 . . . . . … … … … … … … … … …588017605211390138 . . . . . 130 51.2 24.6 43.8 12.1 … … 100 202 618588017114517536797 . . . . . 113 55.1 26.2 46.7 10.9 … … 100 221 646588017116132540589 . . . . . ... … … … … … … … … …588018090541842668 . . . . . … … … … … … … … … …588018090013098618 . . . . . … … … … … … … … … …588016878295515268 . . . . . 259 42.4 24.4 47.1 4.17 … … 100 148 445587735661007863875 . . . . . 182 30.0 22.0 42.7 1.72 … … 100 116 347588016892783820948 . . . . . 225 30.4 22.9 48.0 … … … 100 123 379587735663159738526 . . . . . 287 34.8 25.9 50.0 … … … 100 141 432588018055114784812 . . . . . 245 38.8 23.5 45.8 5.0 … … 100 142 431588018055652769997 . . . . . 223 30.4 23.2 49.2 3.0 3.57 … 100 138 406588017570848768137 . . . . . 180 51.3 25.9 45.0 7.42 … … 100 202 607587736915687964980 . . . . . 103 42.2 24.0 41.5 7.8 … … 100 237 662587736915687375248 . . . . . … … … … … … … … … …587738410863493299 . . . . . 81.4 54.9 25.3 46.6 12.8 3.89 … 100 248 745587735349111947338 . . . . . 63.7 48.9 24.8 46.6 12.9 3.44 1.62 100 220 656587738570859413642 . . . . . 115 50.0 25.0 49.0 9.26 3.27 … 100 225 662587736940372361382 . . . . . … … … … … … … … … …587739153352229578 . . . . . 209 35.0 25.4 45.8 8.32 … … 100 146 457587738947196944678 . . . . . 72.0 53.0 24.7 47.2 13.3 3.39 2.13 100 235 736587738371672178952 . . . . . 192 50.1 27.3 49.9 6.67 4.27 … 100 192 577588017978880950451 . . . . . 242 28.7 … … … … … 100 110 343587739408388980778 . . . . . 183 40.5 24.3 44.1 5.36 2.83 0.89 100 167 511588017977277874181 . . . . . 187 48.2 23.7 46.2 7.45 2.87 … 100 195 576


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The S/H values are more uncertain, due to the weakness ofλ6312 and the effect of the lower S/N in the region of the sulfurlines at λ6717 and λ6731. There are also probable inadequaciesin the ICF to account for nonobserved stages of ionization. Thedegree of ionization of the GPs suggests that there should besignificant sulfur in ionization stages higher than [S III]. Theaverage value of log S=Hþ 12 ¼ 6:83� 0:11 is slightly higherthan is characteristic of H II regions and blue compact galaxieswith O/H similar to the GPs (Milingo et al. 2010). The averagevalue of log S=O ¼ �1:25� 0:14. Infrared observations of[S IV] in the GPs would help to more accurately constrainthe sulfur abundance. I11 found log S=O ¼ �1:64 for theirLCG sample.

Te-based N/H determinations in 57 GPs averagelogN=Hþ 12 ¼ 6:86� 0:19. The N/O versus O/H relation isshown in Figure 4 and is consistent with a constant log N/Oratio at a value of ∼� 1:25� 0:16. The formal fit is

logN=O ¼ ð�0:10� 0:32ÞðlogO=Hþ 12Þ � ð0:44� 2:6Þ;r ¼ 0:08: (4)

The average log N/O agrees with the peak in the histogram inFigure 1 of A10; however, the lack of a trend in N/O with O/H isin contrast with the findings of A10. A10 found an N/O ratiothat increased with O/H using a version of the N2S2 method forestimating N/O calibrated from SDSS SFGs with direct ionicabundance determinations. A possible explanation for the dis-crepancy is discussed in § 4.2. I11 found that N/O shows asignificant spread for their entire sample of LCG and BCD

galaxies, although they found no appreciable increase in N/Owith O/H up to 12þ logO=H ∼ 8:4.

He I λ5876 intensities were measured in 70 GPs. In a subsetof nine, He II λ4686 was weak but measurable, enabling a de-termination of the helium abundance. He I λ6678 was measuredin 23 of the GP spectra, but the line is systematically weakerthan λ5876 and in the portion of the spectrum with consistentlylower S/N. Consequently, He I λ6678 was not used in the he-lium abundance determination.

Heþ abundances were determined from the intensities ofλ5876 utilizing the calculations of Benjamin et al. (1999)and the formulation given by Peimbert & Peimbert (2000):

NðHeþÞ=NðHþÞ ¼ fIðλ5876Þ=IðHβÞg0:735T 0:230�6:3×10�4Ne

4 :

(5)

Te was assumed to be Te [O III]. Heþþ abundances were de-termined from the λ4686/Hβ line ratios and the recombinationcoefficients compiled by Storey & Hummer (1995). In somecases it was possible to calculate the radiation softness pa-rameter, ζ ¼ NðOþÞNðSþþÞ=NðSþÞNðOþþÞ (Vìlchez & Pagel1988). In those objects, and by extension to all other GPs, ζ issmall enough to justify assuming that the contribution ofneutral helium is negligible. The resulting value of He/H is0:091� 0:008, with the contribution of Heþþ to the total he-lium abundance on the order of 1–2%.

