Balmer jump temperature determination in a large sample of low-metallicity H II regions

3.3 MB·English

by  N. G. Guseva

Astronomy&Astrophysicsmanuscriptno.6067 (cid:13)c ESO2008
February5,2008
Balmer jump temperature determination in a large sample of
 ⋆,⋆⋆
low-metallicity H regions
N.G.Guseva1,Y.I.Izotov1,P.Papaderos2,3,andK.J.Fricke3
1 MainAstronomicalObservatory,UkrainianNationalAcademyofSciences,Zabolotnoho27,Kyiv03680,Ukraine
2 InstitutodeAstrof´ısicadeAndaluc´ıa,Apdo.3004,18080Granada,Spain
3 InstituteforAstrophysics,Friedrich-Hund-Platz1,37077Go¨ttingen,Germany
7
0 Received ;Accepted
0
2 ABSTRACT

n
a Aims.ContinuingthesystematicdeterminationoftheelectrontemperatureofHregionsusingtheBalmerand/orPaschendisconti-
J nuitiesbyGusevaetal.(2006)wefocushereon3.6mESOtelescopeobservationsofalargenewsampleof69Hregionsin45blue
2 compactdwarf(BCD)galaxies.Thisdatasetspansawiderangeinmetallicity(Z⊙/60<∼Z <∼Z⊙/3)and,combinedwiththesample
of47HregionsfromGusevaetal.(2006),yieldsthelargestspectroscopicdataseteverusedtoderivetheelectrontemperaturein

theH+zone.
1
Methods.InthesamewayasinGusevaetal.(2006)wehaveusedaMonteCarlotechniquetovaryfreeparametersandtocalculatea
v
seriesofmodelspectralenergydistributions(SEDs)foreachHregion.TheelectrontemperatureintheH+zoneswasderivedfrom
2
thebestfittingsyntheticandobservedSEDsinthewavelengthrange∼3200–5100Å,whichincludestheBalmerjump.
3
0 Results.Onthebaseofthepresentlargespectroscopicsamplewefindthatinhot(Te(H+)>∼11000K)Hregionsthetemperature
of theO2+ zone, determined fromdoubly ionised oxygen forbidden lines, does not differ statisticallyfrom the temperature of the
1
H+ zone. Thus, weconfirmand strengthen thefinding by Guseva et al. (2006). Weemphasize that due toa number of modelling
0
assumptionsandtheobservationaluncertaintiesforindividualobjects,onlyalarge,homogeneoussample,astheoneusedhere,can
7
enableaconclusivestudyoftherelationbetweenT (H+)andT (O).
0 e e
/ Keywords.galaxies:irregular—galaxies:starburst—galaxies:ISM—galaxies:abundances
h
p
-
o 1. Introduction atureofarelativelylargesampleoflow-metallicityHregions
r
t usingtheBalmerandPaschenjumps.
s Along-standingprobleminthestudyofHregionsisconnected
a
tothefactthattheelectrontemperaturesderivedfromthehydro- Differences between T (H+) and T (O ) were first dis-
: e e
v gen Balmer and Paschen discontinuity or from the UV helium cussed by Peimbert (1967), who introduceda conceptof tem-
i
X discontinuity are systematically lower than those derived from perature fluctuations in the nebulae. To quantify temperature
the collisionallyexcitedoptical[O ] lineswhichare mostof- fluctuationsheusedtheparametert2,themeansquaretempera-
r
a ten used for the temperature determination (e.g. see review in turevariation.Itwasshowninmanystudiesofplanetarynebu-
O’Delletal.,2003). laeandHregions(e.g. Peimbertetal.,1995;Estebanetal.,
1998,1999,2002;Liuetal.,2000,2001)thattheheavyelement
Electron temperature determinations based both on colli-
abundancesderivedfromthe recombinationlines and from the
sionallyexcitedlinediagnosticsandontheBalmerandPaschen
collisionallyexcitedlinesareconsistentift2 isintherange0.02
jumps have been mainly applied to planetary nebulae (PNe)
–0.10.Ontheotherhand,typicalvaluesoft2forphotoionization
(e.g. Liuetal., 2000; Luoetal., 2001; Ruizetal., 2003;
models of chemically and spatially homogeneous nebulae are
Zhangetal.,2004;Peimbertetal.,2004;Wessonetal.,2005)
and to nearly solar-metallicity H  regions in the Milky Way significantlylower,t2 = 0.00–0.02(e.g. Kingdon&Ferland,
1995;Pe´rez,1997).
and several nearby galaxies with relatively high metallici-
ties of 1/3 – 1/10 solar (e.g. Peimbert&Torres-Peimbert,
Inrecentyearspoint-to-pointmeasurementsofthe electron
1992; Peimbertetal., 1993, 2000; Peimbert, 2003;
temperature fluctuations have been done in several PNe and H
Estebanetal., 1998; Garc´ıa-Rojasetal., 2004, 2005, 2006;
 regions (Liu, 1998; Rubinetal., 2002; Krabbe&Copetti,
Gonza´lez-Delgadoetal., 1994). A common result of these
2005; Rubinetal., 2003; O’Delletal., 2003). Only low am-
studies is that, generally, T (H+) is lower than T (O ). Only
e e plitude temperature variations were found across the nebulae.
recentlyGusevaetal. (2006)havederivedtheelectrontemper-
This is probably because the projected parameter t2 derived in
s
the point-to-pointmeasurements of the electron temperature is
differentfromthetotalparametert2.Copetti (2006)hasshown
Sendoffprintrequeststo:N.G.Guseva,[email protected]
⋆ Based on observations collected at the European Southern thatt2s givesonlyalowlimitoft2.Additionally,therearemany
Observatory,Chile,ESOprogram76.B-0739. difficulties to obtain the temperature maps of the whole nebu-
⋆⋆ Tables 3 and 4 are only available in electronic form at the laewhenimagesandspectraobtainedwithdifferentspace-born
CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via and ground-based telescopes are combined (Luridianaetal.,
http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ 2003a).
