Table Of ContentAstrophysicsandSpaceScience
DOI10.1007/sXXXXX-XXX-XXXX-X
Chemical abundances in high-redshift galaxies:
A powerful new emission line diagnostic
Michael A. Dopita12, Lisa J. Kewley13,
Ralph S. Sutherland3, & David C. Nicholls1
6
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(cid:13)c Springer-Verlag••••
6
] Abstract ThisLetter presentsanew,remarkablysim- to compare metallicity-sensitive optical emission-lines
A
ple diagnostic specifically designed to derive chemical for ensembles of galaxies at different redshifts. These
G abundances for high redshift galaxies. It uses only the investigations have led to new insights into the rela-
. Hα, [N II]and [S II] emission lines, which can usually tionship between galaxy mass and metallicity at high-
h
p be observed in a single gating stetting, and is almost z (Zahid et al. 2013b,a; Izotov et al. 2015, and refer-
- linear up to an abundance of 12+log(O/H) = 9.05 . ences therein) and new insights into the relationship
o
It can be used over the full abundance range encoun- between galaxy mass, metallicity, and star formation
r
t tered in high redshift galaxies. By its use of emission rate at high-z (Wuyts et al. 2014; Sanders et al. 2015;
s
a lines located close together in wavelength, it is also in- Maier et al. 2015; Salim et al. 2015; de los Reyes et al.
[ dependent of reddening. Our diagnostic depends criti- 2015, and references therein).
1 cally on the calibration of the N/O ratio. However, by Regrettably, such abundance studies have been
v using realistic stellar atmospheres combined with the plagued by both observational and measurement un-
7 N/O vs. O/H abundance calibration derived locally
certainties. The commonly used metallicity calibra-
3
from stars and H II regions, and allowing for the fact
3 tions may disagree by more than an order of mag-
that high-redshift H II regions have both high ionisa-
1 nitude (Kewley et al. 2006). This discrepancy has
0 tion parameters and high gas pressures, we find that many potential causes, including calibration errors re-
. theobservationsofhigh-redshiftgalaxiescanbesimply
1 sulting from the use of local H II regions relying
0 explainedbythe modelswithouthavingtoinvokearbi- on direct measurements of electron temperature Te
6 trary changes in N/O ratio, or the presence of unusual
(Pilyugin & Thuan 2005; Pilyugin & Mattsson 2011),
1 quantities of Wolf-Rayet stars in these galaxies.
: the use of hybrid approaches (van Zee et al. 1998), re-
v
lying only on strong emission line methods (McGaugh
i
X 1 Introduction 1991), or simply by directly calibrating the theoretical
r photoionization models (López-Sánchez et al. 2012).
a
The chemical history of the universe provides a fossil Current high-z metallicity calibrations assume that
record of the generations of star formation in galax- the ISM conditions, such as the ISM pressure and ion-
ies, modulated by both inflow of pristine gas and by ization parameter, are similar to those found in lo-
galactic-scale gas outflows. The most commonly used cal galaxies. These diagnostics are either based on
methodtoprobethechemicalhistoryofthe universeis samples of local H II regions (Denicoló et al. 2002;
Pettini & Pagel2004; Pérez-Monteroet al2009),local
galaxies(Marino et al. 2013; Morales-Luis et al. 2014),
MichaelA.Dopita,LisaJ.Kewley,
RalphS.Sutherland, &DavidC.Nicholls orphotoionizationmodels basedonlocalgalaxycondi-
1ResearchSchoolofAstronomyandAstrophysics,AustralianNa- tions (Kewley & Dopita 2002; Blanc et al. 2015).
tionalUniversity,Cotter Rd.,WestonACT2611, Australia However, it is now clear that the conditions in the
2Astronomy Department, King Abdulaziz University, P.O. Box ISM of galaxies evolve with redshift. Galaxies at high
80203, Jeddah,SaudiArabia
redshift have substantially larger [OIII]/Hβ line ratios
3Institute for Astronomy, Universityof Hawaii, 2680 Woodlawn at high redshift (z ∼ 3) than the present day (e.g.,
Drive,Honolulu,HI96822, USA
Hainline et al.2009; Bian et al.2010;Yabe et al.2012;
Kewley et al. 2013b; Holden et al. 2014; Steidel et al.