A second estimate can be made using the intensity of He I

λ5876 in all 70 objects using Te [O III] where available andassuming 13,000 K otherwise, which is representative of theaverage Te in the GPs. That result is He=H ¼ 0:086� 0:013

TABLE 2 (Continued)

SSDS ID[O II] [Ne III] Hδ Hγ [O III] He I He II Hβ [O III] [O III]λ3727 λ3868 λ4101 λ4340 λ4363 λ4471 λ4686 λ4861 λ4959 λ5007

587739406242742472 . . . . . 114 45.6 24.8 42.7 7.9 … … 100 199 590587739828742389914 . . . . . 99.8 58.2 25.8 46.5 12.3 4.16 0.80 100 246 740587739652107600089 . . . . . 143 42.0 25.4 48.3 8.24 5.41 … 100 203 557587739721387409964 . . . . . 214 30.8 21.4 41.0 2.48 … … 100 150 451587741600420003946 . . . . . 257 37.8 28.9 51.9 … … … 100 119 365587741421099286852 . . . . . 129 36.2 23.7 42.6 5.74 … … 100 174 544587741532770074773 . . . . . 163 49.0 25.8 45.9 8.02 … … 100 193 577587741817851084830 . . . . . 195 30.6 23.7 48.8 3.41 … … 100 117 354587741391573287017 . . . . . … … … … … … … … … …587741392649781464 . . . . . 87.8 58.0 28.7 49.8 12.4 … … 100 218 663587739648351076573 . . . . . 200 38.6 25.5 44.9 … 5.73 … 100 148 434587741490367889543 . . . . . 194 46.3 26.4 47.6 4.89 4.0 … 100 174 522587741532781215844 . . . . . 296 38.4 26.7 50.1 … … … 100 135 430587742014876745993 . . . . . 207 54.9 26.4 52.4 9.29 5.98 … 100 205 608588023240745943289 . . . . . 235 52.6 30.0 45.5 4.04 … … 100 163 513587745243087372534 . . . . . 117 46.6 27.6 40.6 10.9 … … 100 197 602587742628534026489 . . . . . 297 38.3 28.3 42.6 … … … 100 123 371587744874785145599 . . . . . 202 40.9 … 38.3 … … … 100 174 495587742013825941802 . . . . . 232 32.5 25.9 41.8 6.1 … … 100 124 366587742062151467120 . . . . . 194 57.3 24.3 47.3 11.3 3.01 … 100 227 680587741727655919734 . . . . . 159 45.2 26.8 45.8 9.02 … … 100 192 596

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TABLE 3

REDDENING-CORRECTED LINE INTENSITIES RELATIVE TO Hβ λλ5876–6731

SSDS ID

He I [S III] [N II] Hα [N II] He I [S II] [S II] [S II] c

λ5876 λ6312 λ6548 λ6563 λ6583 λ6678 λ6717 λ6731 λ6717+6731

587725073921409255 . . . . . 10.7 … 9.4 287 29.3 … 23.7 24.3 51.1 0.297588848899919446344 . . . . . 11.8 … … 287 13.2 … 16.4 13.3 29.1 0.169587725576962244831 . . . . . 13.3 … … 287 14.5 … … … 16.8 0.292587731187273892048 . . . . . 11.4 … 6.3 287 22.0 … 25.5 20.1 48.8 0.286587731513693503653 . . . . . 13.4 … … 287 30.0 … … … 27.5 0.332587724233716596882 . . . . . 12.6 … 7.92 287 25.1 4.07 21.9 14.8 42.2 0.205587727179006148758 . . . . . 11.4 … … 287 7.83 3.29 6.58 5.68 12.9 0.075587724241767825591 . . . . . 12.4 … 7.24 287 22.8 3.55 18.9 14.5 33.6 0.152587724240158589061 . . . . . 11.4 … 9.11 287 25.4 3.22 23.8 18.1 40.5 0.236587726032778559604 . . . . . 8.46 … … 287 18.5 … 18.2 20.8 38.7 0.121587726032253419628 . . . . . 11.8 1.91 4.95 287 12.9 3.09 15.8 11.9 28.4 0.174588010360138367359 . . . . . 11.4 1.81 3.36 287 11.7 2.53 16.4 12.2 29.1 0.244587726102030451047 . . . . . 7.32 … 5.0 287 16.7 … 15.9 9.73 23.6 0.279587729155743875234 . . . . . 12.0 … ... 287 11.8 … 14.4 12.4 25.2 0.061587728919520608387 . . . . . 12.3 … 5.08 287 15.0 4.39 17.3 12.9 34.0 0.186587729229297090692 . . . . . 10.0 … … 287 10.5 3.39 13.6 11.3 24.7 0.251587725818034913419 . . . . . 10.4 … 14.3 287 40.8 … 28.5 22.8 51.5 0.286587730774416883967 . . . . . 10.5 … … 287 15.4 … 20.0 15.7 35.0 0.256587730774965354630 . . . . . 9.78 … … 287 17.7 … 13.7 10.8 24.6 0.065587728906099687546 . . . . . 6.96 … 11.2 287 32.9 2.51 23.9 19.1 42.1 0.388587725550133444775 . . . . . … … … … … … … … … …588009371762098262 . . . . . 10.2 … … 287 18.7 … 11.8 9.0 26.1 0.229588011122502336742 . . . . . 8.98 … … 287 6.03 … 9.94 5.77 15.9 0.448588011103712706632 . . . . . 13.1 … 12.6 287 47.0 … 21.3 17.7 54.0 0.361588013384341913605 . . . . . 10.6 1.72 … 287 10.8 2.26 14.7 10.7 25.8 0.170587732134315425958 . . . . . 12.2 … … 287 11.9 … 12.5 13.3 25.1 0.114587729777439801619 . . . . . 14.9 … … 287 4.61 … … … … 0.363587729777446945029 . . . . . 6.65 … … 287 6.62 … 5.27 6.09 11.8 0.213587732152555864324 . . . . . 12.4 … … 287 5.66 … 10.9 7.03 16.0 0.192587732578845786234 . . . . . 13.4 … 3.83 287 10.3 … 17.6 17.7 35.4 0.168587733080270569500 . . . . . 11.9 … 7.06 287 22.6 … 19.9 12.9 33.1 0.151588297864714387604 . . . . . 8.83 … 10.7 287 30.0 … 21.6 12.9 38.2 0.282587735695911747673 . . . . . … … … … … … … … … …587735696987717870 . . . . . 9.31 … 3.89 287 13.5 … 15.9 12.1 28.0 0.114587733441055359356 . . . . . … … … … … … … … … …588017605211390138 . . . . . 12.4 1.41 … 287 11.1 … 9.83 8.50 20.2 0.180588017114517536797 . . . . . 12.2 … 5.0 287 12.8 … 9.27 8.61 19.3 0.104588017116132540589 . . . . . … … … … … … … … … …588018090541842668 . . . . . … … … … … … … … … …588018090013098618 . . . . . … … … … … … … … … …588016878295515268 . . . . . 9.03 … 5.36 287 13.1 … 22.0 17.5 42.4 0.206587735661007863875 . . . . . 12.8 … 13.4 287 42.7 … 18.7 14.5 37.3 0.300588016892783820948 . . . . . 10.8 … 14.8 287 32.7 … 22.7 14.1 45.1 0.167587735663159738526 . . . . . 10.2 … 2.68 287 15.5 … 23.6 21.9 43.6 0.278588018055114784812 . . . . . 10.5 1.33 8.02 287 23.6 1.86 21.1 19.0 40.1 0.225588018055652769997 . . . . . 11.3 … 9.44 287 35.8 … 18.9 16.0 34.5 0.332588017570848768137 . . . . . 12.1 … 2.41 287 12.2 … 17.3 12.3 29.0 0.142587736915687964980 . . . . . 11.9 … 3.36 281 12.0 … 12.0 6.31 17.4 0.00587736915687375248 . . . . . … … … … … … … … … …587738410863493299 . . . . . 11.1 1.28 1.93 287 6.25 3.43 6.70 5.87 11.7 0.041587735349111947338 . . . . . 11.1 0.99 3.21 287 10.1 2.55 5.83 5.37 11.8 0.057587738570859413642 . . . . . 10.5 1.7 5.44 287 16.0 3.10 10.5 8.98 19.5 0.229587736940372361382 . . . . . … … … … … … … … … …587739153352229578 . . . . . 12.0 … … 287 15.6 … 26.5 12.5 40.2 0.195587738947196944678 . . . . . 11.1 1.19 … 287 4.2 2.92 8.30 5.17 12.2 0.084587738371672178952 . . . . . 11.4 1.60 … 287 19.2 2.72 14.1 12.5 26.9 0.401588017978880950451 . . . . . 7.13 … … 277 27.4 … … … … 0.00587739408388980778 . . . . . 12.0 0.90 7.68 287 21.0 2.24 17.1 13.0 30.3 0.122588017977277874181 . . . . . 10.7 1.17 3.52 287 11.7 2.24 16.7 10.3 24.8 0.160