2 N.G.Gusevaetal.:Balmerjumptemperatureinlow-metallicityHregions
Thus, the observationaldata for PNe and H  regions with much improved statistics, we took new spectra of H  regions
relatively low electron temperatures ≤11000K certainly show withthe3.6mESOtelescope.Thesedataarecombinedwiththe
systematic differencesbetweenthe temperaturesobtainedfrom onesobtainedbyGusevaetal. (2006).
thecollisionallyexcitedlinesandfromtheBalmerandPaschen InSection2,wedescribetheobservationsanddatareduction
jumps (see Fig.12 in Gusevaetal., 2006, for the data col- ofthenewspectroscopicdata.InSection3wecompareT (H+)
e
lectedfromliterature).On theotherhand,nosignificantdiffer- andT (O)forthenewBCDsampleandthosefromliterature.
e
encesbetween T (H+) and T (O ) were foundfor the higher- OurconclusionsaresummarizedinSection4.
e e
temperatureHregions(Gusevaetal.,2006).Thisconclusion
is in agreement with the results obtained by Peimbertetal.
(2002), who compared T (O ) and the electron temperature 2. Observationsanddatareduction
e
T (He+) in the He+ zone. Adoptingt2 = 0.01– 0.04,these au-
e The sample observed with the 3.6m ESO telescope consists of
thors found the temperature differencesof (1 – 3)% in H  re- 69low-metallicityHregionsin45BCDs.Theywereselected
gions with T (O ) = 20000K and of (3 – 12)% in H  re-
e mainly from the literature. Particularly, two galaxies were se-
gions with T (O ) = 10000K. As T (He+) < T (H+) in PNe
e e e lectedfromtheData Release 2ofthe Six-DegreeField Galaxy
(Zhangetal., 2005) and T (He+) ≈ T (H+) in H  regions
e e Redshift Survey (6dFGRS) (Jonesetal., 2005) and eighteen
(Garc´ıa-Rojasetal., 2005), the differences between T (O )
e galaxies were selected from the Data Release 4 of the Sloan
and T (H+) are expected to be not greater than (1 – 3)% in H
e Digital Sky Survey (SDSS) (Adelman-McCarthyetal., 2006).
regionswiththetemperaturesT (O)∼20000K.
e The BCDs were chosen to span a large range of oxygenabun-
Different mechanismsare proposed to explain the observa-
dance, from about 1/60 to about 1/3 that of the Sun. Thus, we
tionaldifferencesinT .Inparticular,densityandchemicalabun-
e canstudythedependenceoftemperaturevariationswithmetal-
dance inhomogeneities, ionization of nebulae by low-energy licity. Since the electron temperature of a H  region depends
cosmic rays are considered (e.g. Torres-Peimbertetal., 1990; on its metallicity, H  regions in our BCD sample span also a
Viegas&Clegg, 1994; Liuetal., 2000; Tsamisetal., 2004;
large range of electron temperatures. The equivalent width of
Tsamis&Pe´quignot, 2005; Giammanco&Beckman, 2005).
theHβline,EW(Hβ),isameasureoftherelativecontributionof
However, no definite conclusions have been made concerning
theionisedgasemissiontothetotallight.Theselectedgalaxies
themainmechanismforthesedifferences.
coveralargerangeofEW(Hβ)from10Åto382Å.
TheknowledgeofdifferencesinT isimportantfortheele-
e All spectra were obtained with the EFOSC2 (ESO Faint
mentabundancedetermination.Theheavyelementabundances
Object Spectrograph and Camera) mounted at the 3.6m ESO
are usually derived assuming that the temperature of the H+
telescope at La Silla in two observing runs, the first one, dur-
zone is equal to the temperature of the O2+ zone. If instead,
ingApril11–14,2005andthesecondone,duringOctober7–
T (H+) is smaller than T (O ), then the heavy element abun-
e e 9, 2005.The observingconditionswere photometricduringall
danceswouldbeincreased.Forthedeterminationoftheprimor-
nights.Forthespringobservationsweusedthegrism#14andthe
dial He abundance from spectra of low-metallicity BCDs (e.g.
grating600gr/mm.Thelongslitwith1′′×300′′wascenteredon
Izotov&Thuan,2004)theknowledgeofthetemperaturestruc-
the brightestpartofeach galaxy.Theaboveinstrumentalsetup
ture of a H  region is especially important because the high
gave a wavelength coverage of λλ3200–5083, a spectral reso-
precisionof1 –2 %isrequiredfortheHe abundance.Thusto
lutionof ∼6.2 Å (FWHM) anda spatialscale of 0′.′314pixel−1
estimatethesystematicerroroftheprimordialHeabundanceitis
alongtheslitfortheused2×2pixelbinning.
crucialtoinvestigatewhethertemperaturedifferencesareasim-
During the fall observations the grism #07 and the grat-
portantin low-metallicityBCDs as they are in high-metallicity
HregionsandPNe. ing 600 gr/mm were used, resulting in a wavelength coverage
of λλ3250–5200. These observations were carried out with a
Gusevaetal. (2006)havedeterminedtheelectrontempera-
ture of H+ zones from the Balmer jump in 23 H  regionsand 1′.′2×300′′slitcenteredonthebrightestpartofeachgalaxy.The
fromthePaschenjumpin24Hregionsinthemetallicityrange spectralresolutionandspatialscalealongthisslitwere∼6.2Å
(FWHM)and0′.′157pixel−1,respectively.
from1/3to1/60ofsolar,based,respectively,onMultipleMirror
Our sample galaxies were mostly observed at low airmass
Telescope (MMT) and Sloan Digital Sky Survey (SDSS) data.