2
2014, and references therein). It is likely that a change 2013), and includes numerous upgrades to both the in-
in both ionization parameter and in the ISM pres- put atomic physics and the methods of solution.
sureisresponsibleforthisdifference(Brinchmann et al. ThespectrumofanHII regiondependsuponanum-
2008; Liu et al. 2008; Hainline et al. 2009; Bian et al. beroffundamentalparameters. Ofcoursethechemical
2010; Nakajima et al. 2012; Kewley et al. 2013a). Re- abundance set is the most important amongst these,
cently, Kewley et al. (2015) showed that the ionization and since the cooling of the H IIregion is moderated
parameter changes systematically in galaxies between by the gas-phase abundances, it is important to have
0<z <0.6, causing a rise in the [OIII]/Hβ ratio, and a reasonable model for the dust grain content of the
afallinthe[NII]/Hαratio. Therefore,iscrucialtouse H IIregions. The dust grains are also very important
a metallicity calibrationthat either takes these param- in determining the photoelectric heating of the plasma
etersintoaccount,orideallyisindependentofboththe (Dopita & Sutherland2000). Theexcitationoftheneb-
ionization parameter and the pressure in the ISM. ula was generally thought to be moderated by three
Recently Masters et al. (2014) used the Magellan parameters, the ionisation parameter logU, the shape
FIRE instrument to observe a sample of z ∼ 2 galax- of the cluster EUV spectrum, and the pressure in the
ies in which all of the important diagnostic lines are ionised plasma. However, Dopita et al. (2014) showed
observed, and Shapley et al. (2015) has extended this that the pressure in the ionised gas is also impotent.
sample using galaxiesdrawnfromthe MOSFIRE Deep Here, for the first time, we systematically investigate
Evolution Field (MOSDEF) Survey. Both of these au- the effect of this pressure parameter on the strong-line
thors find that their data points are offset in the BPT spectra of H II regions, and demonstrate that this pa-
diagramtowardhigher[OIII]/Hβforagiven[NII]/Hα, rameter provides a “missing link” which finally allows
as had already been noted in z ∼1 star-forminggalax- us to explain the peculiarities of the spectra of high-
ies. However,composite spectra derived from the sam- redshift galaxies.
ples do not show a corresponding offset from the local Recently, Nicholls et al. (2012) have suggested that
star-forming sequence on the [O III]/Hβ vs. [S II]/Hα the electrons in H II regions are characterised by
diagram. Both of these authors interpret this result in a κ−distribution in energy, rather than by a simple
terms of an enhanced population of Wolf-Rayet stars Maxwell-Boltzmann (M-B) distribution. The effect of
at high redshift, leading to an enhancement in the rel- thisontheemissionlinespectraofHII regionswassys-
ative N abundance, and a much hotter ionising spec- tematically investigated by Dopita et al. (2014). Here
trum(modelledbyShapley et al.(2015)asahotBlack wepresentgridswiththeM-Bdistributiononly,butwe
Body). We will critically examine this hypothesis in have also computed the effects of a κ−distribution. As
thisLetter,andshowthatithasasimplerexplanation. we show in Section 4, this makes almost no difference
Here,wepresenta newmetallicitycalibrationwhich to our new abundance calibration presented here.
relies solely on the red Hα, [N II], and [S II] emission- In these models we have adopted the local galac-
lines. Thismetallicitycalibrationiseffectivelyindepen- tic concordance (LGC) abundances based upon the
dent of both ionization parameter and ISM pressure, Nieva & Przybilla (2012) data on early B-star data.