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assuming that the contribution of both neutral helium and Heþþ

is negligible. This corresponds to a helium abundance by mass,Y ¼ 0:255� 0:027. For comparison, the helium abundance forNGC 346 in the Small Magellanic Cloud is Y ¼ 0:2405�0:0018 (Peimbert et al. 2000).

3.2. Abundances Based on Strong-Line Methods

The line intensities were used to estimate O/H from N2O2,N2, and O3N2 and N/O from N2S2. For N2 and O3N2 the ca-libration of PP04 was adopted. The average O/H in the GPs liesat the lower limit of the range of validity for the O3N2 method(logO=Hþ 12 ¼ 8:0–8:1). The behavior of O3N2 and choiceof calibration for the N2 index will be discussed further in § 4.

The O/H values derived from N2O2 reproduce the results ofC09 (correlation coefficient of 0.92), with a small systematicoffset to lower O/H by 0.1–0.2 dex. Table 4 summarizes themass-metallicity relations in the GPs derived from the differentabundance estimating techniques, along with the relation of I11for the GPs and for their complete sample of LCGs. All methodsare in agreement within the errors, with the exception of N2O2.Although the N2O2 method is susceptible to errors in flux cal-ibration or reddening correction, it is possible to demonstratefrom these data that such errors are not responsible for the dis-crepancy between N2O2 and the other methods. A10 suggestthat N2O2 overestimates O/H because the calibration fails totake into account the influence of the systematically higherN/O with O/H they find in the GPs. However, where N/O ratiois constant, as it is in the GPs (Fig. 4), N2O2 does not retain ametallicity sensitivity (Kewley & Dopita 2002; Kewley &

Ellison 2008). The most straightforward explanation is thatthe metallicity in the GPs is below the range in O/H whereN2O2 is applicable.

As Table 4 shows, O/H estimates from N2 and O3N2 showtrends with mass in agreement with trends based on the directmethod and with the results inferred from A10 and I11. One canconclude that N2 gives O/H abundances consistent with thedirect method within the statistical errors. To examine that

TABLE 3 (Continued)

SSDS ID

He I [S III] [N II] Hα [N II] He I [S II] [S II] [S II] c

λ5876 λ6312 λ6548 λ6563 λ6583 λ6678 λ6717 λ6731 λ6717+6731

587739406242742472 . . . . . 14.3 … … 287 9.39 … … … 28.0 0.096587739828742389914 . . . . . 11.6 1.27 2.07 287 6.20 2.69 8.32 6.79 14.6 0.141587739652107600089 . . . . . 13.0 … … 287 11.8 3.36 16.7 7.68 23.9 0.195587739721387409964 . . . . . 13.9 … … 287 18.2 … 17.1 17.0 34.4 0.224587741600420003946 . . . . . 11.3 … … 287 34.2 … 26.5 12.8 38.6 0.297587741421099286852 . . . . . 10.4 … … 287 17.3 … 9.55 8.32 16.0 0.020587741532770074773 . . . . . 13.4 … 4.53 287 13.9 2.30 15.5 12.3 31.6 0.138587741817851084830 . . . . . 11.4 … 11.5 287 39.1 … 10.9 13.6 29.8 0.280587741391573287017 . . . . . … … … … … … … … … …587741392649781464 . . . . . 13.2 … … 287 9.69 … 10.9 5.93 18.9 0.123587739648351076573 . . . . . 13.9 … 9.06 287 28.5 … 21.9 14.1 31.6 0.197587741490367889543 . . . . . 11.0 1.22 5.96 287 18.2 5.29 18.3 15.2 34.7 0.171587741532781215844 . . . . . … … … 287 12.6 … 35.1 23.0 49.5 0.217587742014876745993 . . . . . 13.6 1.59 … 287 12.9 … 16.5 9.56 29.5 0.255588023240745943289 . . . . . 12.1 … … 287 13.4 … … … 29.9 0.440587745243087372534 . . . . . 12.0 … … 287 12.0 … … … 14.6 0.158587742628534026489 . . . . . 15.2 … 10.9 287 26.5 … 27.6 17.9 43.2 0.280587744874785145599 . . . . . 11.7 … … 287 10.5 … … … … 0.773587742013825941802 . . . . . 10.7 … … 287 40.5 … … … 20.7 0.250587742062151467120 . . . . . 13.5 … … 287 7.75 … 15.8 9.10 26.5 0.326587741727655919734 . . . . . 12.9 … … 287 6.98 … 13.5 16.5 32.5 0.120