<1.2. Spectroscopicobservationsat a higherairmass were car-
They used Monte Carlo simulations, varying the electron tem-
peratureintheH+zone,theextinctionoftheionisedgasandthat riedoutalongtheparallacticangle.Thus,nocorrectionsforat-
mosphericrefractionhavebeenapplied.Thetotalexposuretime
ofthestellarpopulation,therelativecontributionoftheionised
of typically40– 60minutespergalaxywas splitup into 2 – 3
gastothetotalemissionandthestarformationhistorytofitthe
subexposures to allow for a more efficient rejection of cosmic
spectral energy distribution (SED) of the galaxies in the large
rayhits.Threespectrophotometricstandardstarswereobserved
wavelengthrange,whichincludestheBalmerandPaschendis-
duringeachnightforfluxcalibration.Thejournaloftheobser-
continuities.Thebestsetsoffreeparametershavebeenobtained
vationsisgiveninTable1.
from the minimizationof the deviationsbetween observedand
modelledSEDs. Itwas foundthatthe temperaturesT (O) of ThedatareductionwascarriedoutwiththeIRAF1 software
e
the O2+ zonesdeterminedfrom the nebular to auroralline flux package. This includes bias subtraction, flat–field correction,
ratio of doublyionised oxygen[O ] λ(4959+5007)/λ4363do cosmic-ray removal, wavelength calibration, night sky back-
notdiffer,inastatisticalsense,fromthetemperaturesT (H+)of ground subtraction, correction for atmospheric extinction and
e
the H+ zonesdeterminedfrom Balmer and Paschen jumps. On absolute flux calibrationof the two–dimensionalspectrum.For
each night the sensitivity curve was derived from averaging of
theotherhand,Gusevaetal. (2006)haveemphasizedthat,due
threestandardstars.
to large observationaluncertaintiesand modellingassumptions
forindividualobjects,onlya statistical studyofa largesample
1 IRAFistheImageReductionandAnalysisFacilitydistributedby
ofHregionscanallowfordefiniteconclusions.
theNationalOpticalAstronomyObservatory,whichisoperatedbythe
Thereforein orderto studytheTe(O) vs.Te(H+) relation AssociationofUniversitiesforResearchinAstronomy(AURA)under
overalargemetallicityrange,betweenZ⊙/60andZ⊙/3,andwith cooperativeagreementwiththeNationalScienceFoundation(NSF).
N.G.Gusevaetal.:Balmerjumptemperatureinlow-metallicityHregions 3
temperatureT (O ). The latter was obtained from the relation
e
betweenT (O)andT (O)ofIzotovetal. (2006b).The3.6m
e e
spectracoveredonlythebluewavelengthregion,so thatthe [S
] λ6717,6731emission lines can notbe used for the electron
number density determination. Therefore we have adopted N
e
= 100 cm−3. The electron temperatures T (O ) and T (O )
e e
for the high- and low-ionisation zones in H  regions respec-
tively,theionisationcorrectionfactors(ICFs)andtheionicand
total heavy element abundances for oxygen, neon and iron are
given in Table 4 excluding data for 16 H  regions published
inthepapersbyIzotovetal. (2006a);Papaderosetal. (2006b)
andPapaderosetal.(2006,inpreparation).
Relevantparameterstothepurposeofthepapersuchasthe
equivalentwidthEW(Hβ)oftheHβemissionline, theelectron
temperature t (O ) of the O2+ zone, the electron temperature
e
t (H+) of the H+ zone, the oxygen abundance 12 + log O/H
e
Fig.1. Best fit model SED to the redshift- and extinction- andtheextinctioncoefficientC(Hβ),obtainedfromtheBalmer
corrected observed spectrum of UM 462c. The model SED is decrement,are collectedin Table2 forthe entiresampleofthe
calculatedassumingthattheHregionisionisation-bounded.It 69Hregionsobservedwiththe3.6mESOtelescope.
isseenthattheredpartofspectrumisfittedquitewellwhereas
the modelled SED underestimates the flux shortward of the
Balmerjump.Thisislikelyduetotheleakageofionisingpho- 3.2.ElectrontemperatureTe(H+)
tonsfromtheHregion.
To derive the electron temperature T (H+) we use the same
e
methodasthatdescribedindetailbyGusevaetal. (2006).The
One-dimensional spectra of the bright H  regions in each method is based on the determination of the electron temper-
ature T (H+) of H+ zones by fitting a series of model SEDs
galaxy were corrected for interstellar extinction using the red- e
to the observed SEDs and finding the best fits. Each fit is per-
dening curve by Whitford (1958) and for redshift, derived
formed over the whole observed spectral range shortward of
fromtheobservedwavelengthsoftheemissionlines.Redshift-
correctedspectraareshowninFig.3. ∼ λ(5100 – 5200)Å, which includes the Balmer jump region
Emission line fluxes were measured using Gaussian pro- (λ3646Å).BesidestheelectrontemperatureTe(H+)whichcon-
file fitting. The errors of the line flux measurements were cal- trols the magnitude of the Balmer jump, the shape of the SED
culated from the photon statistics of non-flux-calibrated spec- dependsonseveralotherparameters.AseachSEDisthesumof
tra. They have been propagated in the calculations of the ele- both stellar and ionised gas emission, its shape dependson the
mental abundance errors. The observed relative emission line relativebrightnessofthesetwocomponents.InBCDs,thecon-
fluxes F(λ)/F(Hβ) and fluxes I(λ)/I(Hβ) corrected for inter- tributionoftheionisedgasemissioncanbeverylarge.However,
stellar extinction and underlying stellar absorption, equivalent the EWs of hydrogen emission lines never attain the theoreti-
widthsEWsofemissionlines,extinctioncoefficientsC(Hβ),ob- cal values for pure ionized gas emission. This implies a non-
servedHβfluxesF(Hβ),andequivalentwidthsofthehydrogen negligible contribution of stellar emission in all sample galax-
absorptionlinesarelisted inTable3.Notallgalaxiesobserved ies.Wethereforeparametrizetherelativecontributionofgaseous
withthe3.6mtelescopeareincludedinTable3,butallofthem emissiontothestellaronebytheequivalentwidthEW(Hβ).