and is valid over the full range of abundance encoun- These have the advantage that they sample the abun-
tered in high redshift galaxies. The line ratios used do dances in the local region of the galaxy (out to 500
notrequireeither flux calibration,orextinction correc- pc), providing the current abundances in this region.
tion. We anticipatethatthiscalibrationwillbe critical Nieva & Przybilla(2012)gavetheN/OandC/Oratios
for high-z metallicity studies where different lines usu- in the range 5.9<12+logO/H<9.0. The determina-
ally have to be observed in different bands, and where tion of these ratios is vital to photoionisation models,
the ionizationparameterand/orthe ISM pressuremay since N and C are important coolants in the nebula,
be significantly different from those in local galaxies. are both in part secondary nucleosynthesis elements,
and are consequently difficult to directly calibrate. In
particular, C is only readily observable in the UV, and
2 Models maybe lockedupindust (whichcannotexistinB-star
atmospheres). The Nieva & Przybilla (2012) data pro-
WehaveusedtheMappings 5.0code(Sutherlandetal. videstheabundancesofthemaincoolants,H,He,C,N.
2015,in prep.) 1 to constructa gridofphotoionisation O,Ne,Mg,SiandFe. Forthelightelementsweusethe
models for HII regions. This code is the latest version Lodders & Palme(2009)abundance,whileforallother
of the Mappings 4.0 code described in (Dopita et al. elements the abundances are based upon Scott et al.
(2015a,b) and Grevesse et al. (2010). Thus, in the
LGC scale, the “local region” reference abundance has
1Availableatmiocene.anu.edu.au/Mappings
Chemicalabundances inhigh-redshiftgalaxies:Apowerfulnewemissionlinediagnostic 3
12+log(O/H)=8.77,asopposedtotheGrevesse et al. H IIregions at 3.0, 2.0, 1.0, 0.5, 0.3, 0.2 and 0,1 times
(2010) solar value of 12+log(O/H)=8.69. this “local region” reference abundance, and covering
Since the calibration presented in this Letter de- an ionisation parameter at the inner edge of the neb-
pends critically on the assumed calibration of N/O vs. ula of −3.5 < logU < −2.0 in steps of 0.25. Be-
O/H,wehavepresentedthecalibrationweusedinFig- cause the pressure in the H II region is an impor-
ure 1, takeen from Nicholls et al. (2015). This calibra- tant parameter which changes the excitation in the
tion is based on a mixture of both stellar and neb- nebula in a similar way the the ionisation parameter
ular sources (Izotov et al. 1999; Israelian et al. 2004; (Dopita et al. 2014), we have computed grids at four
Spite et al. 2005; Nieva & Przybilla 2012). The global pressures, logP/k =5.2,5.7,6.2 and 6.7cm−3K, which
enrichment pattern can be represented by mixture of cover the full range commonly encountered in the HII
primary and secondary nucleosythesis, as can be seen regionsofnormalgalaxiesandstarburstgalaxiesinthe
clearlyinFigure1. Thispointwillbediscussedinmore local universe (Dopita et al. 2014). At high redshifts,
detail in Section 4.2. highvaluesofpressurealsoseemtobeappropriate. For
example, Masters et al. (2014) reports electron densi-
ties in their z ∼ 2 sample of 100 . ne . 400, which
corresponds to 6.2.logP/k .6.7cm−3K.
log(N/O) vs 12+log(O/H)
-0.5
b)
e
v(n 3 Results
o
ot
va Iz -1.0 It is very difficult to obtain meaningful abundances
e
Ni
n using a single line ratio of a forbidden line and a
elia -1.5 Hydrogen recombination line, since all such ratios
a
e Isr are two-valued in abundance space, and are grossly
Spit -2.0 primary affected by both ionisation parameter and pressure.