FIG. 1.—The logO=Hþ 12 values are plotted as a function of Green Peamass. Oxygen abundances are Te-based determinations. The Green Pea massesare taken from Cardamone et al. (2009). Error bars are the uncertainties in O/Hderived from uncertainties in the measured line intensities propagated throughthe ionic and total abundance calculations. The linear least-squares fit is shownwith the fit parameters given in the text and in Table 4.

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conclusion in more detail, Figure 5 shows the differences in O/Hestimated by N2 compared with the direct method for the indi-vidual GPs. The residuals show scatter of ∼0:12 dex, but also asystematic difference in the sense that N2 somewhat overesti-mates O/H for logO=Hþ 12 < ∼8:1 and underestimates O/Hfor logO=Hþ 12 > ∼8:2.

The GPs are predominantly of higher ionization and lowerO/H than the objects on which the original calibrations werebased. Empirical calibrations of N2, O3N2, and N2S2 havebeen generally based on some combination of Galactic and ex-tragalactic H II regions plus H II galaxies. Additionally, the re-levant line ratios are ionization-sensitive. Therefore, it is ofinterest to reevaluate the calibration of the strong-line methods.The calibrations based on GP Te-based abundances are shownin Table 5, along with frequently utilized calibrations from theliterature. PMC09 have investigated the dependence of N/O onthe calibration of N2. The N/O ratios characteristic of the GPs

lie in the region where that effect is small. In fact, the average N/O value for the GPs utilized in equation 13 of PMC09 givesessentially the same calibration as the one shown in Table 5,which lacks the N/O term.

The N2 calibration based on the GPs is most similar to thecalibration of PP04 and more at variance with the calibrations ofDenicoló et al. (2002), Yin et al. (2007), and PMC09. The N2S2calibration based on GP data differs from that used by PMC09and A10. The O3N2 calibration based on the GPs (logO=Hþ 12 ¼ �0:27O3N2þ 8:67, not included in Table 5) hasthe appearance of agreement with that of PP04. However, it willbe seen that the agreement is somewhat coincidental. Additionaldiscussion of the different calibrations is included in § 4.

A10 used the N2S2 method, recalibrated using SDSS SFGswith Te-based estimates of ionic abundances, to derive N/O inall the galaxies used in their analysis. The use of N2S2 has theadvantage of minimizing the dependence of N/O on reddening

TABLE 4

O/H VERSUS MASS FROM DIRECT AND/OR STRONG-LINE METHODS

Method Linear fit Corr. coeff.

N2 . . . . . . . . . . . . . . . . . . log O=Hþ 12 ¼ ð0:067� 0:046Þ logMþ ð7:46� 0:44Þ 0.31Direct + O3N2 . . . . . log O=Hþ 12 ¼ ð0:081� 0:057Þ logMþ ð7:36� 0:54Þ 0.31Direct + N2 . . . . . . . . log O=Hþ 12 ¼ ð0:092� 0:066Þ logMþ ð7:28� 0:63Þ 0.30Direct . . . . . . . . . . . . . . log O=Hþ 12 ¼ ð0:091� 0:068Þ logMþ ð7:26� 0:65Þ 0.33Directa . . . . . . . . . . . . . . log O=Hþ 12 ¼ ð0:096� 0:062Þ logMþ ð7:24� 0:56Þ 0.39Directb . . . . . . . . . . . . . . log O=Hþ 12 ¼ ð0:106� 0:061Þ logMþ ð7:14� 0:56Þ 0.39Directc . . . . . . . . . . . . . . log O=Hþ 12 ¼ ð0:069� 0:047Þ logMþ ð7:475� 0:426Þ …N2O2 . . . . . . . . . . . . . . log O=Hþ 12 ¼ ð�0:024� 0:10Þ logMþ ð8:73� 0:97Þ 0.06

a Mass from Izotov et al. (2011).b Mass and O/H from Izotov et al. (2011).c O/H vs. mass determined from LCG sample by Izotov et al. (2011).

FIG. 2.—logNe=Hþ 12 is plotted as a function of Green Pea mass with abun-dances based on the Te method and masses taken from Cardamone et al. (2009).Error bars derive from the uncertainties in the measured line intensities propa-gated through the ionic and total abundance calculations. The linear least-squares fit is shown with the fit parameters given in the text.

FIG. 3.—logNe=O is plotted as a function of Green Pea mass, with the massestaken from Cardamone et al. (2009). Ne and O are both determined from the Te-based method. Error bars derive from the uncertainties in the measured lineintensities propagated through the ionic and total abundance calculations.The linear least-squares fit is shown with the fit parameters given in the text.


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correction or flux calibration. Utilizing these line-intensity mea-surements and the N2S2 calibration of PMC09 or as modifiedby A10 to calculate N/O values, a trend in N/O versus O/H canbe reproduced with O/H determined from N2 only, or in com-bination with direct determinations where Te is available. Thesetrends have different slopes, but are both in agreement with thefindings of A10 that N/O increases with O/H in a manner con-sistent with their Figure 7. Closer examination of the trend ofN/O with O/H based on these measurements shows that the re-lationship is steeper for O/H estimated entirely from N2, de-creases by utilization of Te-based O/H where availablesupplemented with N2 estimates, and disappears altogetherwhen only the Te-based O/H is utilized. Using Te-based abun-dances for both N and O where available, supplemented withN2 and N2S2 estimates otherwise, gives a shallower trend withslope ∼30% of the slope obtained from using N2S2 versus N2only. However, limiting the sample to direct abundance deter-minations of both N and O, as discussed in § 3.1, shows thatN/O is constant, consistent with the behavior of N/O in theI11 sample of LCGs. A possible explanation to reconcile thedifferent results follows.

