A 2D multiwavelength study of the ionized gas and stellar population in the Giant HII Region NGC 588

0.74 MB·English

by  A. Monreal-Ibero

Mon.Not.R.Astron.Soc.000,1–??(2010) Printed11January2011 (MNLATEXstylefilev2.2)
A 2D multiwavelength study of the ionized gas and stellar
population in the Giant HII Region NGC 588
1 A. Monreal-Ibero1,2⋆†, M. Relan˜o3, C. Kehrig2, E. Pe´rez-Montero1,
1
0 1 2 2 2
J. M. V´ılchez , A. Kelz , M. M. Roth , O. Streicher
2
1InstitutodeAstrof´ısicadeAndaluc´ıa(CSIC),GlorietadelaAstronom´ıa,s/n,18008Granada,Spain
n 2AstrophysikalischesInstitutPotsdam,innoFSPECPotsdam,AnderSternwarte16,D-14482,Potsdam,Germany
a 3Dpto.deF´ısicaTeo´ricaydelCosmos,UniversidaddeGranada,CampusFuentenueva,Granada,Spain
J

0
1

revisedversion
]
O
C
ABSTRACT
h. Giant HII regions(GHIIRs)innearbygalaxiesarealocalsampleinwhichwe canstudyin
p detailprocessesin the interactionofgas, dust, andnewlyformedstars whichare analagous
- to those which occurred in episodes of higher intensity in which much of the current stel-
o lar population was born. Here, we present an analysis of NGC 588, a GHIIR in M33,
r
based on optical Integral Field Spectroscopy (IFS) data obtained with the PMAS instru-
t
s ment at the 3.5 m telescope of Calar Alto Observatory, CAHA, together with Spitzer in-
a
fraredimagesat8µmand24µm.Theextinctiondistributionmeasuredintheopticalshows
[
complex structure, with three maxima which correlate in position with those of the emis-