O) Even with two emission line ratios it is difficult, since
N/
g( the grids of models tend to fold up on themselves
lo -2.5 (Veilleux & Osterbrock1987;Dopita et al.2013). Gen-
erallyspeaking,thestrategyherehasbeentosearchfor
secondary
a line ratio which depends principally on abundance,
6.0 7.0 8.0 9.0 and a second which is sensitive to the excitation, for
12+log(O/H) example McGaugh (1991); Kewley & Dopita (2002);
Kobulnicky & Kewley (2004). Frequently, however the
linesusedarefarapartinwavelength,reddeningcorrec-
Fig. 1.— The calibration of N/O vs. O/H used in this tions large, and all lines required may not be available
paper. The data points are derived from the follow- to be observed in the case of high-redshift galaxies.
ing papers: Izotov et al. (1999); Israelian et al. (2004); Vogt et al. (2014) demonstrated the utility of clas-
Spite et al. (2005) and Nieva & Przybilla (2012) sification of galaxies using 3D emission line ratio di-
agrams, and this is the approach we have adopted
here. Specifically, we have searched amongst the set
The depletion factors of the heavy elements onto
of commonly-used strong line ratios, choosing 3 sets
dust are derived from the formulae of Jenkins (2009),
extended to the other elements on the basis of conden- of line ratios which are either known to be excitation
dependent, or else more abundance-sensitive. We also
sationtemperaturesand/orpositionontheperiodicta-
applied the criterion that the emission line ratios used
ble. We have investigatedthe effect of changing the Fe
depletion,logDFe(intherange−1.0>logDFe >−2.5, be close together in wavelength to eliminate, as far as
possible, the effects of differential reddening. We then
and have ascertained that it makes no difference to
abundancediagnosticsatsolarabundance,butmayin- rotatedthetheoreticalgridsinpitch,yawandrollinor-
troduceanuncertaintyof±0.12dex. attheextremities der to separate the abundance sensitivity into one axis
ofprojection,andtheexcitationsensitivityintheother
of the abundance range computed here.
FortheEUVclusterspectraweusetheStarburst99 axis.
models (Leitherer et al. 1999) interpolated in abun- Wediscoveredthattheuseofthe[NII]λ6484/Hαthe
dance as described in (Dopita et al. 2013), and for [N II]λ6484/[S II]λλ6717,31 and the [O III]λ5007/Hβ
line ratios provided an excellent result. The first two
these we have computed grids of spherical, isobaric
4
ratios are abundance sensitive, and the third is more
sensitivetoexcitation. TheresultisshowninFigure2, log U
-2.5-2.0
wherethe y−axisgivestheabundance(withverysmall 0.50 -3.0
-3.5
residualsensitivitytoeitherlogU orlogP/k,whilethe
9.24
x−axis demonstrates sensitivity to both ionisation pa-
9.06
rameter and pressure.
0.00
In Figure 2, apart from the high-redshift (mean)
8.76
pointsfromtheMasters et al.(2014)andShapley et al.
(2015) samples, we have used the observations of indi- 8.46
vchidouseanlHtoIuIsreegthioenvsafnroZmeevasnamZpeleeertatahl.e(r1t9h9a8n).thWeeShDaSvSe -0.50 12 + log( 8.24
O/
nucleardatatoavoidtheapertureeffects,the inclusion H) 8.06
log P/k = 6.7
of diffuse emission, and the bias towards high abun- log P/k = 6.2
-1.00
dance inherent in the SDSS sample. In addition, the log P/k = 5.7 7.96
log P/k = 5.2
van Zee sample represents a homogenenous sample of
integral H II region spectra, observed and reduced in -0.50 0.00 0.50 1.00 1.50 2.00
the same way, with the same instrument. This min-
imisessystematicerrors. Inaddition,itprovidescondi-
tions in the ionised gas which are more comparable to
Fig. 2.— The 3D projection of the [NII]λ6484/Hαthe
the high-redshift galaxies because:
[N II]λ6484/[S II]λλ6717,31 and the [O III]λ5007/Hβ
1. It samples HII regions across the faces of galax- line ratios which cleanly separates the abundance
ies, ensuring that a wide range of metallicities (y−axis) from the effects of pressure and ionisation
and ionisation parameters are sampled. In par- pressure(x−axis). Forcomparisonwiththetheoretical
ticular, HII regions with abundances similar to grid, the observations of van Zee et al. (1998) of indi-
the high-redshift sample are well-represented. vidual H IIregions in local spiral galaxies are shown
as grey circles, and for high-redshift galaxies we plot
2. Byits choiceofhighsurfacebrightnessandhigh the z ∼ 2 MOSDEF stacks from Shapley et al. (2015)
luminosity HII regions, the van Zee sample is (stars), and the z ∼ 2 composite spectrum from the
biased towards selecting HII regions with both MagellanFIREsurveyfromMasters et al.(2014)(filled
highlocalspecificstarformationrates,andwith blackcircle). Thesedatapointsaregenerallyconsistent
highISMpressure,bothofwhicharebelievedto with sub-solar metallicity, high pressure, and high ion-
apply to high-redshift galaxies. isation parameter.