4. DISCUSSION

4.1. Properties of the Green Pea Galaxies

Te-based abundances in the GPs confirm an average O/H anda mass-metallicity relation in agreement with the findings of I11for their LCGs. Average abundances of O, N, Ne, and He aresimilar to abundances in Small Magellanic Cloud H II regions(Peimbert et al. 2000). The S estimate in the GPs is more sus-ceptible to systematic errors, due to greater uncertainties in the

relevant line ratios and the ICF. N/O is independent of O/Hwithin the errors over the range of 7:9 < logO=Hþ 12 < 8:4.

Izotov & Thuan (1999) studied a sample of approximately50 blue compact galaxies with 7:6 < logO=Hþ 12 < ∼8:1and determined logN=O ¼ �1:46� 0:14 and logNe=O ¼�0:72� 0:06. Estimates from Figure 7 of I11 for the LCGswith 7:9 < logO=Hþ 12 < 8:3 give logN=O ∼�1:4 to �1:3and logNe=O ∼�0:7. Values for the GPs are logN=O ¼�1:25� 0:16 and logNe=O ¼ �0:68� 0:06. Although theN/O ratio in the GPs is evidence of, at most, only modest Nenrichment, a larger N/O ratio could result from nitrogen en-hancement by Wolf-Rayet stars. A larger N/O ratio would alsobe expected if the GPs recently began the intense star-formingphase, perhaps for only a few million years, given the presence

FIG. 4.—logN=O is plotted against logO=Hþ 12 for Green Peas with O andN determined by the Te-based method. Error bars derive from the uncertaintiesin the measured line intensities propagated through the ionic and total abundancecalculations for O and N. The formal linear least-squares fit is shown with the fitparameters given in the text. N/O is approximately constant within the errorsover the range 7:9 < logO=Hþ 12 < 8:4.

FIG. 5.—Differences in logO=Hþ 12 between the N2 method with the PP04calibration and the Te method are plotted against the Te-based oxygen abun-dances. Error bars shown for logO=Hþ 12 derive from the uncertainties in theline intensities propagated through the ionic and total abundance calculations.Error bars in ΔðN2-TeÞ are representative of the statistical errors from the uti-lization of the N2 method. The residuals indicate that N2 overestimates O/H atlogO=Hþ 12 < ∼8:1 and underestimates O/H for logO=Hþ 12 > ∼8:2.

TABLE 5

CALIBRATIONS OF N2 AND N2S2 STRONG-LINE METHODS

Calibration Reference

N2 Strong-Line MethodlogO=Hþ 12 ¼ ð0:35� 0:13ÞN2þ ð8:57� 0:17Þ r ¼ 0:59 This articlelogO=Hþ 12 ¼ ð0:57� 0:04ÞN2þ ð8:90� 0:03Þa PP04logO=Hþ 12 ¼ 0:79N2þ 9:07 PMC09logO=Hþ 12 ¼ ð0:73� 0:10ÞN2þ ð9:12� 0:05Þ D02logO=Hþ 12 ¼ 9:514þ ð0:916� 0:15ÞN2, (P > 0:80) Y07

N2S2 Strong-Line MethodlogN=O ¼ ð0:76� 0:13ÞN2S2� ð1:05� 0:041Þ r ¼ 0:84 This articlelogN=O ¼ 1:26N2S2� 0:86 PMC09logN=O ¼ �0:76þ 1:94N2S2þ 0:55ðN2S2Þ2 A10

a Error quoted is the formal statistical error.REFERENCES.—(A10) Amorìn et al. 2010; (D02) Denicoló et al. 2002; (PMC09)Perez-Montero & Contini 2009; (PP04) Pettini & Pagel 2004.

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of Wolf-Rayet stars. In this delayed-release scenario a largerN/O ratio would be expected at the beginning of a starburst in-terval as the result of a preceding quiescent phase where nitro-gen enrichment from 4–8 M⊙ stars was occurring (Kobulnickyand Skillman 1999). In this model, the N/O ratio subsequentlydecreases as supernovae enhance O and other α-elements. I11find that star formation must be occurring in bursts in the LCGs,with the bursting nature more pronounced in the LCGs withhigher Hβ equivalent widths and lower masses, the subset mostsimilar to the GPs.

If nitrogen enhancement were predominantly by Wolf-Rayetstars, there should be evidence of helium enrichment as well(Kobulnicky et al. 1997). The uncertainties in the He and Nabundances in the individual GPs are large enough to mask asmall trend. The average values of He/H and N/H for theGPs can be plotted on Figure 9 of Kobulnicky et al. (1997),which depicts He/H versus N/H for 60 low-metallicity galaxies.The result is inconclusive, as that one data point lies near theupper limits of both He/H and N/H, but still within the distribu-tion of the low-metallicity galaxies.

Although A10 find that their results are consistent with thetrend in O/H versus mass that they see in their sample of SDSSstar-forming galaxies; the mass-metallicity relationship inferredfrom these measurements for the GPs is shallower over therange 8:5 < logM⊙ < 10:5 than found in the galaxies studiedby Tremonti et al. (2004). The criteria used to identify the orig-inal GPs suggest the possibility of a selection effect in the de-duced mass-metallicity relation. The galaxy sample of Tremontiet al. (2004) includes a range in [O III]/Hβ, while the GPs wereoriginally chosen by C09 based on strong [O III]. Consequently,the GPs constitute a sample with an artificial metallicity cutoffthat is biased toward low metallicity. This could in turn result ina shallower mass-metallicity relationship for the GPs than ischaracteristic of the large sample of Tremonti et al. (2004)introduced solely by using strong [O III]/Hβ as the selectioncriterion. It is interesting that the mass-metallicity relation foundby I11 for their 803 LCGs agrees with that found here for theGPs, even though the I11 sample was selected by Hβ equivalentwidth and not the strength of [O III], specifically to avoid biasingtheir sample to low-metallicity objects. However, one of theirother selection criteria was the presence of measurable λ4363necessary to enable the Te-based abundance determinations.That criterion could also have the effect of selecting forlower-metallicity objects. I11 judged that not to be the casebased on the argument that removing that constraint only in-creased their sample by ∼20%.