are usedin the subsequentanalysis.We excludedfromTable 3 The shape of the spectrumdependsalso on reddening.The
eight H  regions with an oxygen abundance of 12 + log O/H extinction coefficient for the ionised gas C(Hβ) has been ob-
≤7.6.Theemissionlinefluxes,electrontemperatures,ionicand tainedfromtheobservedhydrogenBalmerdecrement.Herewe
totalelementabundancesfortheseHregionsarepresentedin assumethatthedifferencesbetweentheobservedandtheoretical
Papaderosetal.(2006,inpreparation),aspartofadetailedspec- recombinationhydrogenBalmerlineratiosareonlyduetored-
troscopic and photometric study of extremely metal-deficient dening and underlying stellar absorption. However, additional
star-forming galaxies. We neither include in Table 3 the two source of differences, collisional excitation of hydrogen lines,
extremelymetal-pooremission-linegalaxiesSDSSJ2104–0035 couldplaysomeroleinthehotHregions(Stasin´ska&Izotov,
andJ0133+0052discoveredinSDSS.Allobservationaldatafor 2003; Luridiana,etal., 2003b). Collisional excitation may in-
thesegalaxieswerepublishedbyIzotovetal. (2006a).Finally, crease the flux of the Hα emission line by <∼ 5% above the re-
Table 3 does not include the extremely metal-deficient BCDs combinationvalue.Theeffectislessimportant,<∼2%,fortheHβ
SBS 0335–052Wand SBS 0335–052E. A spectroscopic study emissionline.Wefindthatignoringcollisionalexcitationofhy-
ofthesetwosystemsispresentedinPapaderosetal. (2006b). drogen lines does not change significantly the derived T (H+).
e
As the Hα line is not present in our spectra, we decreased by
2%thefluxofHβlinemeasuredinspectraofseveralhottestH
3. Results  regions from our sample. Such correction results in decreas-
3.1.ElectrontemperatureT (O)andelementabundances ing C(Hβ) by ∼ 0.07. Then the derived Te(H+) is not changed
e morethanby∼1%ascomparedtothecasewiththemeasured
TheelectrontemperatureT ,ionicandtotalheavyelementabun- fluxoftheHβline.Therefore,forthewholesamplewedecided
e
danceswere derived followingIzotovetal. (2006b). In partic- nottotakeintoaccountcollisionalexcitationofhydrogenlines
ular,fortheO2+ andNe2+ ionswe adoptthe temperatureT (O and use the C(Hβ) obtained from the measured hydrogen line
e
) derived from the [O ] λ4363/(λ4959 + λ5007) emission fluxes. We have no direct observational constraint for the red-
line ratio. The O+ and Fe++ abundanceswere derived with the deningforthestellarcomponent,whichcoulddifferfromC(Hβ).
4 N.G.Gusevaetal.:Balmerjumptemperatureinlow-metallicityHregions
Fig.2.(a)ExampleoftheparameterspaceexploredwiththeMonteCarlotechnique(2253solutionsoutof105 simulations)tofit
the spectrumofthe BCD UM 311.The parametersshownare the equivalentwidth ofthe Hβ emissionline EW(Hβ),the agesof
the old and youngstellar populations,t(old) and t(young),the mass ratio b of the old-to-youngstellar population,the extinction
coefficientforthestellaremissionC(Hβ) ,theextinctioncoefficientfortheionisedgasC(Hβ) ,andtheelectrontemperature
stars gas
t (H+)=10−4T (H+)intheH+zone.Theparameterσisanestimatorofthegoodnessofthefit.(b)and(c)arethesameas(a)except
e e
thatonlythe64and10bestMonteCarlorealizationsareshown.
Therefore,forsimplicity,weadoptincalculationsthatbothex- the EW(Hβ) expected for pure gaseous emission. The gaseous
tinctions C(Hβ) and C(Hβ) , respectively for the ionised continuum emission is calculated following Aller (1984) and
gas stars
gasemissionandthestellarcomponentemission,arerandomly includes hydrogen and helium free-bound, free-free, and two-
variedin thenarrowrangearoundC(Hβ).Finally,the SED de- photonemission.Inourmodelsitisalwayscalculatedwiththe
pendsonthestarformationhistoryoftheBCD. electrontemperatureT (H+)andwiththeHe/Habundanceratio
e
We have carried out a series of Monte Carlo simula- derived from the H  region spectrum. The observed emission
tions to reproduce the SED in each H  region of our sam- linescorrectedforreddeningandscaledusingtheabsoluteflux
ple. To calculate the contribution of stellar emission to the of the Hβ emission line were added to the calculated gaseous
SEDs, we have adopted a grid of the Padua stellar evo- continuum.
lution models by Girardietal. (2000)2 with heavy element Given that T (H+) is not necessarily equal to T (O ), we
e e
mass fractions Z = 0.0001, 0.0004,0.001, 0.004,0.008. Using chosetovaryitintherange(0.7–1.3)×T (O).Weadoptfor
e
these data we have calculated with the package PEGASE.2 therangeoftheallowedEW(Hβ)valuesbetween0.95and1.05
(Fioc&Rocca-Volmerange,1997)agridofinstantaneousburst times its nominal value. As for the extinction, we allow both
SEDsinawiderangeofages,from0.5Myrto15Gyr.Wehave C(Hβ) and C(Hβ) to vary in the range of (0.8 – 1.2) of
stars gas
adopteda stellar initialmassfunctionwith a Salpeter slope,an C(Hβ),obtainedfromtheBalmerdecrement.
uppermass limitof100 M⊙ anda lowermass limitof0.1 M⊙. We assumethattheyoungstellar populationwasformedin
Following Gusevaetal. (2006) we could approximate the star a recent burst of star formation with an age t(young) between
formationhistory in BCDs by two short bursts of differentage 0.3 and 10 Myr. For the age of the older stars t(old) we adopt
butequaldurationsandwith differentstrengths.Theoldstellar values between 10 Myr and 15 Gyr. The contribution of each
populationisusuallyapproximatedbycontinuousstarformation burst to the SED is defined by the ratio of the masses of stel-
(e.g. Gusevaetal.,2001,2003a,b,c).However,thecontribution larpopulationsformedrespectivelyintheoldandyoungbursts,
oftheoldstellarpopulationtothelightofthebrightHregions b = M(old)/M(young), which we vary between 0.01 and 100.