From Figure 2 it is evident that only the line ratios
[N II]λ6484/Hα the [N II]λ6484/[S II]λλ6717,31 need modelsthemselves,includingtheuncertaintyinthede-
to be used to obtain a good estimate of the chemical pletion factor discussed above). With:
abundance. Our use of only the red lines allows ob-
y =log[NII]/[SII]+0.264log[NII]/Hα, (1)
servers to effectively ignore reddening corrections. In-
deed, these lines may be the only ones available to be 12+log(O/H)=8.77+y (2)
observed at certain red-shifts.
This linear fit is shown on Figure 2. If so desired, a
This emission line combination has notable advan-
somewhat improved fit can be got by adding a 5th.
tages over the calibration of Pettini & Pagel (2004),
order correction term:
which uses the [O III]λ5007/Hβ and [N II]λ6484/Hα
line ratios, through the compound so-called O3N2 ra- 5
12+log(O/H)=8.77+y+0.45(y+0.3) , (3)
tiointroducedbyAlloin et al.(1979). Forhigh-redshift
galaxies, the use of this requires observations in two
which is shown as the curved line on Figure 3. How-
wavelength bands, and is strongly affected by both
ever,othererrorsbothobservationalandtheoreticalare
the pressure and ionisation parameter issues discussed
likely to mask the effect of such a correction.
above.
Forthesetwolineratiosthecalibrationtoabundance
isparticularlysimple. ThisisshowngraphicallyinFig-
ure 3. A linear fit is good up to 12+log(O/H)∼9.05
(within the limits imposed by the uncertainties in the
Chemicalabundances inhigh-redshiftgalaxies:Apowerfulnewemissionlinediagnostic 5
Fishlock et al. (2014). Recently Nicholls et al. (2015)
0.5 log P/k = 6.2 has examined the calibration of the N/O vs. O/H cal-
log P/k = 5.2 ibration using both H II regions and results from old
stars in the Galaxy (galactic archeology). These au-
(cid:95)
H
g[N II]/ 0.0 thhaolorsmuesteadl-pdoaotraufnrommixeSdpigteiaenttsa,l.Fa(b20b0ia5n)deetriavle.d(2f0r0o9m)
o
4 l from halo solar type dwarfs and sub-giants, and from
6
2
0. Nieva & Przybilla(2012)forthe localBstars. Inaddi-
+
N II]/[S II] -0.5 tieiosnb,ytIhzeoytouvseedtdaal.ta(1d9e9r9i)v.edTfhreosmeaBultuheoCrsopmopinatctouGtatlhaaxt-
g[ thereislittleevidencefordustintheseobjects,and,by
o
l implication, that there is relatively little oxygen or ni-
trogen depletion into dust. Together, these data are
-1.0
remarkably consistent, and provide the tight calibrate
7.5 8.0 8.5 9.0
whichwehaveusedinthispaper. Thiscalibrationisin
12+log(O/H)
fairagreementwiththatof Pérez-Montero et al(2013)
for intermediate-redshift galaxies which, however, dis-
Fig. 3.— Calibration of the [N II]/[S II]and [N II]/Hα play a broader scatter in N/O at the low-abundance
emissionline ratiocombinationagainst12+log(O/H). end.