4.2. Assessing the Strong-Line Methods

As shown in Table 4, N2 with the PP04 calibration producesa mass-metallicity relation consistent with the Te-based rela-tion. N2O2 does not, because O/H is outside the valid rangefor that method. The behavior of the O3N2 method will bediscussed further subsequently. It remains to explain why the

N/O trend with O/H based on the N2S2 method in combinationwith the direct plus N2 method gives a different result from thedirect-only method. The following discussion shows that the be-haviors of N2 and N2S2 are coupled in an unexpected way asthe result of the relative uniformity of the emission-line spectrain the GP sample.

Combining the definitions of N2S2 and N2 with the pub-lished calibrations demonstrates that the two methods are notindependent, at least in the GPs. A conversion relation betweenN/O (N2S2) and O/H (N2) can be derived from the calibrationof PMC09 for N2S2 and PP04 for N2:

logN=ON2S2 ¼ 2:22ðlogO=Hþ 12ÞN2 � 20:6

� 1:27 logð½S II�=HαÞ: (6)

In 51 of the 71 GPs with measured line intensities, the [S II]/Hβratios are between ∼0:25 and ∼0:50, reducing log N/O (N2S2)to a linear relation with O/H (N2) plus some amount of scatter.Consequently, if the N2 and N2S2 methods are utilized together,they will introduce a bias and N/O will show a trend with O/Hbased solely on the definitions of the indices and the calibra-tions. A similar conversion relation results by utilizing theN2 calibration of PMC09.

Figure 6 illustrates the specific case where N/O is obtainedfrom N2S2 and O/H is determined solely by the N2 method.The solid line is the least-squares fit to the measurements in thisarticle, which is consistent with the trend found by A10. Thedashed lines represent the conversion relationship for [S II]/Hβ ratios of 0.10, 0.25, and 0.50. The observed variation in[S II]/Hβ flattens the trend compared with the slope of the con-version relation, but the net effect shows N/O increasing with

FIG. 6.—The logN=O determined from the N2S2 method using the PMC09calibration is plotted against logO=Hþ 12 determined from the N2 methodusing the PP04 calibration. The solid line is the least-squares fit to the GreenPea data. The error bars have been suppressed to better display the trends. Thedashed lines represent the conversion relationship between N2O2 and N2, asdescribed in the text, for [S II]/Hβ ratios of 0.10, 0.25, and 0.50 from left to right.


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O/H. As more objects are included with directly determinedO/H, this trend moderates until it disappears when limitingthe sample to only Te-based O/H. N2S2 or N2 would still givevalid results if used independently when Te-based abundancesare not possible, however, plotting N2S2 against N2 in the GPsleads to the appearance of a relationship different from the ac-tual abundance trend.

Similarly, the N2 and O3N2 methods are not independent inthe GPs. The O3N2 method gives oxygen abundance estimatesthat are more tightly correlated with those from the N2 method(correlation coefficient of 0.99) than one might expect given thesystematic uncertainty in the calibrations of O3N2 and N2 asshown in Table 1. Based on the definition of the indices andcalibrated relationships from PP04, there is a conversion rela-tionship:

12þ log O=HN2 ¼ 1:78ðlogO=Hþ 12ÞO3N2 � 6:64

þ 0:57 logðλ5007=HβÞ: (7)

The λ5007/Hβ ratio varies by just over a factor of 2 in the71 GPs used in this study. As a result, in the GPs O3N2 andN2 are equivalent to within a roughly constant offset, withO3N2 giving a systematically lower estimate of O/H, whichis the observed behavior. One consequence is that the agreementbetween the calibration of O3N2 based on the GP abundancesand the calibration of PP04 is, to some extent, coincidental andderives from the GP-based calibration of N2. These data cannotbe used to imply that O3N2 should independently be assumedto be valid in this low-metallicity, high-ionization regime(O3N2 > 1:9, P > 0:80); although, López-Sánchez & Esteban(2010) find that O3N2 with the PP04 calibration is applica-ble for 7:70 < logO=Hþ 12 < 8:68 in their sample of star-forming galaxies, some of which were classified as Wolf-Rayetgalaxies.

As expected, the higher ionization level in the GPs intro-duces additional systematic error into O/H and N/O estimatesfrom the N2 and N2S2 methods, respectively. The presenceof λ4686/Hβ plus strong [O III]/Hβ is more typical of planetarynebulae than H II region spectra, and the GPs generally occupythe locus of planetary nebulae on a Baldwin-Phillips-Terlevich(BPT; Baldwin, et al. 1981) plot of λ5007/Hβ versus λ3727/λ5007. Strong-line methods are not expected to work in plane-tary nebulae because of the higher effective temperatures(Stasińska 2002). Relaño et al. (2010) have found that the[N II]/Hα and [N II]/[O III] line ratios show variations as a func-tion of ionization in NGC 595, the most luminous H II region inM33,which containsWolf-Rayet stars. Yin et al. (2007) analyzedthe N2 method and found the calibration to be ionization-dependent, with the slope of the relation becoming slightlyflatter as the excitation parameter, P , increased. Table 5 in-cludes the calibration based on 149 of their galaxies thathad P > 0:80, which is also the average value for the excita-tion parameter in the GPs. The slope of the GP-based calibra-tion of N2 is shallower than the slopes of the other calibrations

in Table 5, with the exception of PP04, by greater than twicethe statistical errors. The slope of the N2S2 calibration basedon GP abundances is shallower by more than 3 times the sta-tistical error compared with the slopes of the calibrations ofPMC09 and A10.

At some point, ionization effects could invalidate thesestrong-line methods. To isolate the effect of even higher ioniza-tion on the behavior of the N2 and N2S2 methods, a cursoryexamination was made of emission-line strengths taken fromthe literature for approximately 50 SMC and LMC planetarynebulae, chosen with Te-based abundances and O/H compar-able with the GPs. No relationship was evident in N2 withO/H over the range of �2:5 < N2 < �0:3, although a trend ex-ists between N2S2 and log N/O in the planetary nebulae, with aslope similar (within the errors) to the trend found in the GPs.