considered in this paper is always small, not exceeding a few For each H  region we computed a series of synthetic SEDs
percentin the optical range. Therefore,the approximationof a usingagridofPaduamodelswiththeheavyelementmassfrac-
short burst for the old stellar population is acceptable for our tion, that is closest to the heavy element mass fraction of the
modelling,asitwasshownbyGusevaetal. (2006).Thecontri- ionisedgas.Werun105MonteCarlomodelsforeachHregion
butionofgaseousemissiontothetotalemissionisscaledbythe varyingsimultaneouslyt(young),t(old),b,T (H+),C(Hβ) and
e gas
ratiooftheobservedequivalentwidthoftheHβemissionlineto C(Hβ) . The first four parameters are used to calculate the
stars
model EW(Hβ). Only those solutions were further considered
2 http://pleiadi.pd.astro.it. for the SED fitting in which the modelled EW(Hβ) falls in the
N.G.Gusevaetal.:Balmerjumptemperatureinlow-metallicityHregions 5
Fig.4. (a)Comparisonofthet (H+)derivedfromfittingtheBalmerjumpandtheSEDwiththet (O)=10−4T (O)derivedfrom
e e e
thenebulartoaurorallinefluxratio[O]λ(4959+5007)/λ4363.Ineachpanel,thedashedlinedenotesequaltemperatures,thesolid
lineisthelinearregressionobtainedbythelikelihoodmethod(Pressetal.,1992),dottedlinesdenote1σdispersionsofoursample
H  regionsaroundthe regressionline. Error barsare the rootmeansquare temperaturedeviationsfromthe mean of the 10 best
MonteCarlosimulations.3.6mdataareshownbylargefilledcircles.Twoobjects,Pox186andUM462c,whichcouldnotbefitted
byionisation-boundedmodelaredenotedbysquares.Foreachofthesetwogalaxiesweshowbyopensquaresthesolutionsfrom
the ionisation-boundedmodeland by filled squaresthe solutionsfromthe modelwith escapingLyc photons.The derivedt (H+)
e
fromtheBalmerjumpin23low-metallicityHregions(MMTdata)andthePaschenjumpin24low-metallicityHregions(SDSS
data)areshownbylargeandsmallopencircles,respectively(Gusevaetal.,2006).Forcomparisonwehavealsoplottedbydotsin
(a)thedataofLiuetal. (2004),Zhangetal. (2004),Wessonetal. (2005)andKrabbe&Copetti (2005)for58GalacticPNe.Stars
showthedataforH regionsfromEstebanetal. (1998),Garc´ıa-Rojasetal. (2004, 2006),Peimbert&Torres-Peimbert (1992),
Peimbertetal. (2000),Peimbert (2003)andGonza´lez-Delgadoetal. (1994).(b)Thispanelisidenticaltopanel(a)exceptthatonly
H regionswithsmalldispersionsfort (H+)(σ[t (H+)]/t (H+) <0.1)for3.6mdata(presentpaper)anddatafromGusevaetal.
e e e
(2006)(MMTandSDSSdata)areshown.(c)Thesameasin(a)butonlyHregionswithlargeequivalentwidthsandsmallerrors
fort (H+)andt (O)areincluded(σ[t (H+)]/t (H+)<0.1;σ[t (O)]/t (O)<0.013;EW(Hβ)>80Å).
e e e e e e
range 0.95 – 1.05 of the observed one. Typically, the number we choose to average. No systematic differences were found
of such solutionsis ∼ 10 – 100 times less than the total of 105 when the number of best fits varies between a few and a cou-
simulations. ple of ten (Gusevaetal., 2006). Hence, we adoptfor t (H+) =
e
10−4T (H+) the mean of the 10 best-fitting solutions weighted
In the same way as in Gusevaetal. (2006) we used the σ e
byσ.
statistic to quantifythe goodnessof each model’sfit to the ob-
WenotethatthebimodalσdistributionforsomeHregions
served SED. For each Monte Carlo realization we computed a
does notallow to derivea uniqueT (H+) in those objects. The
mean deviation σ between the observed and modelled spectra. e
averageT (H+)ofthe10best-fittingsolutionsfortheseHre-
Wecalculateineachspectrumtherootmeansquaredeviationσ e
j
gionscan significantly differ from the actualone. On the other
foreach of the five continuumregionsselected to be devoidof
hand,thedispersionoftheaverageT (H+)sinHregionswith
lineemissionandabsorption,asσj = qPiN=j1(foibs− fmiod)2/Nj, thebimodalσdistributionislargerefleectingtheuncertaintiesof
whereN isthenumberofthepointsineachparticularspectral the temperaturedetermination.Therefore,in the followingdis-
j
interval.Thenσ= σ /5. cussionwewillusedifferentsamples:thetotalsample(Fig.4a)
j
P and the samples with the sharp minimum in the σ distribution
AsshowninFig.2a,forallMonteCarlorealizationstheage
(Figs.4b,c).
t(old)andt(young)andmassratioboftheoldandyoungstellar
populations,andtheelectrontemperatureT (H+)intheH+zone
e
spanawiderange.However,ifonlysolutionswiththelowestσs 3.3.Comparisonoft (H+)andt (O)
e e
aretakenintoaccountthentherangeoftheelectrontemperature
intheH+ zoneformanyHregionsfromoursampleisconsid- Figure3showsbyathicksolidlinethebest-fitSED(SEDwith
erablynarrowed(Fig.2band 2c).Ontheotherhand,forsome the smallest σ) superimposed on the redshift- and extinction-
H  regions the minimum in the distribution of σ is not sharp correctedobservedspectrumof eachH  regionin the sample,
andeventwominimainthedistributionofσareobtainedwith includingtheHregionsdiscussedbyIzotovetal. (2006a)and
significantlydifferentT (H+)(see alsoFig.9 in Gusevaetal., Papaderosetal.(2006,inpreparation).