The red group of points represent the models having Sulphur, like Oxygen is an α−process element, and
−3
logP/k = 6.2cm K, while the blue points are for bothareenrichedinasimilarwayandonasimilartime-
logP/k =5.2cm−3K.The lines shownarethe best lin- frame. Because the time delay required to transition
ear fits and 5th. order fits as described in the text. from primary to secondary Nitrogen production, the
N/OratiomaybeaffectedbyscatteratagivenO/Hra-
tio,dependingontherateofstarformationenrichment
4 Discussion
rate, and the mass of the parent galaxy (Pettini et al
2002; Pérez-Monteroet al 2009, 2013).
4.1 The new diagnostic
TherecentobservationsbyMasters et al.(2014)and
Shapley et al. (2015) have cast some doubt on the va-
From an investigation of triplets of emission line ra-
lidityoftheN/Ovs. O/Hcalibration(althoughregret-
tios in 3D, we have discovered a simple, linear diag-
tably they did not give the actual scaling they used in
nostic with a wide dynamic range which uses only the
theirmodels). Theseauthorsfindthat[OIII]/Hβ is el-
red lines; Hα, [N II]λ6484 and the [S II] doublet at
evatedabove whatcan simply be explainedin terms of
λλ6717,31 to determine the O/H ratio. This should
an elevated ionisation parameter. Furthermore, on the
proveveryvaluableinthe investigationofthe chemical
Phillips Baldwin and Terlevich (BPT) diagrams they
evolution of the Universe, as frequently these lines are
find that [NII]/Hα is offset towards higher values rela-
the only ones observable in high redshift galaxies, all
tive to [S II]/Hα. They tentatively identify this as the
can be observed together with a single instrument set-
effect of hot Wolf-Rayet stars with high effective tem-
ting, andreddeningcorrectionsaresufficientlysmallto
be neglected. peratures (Te ∼ 80000 K), required to produce high
[O III]/Hβ ) and which enrich the surrounding ISM
with N (producing the high observed [NII]/Hα).
4.2 Calibration of the N/O ratio
Is this hypothesis correct? A major problem with
it is that the important effects of pressure on the H II
Because both of the ratios used are dependent on the
region spectrum is not taken into account. This has a
Nitrogen abundance, clearly our calibration is strongly
strong influence on the emission line ratios used in the
dependent on the correct calibration of the N/O ra-
BPTdiagrams. Thiseffectwasinitiallydiscussedinthe
tio with O/H. In particular, Nitrogen behaves in part
context of starburst galaxies by Dopita et al. (2014),
as a secondary nucleosynthetic element because at low
who showed that pressure had a similar effect to ioni-
abundance N is promptly enriched as a result of CN
sation parameter, at least at moderate pressures. The
processingandmass-lossinmassivestars,whileatlater
models presented here are the first which investigate
epochs it is more produced by hot-bottom burning
the effect of interstellar pressure up to the range ob-
in intermediate-mass stars. For low-abundance stars
(appropriate to the high-redshift regime) the effect of served in these high-redshift galaxies (100.ne .400,
or 6.2 . logP/k . 6.7cm−3K. This has two principal
hot-bottom burning has recently been investigated by
6
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Chemicalabundances inhigh-redshiftgalaxies:Apowerfulnewemissionlinediagnostic 7
-2.0 -2.0
-2.5 -2.5
log U
-3.0 -3.0
log U
8.24 8.46 8.248.46
-3.57.76 128+.0lo6g(O/H) 1 2 +lo g(-O3/.5H)7.76 8.06
6.7 6.7
6.2 6.2
5.7 5.7
log P/k = 5.2 log P/k = 5.2
Fig. 4.— BPT diagrams showing the effect of pressure on the shape of the theoretical grids. The meaning of
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