Table 6 lists three abundance-related parameters for severalN2 calibrations over the range of the N2 index in the GPs(∼� 1:8 < N2 < �0:8). Both the Yin et al. (2007) and PMC09calibrations yield a wider range in O/H while, respectively, over-estimating and underestimating the absolute O/H values. ThePMC09 calibration utilizing the N/O term is included for com-pleteness and was evaluated using N/O values for the individualGPs. The absolute value of ΔO/H (Te � N2) is reduced,

TABLE 6

COMPARISON OF RESULTS USING DIFFERENT CALIBRATIONS OF N2STRONG-LINE METHOD APPLIED TO GREEN PEAS

Value in Green Peas Calibration

Range7:84 < logO=Hþ 12 < 8:42 PP047:76 < logO=Hþ 12 < 8:48 D027:60 < logO=Hþ 12 < 8:41 PMC097:70 < logO=Hþ 12 < 8:33 PMC09 (N/O)a

7:81 < logO=Hþ 12 < 8:74 Y07b

7:92 < logO=Hþ 12 < 8:28 Green Peas (this article)7:88 < logO=Hþ 12 < 8:41 Te (this article)

Average ValuelogO=Hþ 12 ¼ 8:15� 0:13 PP04logO=Hþ 12 ¼ 8:16� 0:16 D02logO=Hþ 12 ¼ 8:03� 0:18 PMC09logO=Hþ 12 ¼ 8:07� 0:14 PMC09 (N/O)a

logO=Hþ 12 ¼ 8:31� 0:20 Y07b

logO=Hþ 12 ¼ 8:11� 0:08 Green Peas (this article)logO=Hþ 12 ¼ 8:11� 0:13 Te (this article)

Absolute Value Δ logO=HðTe � N2Þ0.09 ± 0.08 PP040.12 ± 0.09 D020.13 ± 0.11 PMC090.08 ± 0.06 PMC09 (N/O)a

0.21 ± 0.14 Y07b

0.08 ± 0.07 Green Peas (this article)

a The calibration uses the term accounting for the dependence on N/O.b The calibration chosen for comparison is for a value of the excitation

parameter, P > 0:80.REFERENCES.—(D02) Denicoló et al. 2002; (PMC09) Perez-Montero & Contini2009; (PP04) Pettini & Pagel 2004; (Y07) Yin et al. 2007.

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consistent with the findings of PMC09 that including the N/Odependence reduces the dispersion in the N2-based metallicityestimates, although this is a small effect in the GPs.

The calibration of PP04 gives the smallest average value ofthe residual compared with Te-based O/H, comparable withthe residual produced by the GP-based calibration. López-Sánchez & Esteban (2010) examined the N2 method with thePP04 calibration in their study of star-forming galaxies, includ-ing Wolf-Rayet galaxies, and found an absolute value ofΔO=HðN2� TeÞ ¼ 0:12� 0:13. The PP04 calibration intro-duces the smallest systematic effect in the mass-metallicity rela-tion for the GPs, giving a shallower slope within 10% of theTe-based value,while the calibration of PMC09 gives a slope thatis steeper than the Te-based value by slightly more than ∼30%.

Table 7 shows the same abundance parameters for the differ-ent N2S2 calibrations applied to the GP line intensities. Thegreater slope of the calibrations based on H II regions andH II galaxies overestimates log N/O compared with Te-basedlog N/O for values of N2S2 > ∼� 0:4 and underestimateslog N/O otherwise. The A10 calibration exhibits a largeramount of scatter than the calibration of PMC09; however,the residuals in log N/O estimates from each calibration arewithin the estimated error cited in Table 1.

It is not unexpected that the calibration of N2S2 would bedifferent in the GPs. First, given the estimated error in the pub-lished calibrations, some variance could be expected solely be-cause the GPs have a smaller range in both N2S2 and log N/Othan the larger sample of objects on which the other calibrationsare based. More importantly, however, the relation between N/Oand [N II]/[S II] has an ionization dependence. PMC09 were mo-tivated to use N2S2 for N/O estimates to minimize the effects oferrors in reddening correction or flux calibration. To first order,the [N II]/[S II] line ratio would be sensitive to Nþ=Sþ and,

therefore, N/S, just as the [N II]/[O II] line ratio is sensitiveto Nþ=Oþ and, therefore, N/O. Since sulfur is also an α-element, N/O would scale with N/S. The ionization potentialsof Nþ and Oþ are similar, a fact that constitutes the rationale forthe ICF used to calculate the total nitrogen abundance. How-ever, the ionization potential of Oþ is more similar to Sþþ thanSþ, again the basis for the traditional ICF used to calculate totalsulfur abundance (Peimbert & Costero 1969). N/S and, there-fore, N/O would not necessarily maintain the same relationshipwith [N II]/[S II] as ionization increases.

Based on this argument and as a consistency check, ½N II�=ð½S II� þ ½S III�Þ should scale directly with N/O. There are 15 ob-jects with both Te-based abundances and a measurement of[S III] λ6312, so a calibration can be determined for this“N2S23” index, which would be expected to have a slope onthe order of unity. The [S III] line is weak and the number ofdata points is limited, but the result is

logN=O ¼ ð0:99� 0:30ÞN2S23� ð0:95� 0:10Þ;r ¼ 0:89: (8)

As ionization increases at a given N/S ratio, one would expectthe fraction of S that is Sþ to be less than the fraction of N that isNþ. Therefore, the [N II]/[S II] ratio would overestimate N/S(and, equivalently, N/O), and the slope of the relation betweenN2S2 and N/O in the GPs would be expected to be less thanunity, as is the case. Kewley & Dopita (2002) have previouslynoted that the [N II]/[S II] ratio is strongly dependent on ioniza-tion making it less useful than [N II]/[O II] for estimating O/H.