e
2006). The T (H+) of each H  region is derived as the aver- Thegalaxiesare arrangedin orderof decreasingHβequiv-
e
ageoftheelectrontemperaturesobtainedforseveralbest-fitting alent width, i.e. in order of decreasing contribution of gaseous
SEDs.WealsoderivethedispersionaroundtheaverageT (H+) emissionrelativetostellaremission.Itisseenthat,ineverycase,
e
oftheelectrontemperaturesobtainedfromthesamebest-fitting themodelSEDfitswelltheobservedoneoverthewholespectral
SEDs. Thensmalldispersionscorrespondto sharpsinglemini- range,includingtheBalmerjumpregion.Theseparatecontribu-
muma in the distributions of σ. We have checked how the de- tionsfromthestellarandionisedgascomponentsareshownonly
rivedT (H+)doesdependontheexactnumberofbestsolutions forSBS0335–052ESEbythinsolidlines.
e
6 N.G.Gusevaetal.:Balmerjumptemperatureinlow-metallicityHregions
InFig.4a wecomparet (H+) andt (O) =10−4 T (O). andlikelyshowsLyαemissioninthe UV. Thesepropertiesled
e e e
The 3.6m data for the sample of 69 H  regions in 45 BCDs Gusevaetal. (2004)toconcludethattheHregioninPox186
areshownbylargefilledcirclesand,inthecaseofPox186and maybedensity-bounded.IfthisisindeedthecasethenEW(Hβ)
UM462c,bysquares.Theelectrontemperaturet (H+)foreach does not measure the starburst age anymore. Its inclusion into
e
galaxyisthemeanofthetemperaturesderivedfromthe10best ourmodellingprocedurewouldthenresultinanincorrectrepro-
Monte Carlo realizationsweightedby their σs (Table 2). Error ductionofthestarformationhistoryandthestellarSED.While
bars are the root mean square temperature deviations from the this assumption is less critical in the case of Pox 186 because
mean of the 10 best Monte Carlo simulations. The dashed line of the high EW(Hβ) and hence the dominant contribution of
denotes equal temperatures, while the solid line is a linear re- gaseous emission to the total emission of this system, it is im-
gressiont (H+)=1.078×t (O)–0.054obtainedwiththelike- portantforthecaseofaHregionwithlowerEW(Hβ),suchas
e e
lihood method (Pressetal., 1992) for all 116 H  regionsthat UM462c.ThisisillustratedinFig.1whereweshowthemodel
takes into account errors in t (H+) and t (O ) in each object. SED which best fits the observed spectrum of UM 462c. It is
e e
Dotted lines illustrate 1σ deviationsof the sample H  regions seenthattheionisation-boundedmodeloftheHregionfailsto
from the linear regression. The linear regression is almost not reproduce the observations regardless of the adopted synthetic
changediftheoutlyingobjectwiththehighestelectrontemper- SED.Specifically,themodelsunderestimatetheSEDblueward
aturet (O) isexcluded.ItisseenfromFig.4athatthelinear oftheBalmerjump,whereasredwardofittheyprovideasatis-
e
regression(solidline)isslightlydifferentfromthelineofequal factorilyfit.Themostlikelyreasonforthisdiscrepancyisleak-
temperatures (dashed line) because of the small dispersion of ageofionisingphotonsfromtheHregion.
t (H+)fortheHregionMrk71No.1(Gusevaetal.,2006)and, ThediscrepancybetweenobservedandmodelledSEDscan
e
therefore,itslargeweightinthelinearregressiondetermination. be eliminated,however,assuming a H  regionmodelwith es-
Ontheotherhand,ifequaldispersionsoft (H+)areadoptedfor capingLyc photons,which impliesa largernumberof massive
e
allHregions,thelinearregressiondeviatesverylittlefromthe ionising stars than the number needed to account for the ob-
lineofequaltemperatures.Allthesedifferencesaresmallerthan servedEW(Hβ)inthecaseofanionisation-boundedHregion.
the1σdeviationsofHregionsfromthelinearregression(dot- Thus,MonteCarlosimulationsassumingaH regionwith es-
tedlines).Therefore,weconcludethatnosystematicdifference capingLyc photonsrequirea youngerstellar populationwith a
betweent (O)andt (H+)isevidentfromFig.4a. larger production rate of ionising photons, compared to mod-
e e
Our 3.6m observations in Fig. 4a are supplemented by the elsassuminganionisation-boundedH region.Asa resultthe
data frompreviousstudies.The resultsoft (H+) determination stellarSEDbluewardoftheBalmerjumpisactuallylargerthan
e
from Balmer jump for 23 low-metallicity H  regions (MMT what fits to EW(Hβ) imply.In practice,in orderto accountfor
data) and Paschen jump for 24 low-metallicity H  regions thefractionofphotonsescapingfromtheHregionweneedto
(SDSSdata)fromGusevaetal. (2006)areshownbylargeand introduceanadditionalparameter f.Then,themodelledSEDof
small open circles respectively. In Fig. 4a we also plot some the ionised gas emission and EW(Hβ) must be multiplied by a
other data from the literature. By stars are shown the H  re- factor(1– f)tomatchtheobservedones.
gions:M17(Peimbert&Torres-Peimbert,1992),Orionnebula InFig.4awe showbyopensquaresthet (H+) forPox186
e
(Estebanetal.,1998),NGC3576(Garc´ıa-Rojasetal.,2004),S andUM462cobtainedfromtheionisation-boundedmodeland
310(Garc´ıa-Rojasetal.,2004, 2006) intheGalaxy;30Dorin by filled squares the t (H+) obtained from the model with es-
e
LargeMagellanic Cloud (Peimbert, 2003); NGC 346 in Small capingLycphotons.Itisseenthatthet (H+)obtainedfromthe
e
Magellanic Cloud (Peimbertetal., 2000); and H  regions A latter models for both H  regions (filled squares) are in bet-
and B in the BCD Mrk 71 (Gonza´lez-Delgadoetal., 1994). ter agreement with the data for other H  regions. This is also
By dots in Fig. 4a we plot the data for 58 Galactic PNe by evident from Fig. 3 where we plot the best-fit SEDs obtained
Liuetal. (2004),Zhangetal. (2004),Wessonetal. (2005)and forthese twosystemsfrommodelswithescapingLycphotons.
Krabbe&Copetti (2005).Thedatacollectedfromliteratureex- TheseSEDsreproducequitewelltheobservedspectraovertheir
tendthet (H+)–t (O)relationtolowertemperatures. whole wavelength range, including the region blueward of the
e e
InFig.4bareshownonly41Hregionswithsmalldisper- Balmerjump.
sionsoft (H+) (σ[t (H+)]/t (H+) <0.1)for3.6mdata(present The leakage of Lyc photons could be present in other H
e e e
paper)andfordatafromGusevaetal. (2006)(MMTandSDSS  regions from our sample. This may introduce an additional
data). These data are fitted by a linear regression t (H+) = sourceofuncertaintiesinthedeterminationoft (H+).However,
e e
1.085×t (O ) – 0.050 (solid line) that is very similar to the thefactthatmajorityofHregionsarewellfittedbythemod-
e
linear regressionobtainedfor the whole sample of 116 H  re- elswithnon-escapingLycphotonssuggeststhatingeneral f is
gion.Ifthe outlyingpointwith the highestt (O ) is excluded small.Ourconclusionissupportedbymulti-wave-bandstudies
e
thelinearregressionisalmostnotchanged.Thus,fromtheanal- byLeithereretal. (1995).Theyfindthatlessthan3%ofioniz-
ysisofthesubsampleofHregionswithsharpestminimainthe ingphotonsescapefromlocalstarburstgalaxies.Morerecently,
σdistributionwereachthesameconclusionasthatbasedonthe forseverallocalstarburstgalaxies,Heckmanetal. (2001)esti-
total sample: there is no significant differencesbetween te(H+) mate f <∼6%andBergvalletal. (2006)find f ∼4%–10%for
andt (O).However,wecannotexcludesmalldifferencesof≤ anotherlocalstarburst.ThedirectdetectionofLymancontinuum
e
1000K,correspondingtothetemperaturefluctuationparameter emissionfrom14high-redshiftstar-forminggalaxiesshowsthat
t2≤0.02. only2of14studiedgalaxieshaveasignificantemissionbelow
SpecialemphasiswasgiventotheanalysisofafewHre- theLymanlimit(Shapleyetal.,2006).Therefore,forallourH
gions which are most deviant from the line of equal electron regionsexceptforPox186andUM462cwedecidedtoapply
temperatures in Fig. 4a. Two of these H  regions, UM 462c themodelswiththesmallestnumberoffreeparametersadopting
and Pox 186,are labeled in Fig. 4a. One of these sources, Pox f =0.
186, has been studied previously by Gusevaetal. (2004). No Inspection of Fig. 4b also reveals that some of the H  re-
H  21 cm emission was detected in this galaxy.Its H  region gionsfromthe3.6mdatasamplewiththehighesttemperatures
is characterisedbya veryhighO2+/O+ abundanceratioof ∼20 [(te(H+)∼1.9andte(O)>∼2.0)]areoffsetfromthelinearre-
N.G.Gusevaetal.:Balmerjumptemperatureinlow-metallicityHregions 7
gressionandfromthelineofequaltemperatures,atvarianceto We also identify and discuss cases of H  regions where
H regionswith alowert (H+).Theseoffsetgalaxiesarelow- ionisation-boundedmodels fail to fit the observed SEDs. Such
e
excitation H  regions with weak [O ]λ4363 emission lines. modelstypicallyunderestimatetheobject’sSEDbluewardofthe
Therefore, the determination of t (O ) is more uncertain in Balmerdiscontinuity.Thiscanbe,however,plausiblyaccounted
e
those galaxies. Indeed, the number of deviant points from the forbyphotoionisationmodelsallowingforthepartialleakageof
line of equal temperatures are decreased if only galaxies with LycphotonsfromaHregion.
high equivalentwidths of Hβ (EW(Hβ) > 80Å) and small t (O
e
)andt (H+)errorsareconsidered(Fig.4c). Acknowledgements. Y.I.I.andN.G.G.thankthehospitality oftheInstitute
e forAstrophysics(Go¨ttingen),andthesupportoftheDFGgrantNo.436UKR
WiththeadditionalPNedataaninterestingtrendappearsin
17/25/05.P.P.wouldliketothankGaspareLoCurto,LorenzoMonaco,CarlosLa
Fig.4a.WhilethePNedatascattersnicelyoneithersideofthe Fuente,EduardoMatamorosandthewholeESOstaffattheLaSillaObservatory
line of equal temperatures for te(O ) >∼ 1.1, just as the BCD fortheirsupport.AlltheauthorsacknowledgetheworkoftheSloanDigitalSky
data, the t (H+) – t (O ) relationshipcurvesdownfor objects Survey(SDSS)team.FundingfortheSDSShasbeenprovided bytheAlfred
withte(Oe)<∼1.1,ewithte(H+)beingsystematicallylowerthan PS.pSalcoeaAndFmouinnidsatrtaiotino,nt,htehePaNrtaitciiopnaatilnSgcIiennsctietuFtioounnsd,athtieonN,atthioenUa.lSA.eDreopnaaruttmicesnatnodf
te(O).ThereareseveralcoolHregionscollectedfromliter- Energy,theJapaneseMonbukagakusho,andtheMaxPlanckSociety.TheSDSS
atureandshownbystars,whichalsofollowthetrenddelineated Websiteishttp://www.sdss.org/.