In summary, the N2 and N2S2 methods are still valid for star-forming galaxies ionized by Wolf-Rayet stars, although use ofthe calibrations based on GPs should give more accurate O/Hand N/O estimates in galaxies with harder SEDs. There is evi-dence that the slope of both the N2 and N2S2 calibrations de-creases as ionization increases in star-forming galaxies. Thehigher ionization exhibited by the GPs is sufficient to alterthe calibration of these two strong-line methods, but is not highenough to invalidate them.

I11 concluded that oxygen abundances from strong-linemethods should not be used in galaxy evolution studies withouta careful analysis of the errors. The GP results confirm that careshould be taken when applying strong-line methods to star-forming galaxies where ionization or metallicity is substantiallydifferent from the objects on which the calibrations are based.Caution may be warranted at either extreme of ionization seen instar-forming galaxies. Berg et al. (2011) find that the commonstrong-line methods, including N2 with the PP04 calibration,overestimate O/H in their study of four very low mass, low-ionization galaxies. However, Berg et al. (2011) were not ableto determine Te-based abundances, so O/H was estimated fromcomparison with photoionization models. Nevertheless, withoutinformation inferring a high level of ionization where the GP-based calibration would apply, the N2 method as calibrated by

TABLE 7

COMPARISON OF RESULTS USING DIFFERENT CALIBRATIONS OF N2S2STRONG-LINE METHOD APPLIED TO GREEN PEAS

Value in Green Peas Calibration

Range�1:64 < logN=O < �0:73 PMC09�1:90 < logN=O < �0:23 A10�1:62 < logN=O < �0:86 Green Peas (this article)�1:55 < logN=O < �0:92 Te (this article)

Average ValuelogN=O ¼ �1:19� 0:23 PMC09logN=O ¼ �1:21� 0:30 A10logN=O ¼ �1:25� 0:14 Green Peas (this article)logN=O ¼ �1:25� 0:14 Te (this article)

Absolute Value Δ logN=OðTe � N2S2Þ0.10 ± 0.09 PMC090.13 ± 0.14 A100.08 ± 0.07 This article

REFERENCES.—(A10) Amorìn et al. 2010; (PMC09) Perez-Montero &Contini 2009.


2012 PASP, 124:21–35



PP04 would still be recommended based on its ability to givereasonable O/H estimates over a wide range of ionization andabsolute abundances.

Finally, looking for trends in N/O with O/H based solely onstrong-line methods can lead to biased results, depending on thedetails of the spectra being analyzed. Using N2S2 with N2,given the relatively small range in the [S II]/Hβ ratio in theGPs, leads to the conclusion that nitrogen is enhanced with re-spect to oxygen at these low metallicities, which is not the case,as seen from the Te-based abundances.

5. SUMMARY

This article presents emission-line measurements for 71Green Pea star-forming galaxies. Analysis of the measurementsleads to the following results:

1. He II λ4686 is detected in nine Green Peas, which has notbeen previously reported. The measured He II λ4686/Hβ ratiosare consistent with photoionization due to Wolf-Rayet stars.

2. Te-based abundance determinations show the Green Peasto have average abundances of logO=Hþ 12 ¼ 8:11� 0:13,logNe=Hþ 12 ¼ 7:43� 0:18, and logN=Hþ 12 ¼ 6:86�0:19. The sulfur abundance, log S=H ¼ 6:83� 0:11, is subjectto greater systematic error. The average He=H ¼ 0:091� 0:008in the galaxies with measurable λ4686. The mass-metallicityrelation based on O/H, confirmed by Ne/H, is consistent withthe results of Amorìn et al. (2010) for the Green Peas and inagreement with Izotov et al (2011) for ∼800 luminous compactgalaxies. The fact that the mass-metallicity relation is differentfrom that found in the star-forming galaxies studied by Tremontiet al. (2004) is most likely a selection effect.

3. Te-based nitrogen abundances show that N/O is indepen-dent of O/H, in contrast to the finding of A10, although con-sistent with the properties of the much larger luminouscompact galaxy sample of I11. The observed log N/O ratioof ∼� 1:25 is slightly larger than that seen in a sample of bluecompact dwarf galaxies and the LCG sample of I11 with com-parable O/H. The modest nitrogen enhancement compared with

other low-metallicity galaxies could be due to enrichment byWolf-Rayet stars or by intermediate-mass stars during a pre-vious quiescent period.

4. The relatively restricted range of [O III]/Hβ ratio in theGreen Peas results in reducing the O3N2 method to the equiva-lent of the N2 method with a systematic offset to lower O/H.Similarly, the modest variation in the [S II]/Hβ ratio introducesa bias when combining the N2S2 and N2 methods, resulting inthe appearance of a trend in N/O with O/H. The failure of N2O2to give reliable O/H estimates in the Green Peas is becauselogO=Hþ 12 < ∼8:3, outside the applicable range for thatmethod.

5. The presence of He II λ4686 implies that the ionizingsources must have higher effective temperatures than are gen-erally seen in star-forming galaxies. Green Peas are more similarto planetary nebulae than to H II regions in terms of spectralfeatures and location on a BPT diagram. The higher ionizationintroduces additional systematic error into abundance estimatesfrom application of the existing calibrations of the N2 and N2S2methods. Still, N2 and N2S2 remain valid for estimating O/Hand N/O at these levels of ionization and metallicity when Te-based abundances are not available. However, the calibrationsderived from the Green Peas differ from those commonly uti-lized and would be useful if star-forming galaxies like the GreenPeas with extremely hot ionizing sources are found to be morecommon.

I am grateful to Greg Rudnick for helpful discussions and toDick Henry for useful comments on a draft of the article. Thereferee’s comments and suggestions were very helpful with im-proving the organization and presentation of these results. Fund-ing for the Sloan Digital Sky Survey (SDSS) and SDSS-II hasbeen provided by the Alfred P. Sloan Foundation, the Partici-pating Institutions, the National Science Foundation, theNational Aeronautics and Space Administration, the US Depart-ment of Energy, the Japanese Monbukagakusho, the MaxPlanck Society, and the Higher Education Funding Councilfor England.

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abundances in “green pea” star-forming galaxies

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