by PNe. All other H  regionswith t > 1.0 collected fromlit-
e
erature do not deviate from line of equal temperatures, similar
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N.G.Gusevaetal.:Balmerjumptemperatureinlow-metallicityHregions 9
Table2.Comparisonoft (O)aandt (H+)bforthesamplegalaxies
e e
Galaxy EW(Hβ) te(O) te(H+) 12+logO/H C(Hβ)c
SBS0335-052ESE 382. 1.98±0.05 1.90±0.10 7.28 0.00
Pox186 358. 1.67±0.02 2.00±0.07 7.75 0.08
HS2236+1344a 343. 2.05±0.02 1.96±0.06 7.49 0.14
UM461a 335. 1.66±0.02 1.77±0.32 7.76 0.22
J104457.80+035313.1a 318. 1.96±0.02 1.91±0.08 7.44 0.31
Tol1214-277 314. 2.04±0.03 1.92±0.06 7.50 0.14
UM570 310. 1.83±0.02 1.83±0.13 7.72 0.14
CGCG032-017 308. 1.38±0.01 1.48±0.21 8.09 0.27
Pox108 306. 1.31±0.02 1.31±0.17 8.11 0.08
UM559 306. 1.60±0.02 1.55±0.12 7.72 0.06
SBS0335-052#7 299. 1.97±0.06 1.91±0.06 7.21 0.14
UM442 289. 1.30±0.01 1.29±0.09 8.10 0.14
Mrk1329 288. 1.05±0.01 1.26±0.06 8.33 0.03
Mrk1236a 283. 1.21±0.01 1.30±0.08 8.17 0.14
Tol2146-391 277. 1.63±0.02 1.59±0.32 7.78 0.18
CGCG007-025a 274. 1.58±0.02 1.83±0.27 7.81 0.20
HS2134+0400 262. 2.02±0.04 1.86±0.20 7.41 0.25
J001434.98-004352.0 260. 1.39±0.04 1.31±0.28 7.97 0.12
J232420.34-000625.0 249. 1.41±0.01 1.55±0.22 7.95 0.20
SBS0335-052E#3b 249. 2.04±0.03 2.07±0.22 7.31 0.45
J125305.96-031258.9 248. 1.38±0.01 1.34±0.21 8.01 0.36
J120122.30+021108.3 248. 1.81±0.03 1.70±0.27 7.49 0.09
SBS0335-052E#3a 246. 2.02±0.03 2.05±0.22 7.31 0.43
Tol65a 244. 1.76±0.02 1.83±0.17 7.54 0.10
UM311 243. 0.99±0.01 1.00±0.04 8.36 0.22
J130432.27-033322.1 233. 1.08±0.01 1.20±0.08 8.24 0.12
J024815.95-081716.5a 233. 1.32±0.01 1.13±0.19 8.04 0.04
Pox4a 220. 1.39±0.01 1.11±0.23 8.01 0.08
Pox120a 215. 1.53±0.01 1.76±0.22 7.88 0.21
HS2236+1344b 215. 1.86±0.02 1.80±0.10 7.58 0.22
CGCG007-025c 214. 1.56±0.02 1.49±0.25 7.78 0.02
UM462a 212. 1.39±0.01 1.45±0.22 8.00 0.52
J230210.00+004938.8 212. 1.80±0.03 1.91±0.09 7.63 0.00
Tol2138-405 208. 1.39±0.02 1.08±0.12 7.99 0.39
J210455.31-003522.2N 203. 2.01±0.08 1.87±0.18 7.26 0.00
J011340.44+005239.1 193. 2.29±0.30 2.55±0.04 7.17 0.00
J013352.56+134209.3 178. 1.74±0.02 1.62±0.33 7.56 0.04
Pox4b 177. 1.43±0.01 1.51±0.18 7.87 0.00
IC4662a 172. 1.27±0.01 1.34±0.19 8.09 0.44
J143053.50+002746.3 168. 1.30±0.01 1.21±0.15 8.07 0.15
IC4662c 166. 1.25±0.01 1.25±0.14 8.16 0.33
Pox120b 158. 1.37±0.02 1.18±0.20 7.99 0.14
g2252292-171050 158. 1.59±0.02 1.74±0.24 7.82 0.10
UM461b 147. 1.58±0.02 1.79±0.23 7.77 0.18
Mrk1236c 147. 1.22±0.04 0.98±0.03 8.05 0.00
J144805.36-011057.7 146. 1.39±0.01 1.44±0.24 8.00 0.21
IC4662b 140. 1.27±0.01 1.32±0.07 8.10 0.14
J223036.79-000636.9 133. 1.71±0.03 1.86±0.30 7.64 0.34
Pox105 131. 1.53±0.02 1.49±0.27 7.85 0.04
UM462d 130. 1.45±0.03 1.52±0.26 7.86 0.23
UM462c 127. 1.48±0.02 1.11±0.15 7.88 0.00
J024815.95-081716.5b 125. 1.35±0.04 1.36±0.18 7.99 0.00
CGCG007-025b 122. 1.58±0.02 1.67±0.19 7.75 0.06
J162410.12-002202.5 122. 1.20±0.01 1.21±0.15 8.16 0.16
UM462e 112. 1.59±0.05 1.67±0.26 7.77 0.00
J013452.00-003854.4 111. 1.49±0.15 1.53±0.26 7.85 0.16
SBS0335-052W 109. 1.97±0.25 2.01±0.25 7.13 0.02
Tol65b 96. 1.69±0.04 1.74±0.31 7.59 0.14
SBS0335-052E#4,5 89. 2.11±0.04 1.90±0.13 7.27 0.26
UM462b 85. 1.42±0.02 1.44±0.15 7.92 0.21
CGCG007-025d 69. 1.68±0.04 1.72±0.27 7.65 0.00
ESO338-IG04 59. 1.46±0.02 1.44±0.27 7.90 0.02
Mrk1236b 56. 1.17±0.02 1.14±0.22 8.12 0.07
J141454.13-020822.9 48. 1.89±0.01 1.98±0.23 7.32 0.08
IC4662d 44. 1.25±0.02 1.12±0.28 8.07 0.14
J010746.56+010352.0 40. 1.56±0.23 1.57±0.25 7.68 0.12
g0405204-364859 23. 2.08±0.25 1.95±0.21 7.34 0.02
J104457.80+035313.1b 20. 1.94±0.21 1.81±0.32 7.45 0.14
IC4662e 10. 1.31±0.01 1.30±0.23 8.08 0.06
ate(O)=Te(O)/10000K,derivedfromthe[O]λ(4959+5007)/λ4363emissionlineratio.
bte(H+)=Te(H+)/10000,derivedastheweightedmeantemperaturefromthe10bestfitsoftheSED.
cExtinctioncoefficientisderivedfromobservedhydrogenBalmerdecrement.
N.G.Gusevaetal.:Balmerjumptemperatureinlow-metallicityHregions,OnlineMaterialp1
Online Material