1 sion at 24 µm and 8 µm. Furthermore, the Hα luminosity absorbed by the dust within
v the HII region reproduces the structure observed in the 24 µm image, supporting the use
3
of the 24 µm band as a valid tracer of recent star formation. A velocity difference of
5 ∼50 km s−1 was measured between the areas of high and low surface brightness, which
8
would be expected if NGC 588 were an evolved GHIIR. We have carefully identified the
1
areas which contribute most to the line ratios measured in the integrated spectrum. Those
.
1 line ratios which are used in diagnostic diagrams proposed by Baldwinetal. (1981) show
0
a larger range of variation in the low surface brightness areas. The ranges are ∼0.5 to
1
1.2 dex for [NII]λ6584/Hα, 0.7 to 1.7 dex for [SII]λλ6717,6731/Hα, and 0.3 to 0.5 dex
1
: for[OIII]λ5007/Hβ,withhighervaluesof[NII]λ6584/Hαand[SII]λλ6717,6731/Hα,and
v lowervaluesof[OIII]λ5007/Hβ intheareasoflowersurfacebrightness.Ratioscorrespond-
i ing to large ionization parameter (U) are found between the peak of the emission in Hβ
X
andthemainionizingsourcedecreasingradiallyoutwardswithintheregion.Differencesbe-
r
a tween the integrated and local values of the U tracers can be as high as ∼0.8 dex, notably
whenusing[OIII]λλ4959,5007/[OII]λλ3726,3729andinthehighsurfacebrightnessspax-
els. [OII]λλ3726,3729/Hβ and [OIII]λλ4959,5007/[OII]λλ3726,3729 yield similar local
valuesfor theionizationparameter,whichare consistentwith those expectedfromthe inte-
grated spectrumof an HII regionionized by a single star. The ratio [SII]λλ6717,6731/Hα
departssignificantlyfromtherangepredictedbythisscenario,indicatingthecomplexioniza-
tionstructureinGHIIRs.Thereisasignificantscatterinderivationsofthemetallicityusing
stronglinetracersasafunctionofposition,causedbyvariationsinthedegreeofionization.
ThescatterissmallerforN2O3whichpointstothistracerasabettermetallicitytracerthan
N2. One interesting result emerges from our comparison between integrated and local line
ratiovalues:measurementsofthelineratiosofGHIIRingalaxiesatdistances>∼25Mpcmay
bedominatedbytheionizationconditionsintheirlowsurfacebrightnessareas.
Key words: HII regions:individual:NGC 588 – galaxies: individual:M33 – stars: Wolf–
Rayet–ISM:abundances–ISM:kinematicsanddynamics–dust,extinction.
⋆ E-mail:[email protected](AM-I) Center, Calar Alto, jointly operated by the Max-Planck-Institut fu¨r As-
† Based on observations collected at the German-Spanish Astronomical tronomieHeidelbergandtheInstitutodeAstrof´ısicadeAndaluc´ıa(CSIC).
2 A. Monreal-Iberoet al.
1 INTRODUCTION ogy.Moreover, weevaluatedthereliabilityofdifferentlineratios
as metallicity (Z) tracers. In a companion paper, we presented a
Large areas of ionized gas known as Giant HII regions (GHI- novel approach tomodel these complex structures. There, were-
IRs) constitute the most conspicuous places of star formation
produced our observations by jointly fitting the radial profiles of
in normal galaxies (see Shields 1990, for a review). Their di-
different optical (i.e. Hα, [OII]λλ3726,3729, [OIII]λ5007) and
ameters typically range between ∼100 pc to ∼800 pc (e.g.
infrared(i.e.8µm,24µm)magnitudestoasetofCLOUDY-based
Kennicutt1984;Alonso-Herreroetal.2002)whiletheirHαlumi- photoionizationmodels(Pe´rez-Monteroetal.2010).
nosity range expands up to 3 orders of magnitudes (∼ 1038 − OurexperiencewithNGC595showstheimportanceofcarry-
1040 erg s−1, e.g. Kennicutt 1984; Rozasetal. 1996; Firpoetal.
ingoutananalysisusingthecombinedinformationofopticaland
2005; Monreal-Iberoetal. 2007; Relan˜o&Kennicutt 2009). Re-
infrareddatatogetherwithmodelling.However,giventheirdiver-
gardingtheirmorphologies, someof thempresentacompact dis-
sity,thesampleofGHIIRsstudiedbymeansofthismethodology
tributionwithhighsurfacebrightnesswhileothershaveamoredif-
cannot be reduced to only one example. Instead, studies of other
fuseemission.Also,theycanpresentmultiplecoresand/orshells
regionssamplingadifferentrangeintheparameterspacewouldbe
or ring-like features. In the same manner, they present very dif-
desirable.Wepresenthere,theanalysisofaregionwithadifferent
ferent content of gas, varying between ∼ 103 M⊙ to almost morphology,andrelativelylowermetallicityandhighHαluminos-
107 M⊙ whiletheyusually have∼ 102 −105 M⊙ instars(e.g. ity:NGC588.ThisregionislocatedintheoutskirtsofM33,atthe
Castellanosetal. 2002b). Finally, GHIIRs are small-scale exam- endofaspiralarm,ataradiusof∼14′ (i.e.∼ 3.42kpcforadis-
ples of the extreme events of star formation occurring in star-
tancetoM33of840kpc;Freedmanetal.1991).Thisareapresents
burstgalaxies(e.g.Alonso-Herreroetal.2009;Garc´ıa-Mar´ınetal.
HIemission(Verleyetal.2010;Gratieretal.2010).However,no
2009).Thus,agoodknowledgeoftheseobjectsishighlyvaluable
local H (or CO) emission has been detected towards NGC 588
2
forabetterunderstandingofthesemoreviolentphenomena. (Verleyetal.2010;Israeletal.1990).Withasizeof∼30′′×50′′
ThevarietyofpropertiesinGHIIRsimpliesacomplexstruc- (i.e.∼120pc×200pcatourassumeddistance),NGC588hasbeen
ture,farfromthetextbook-likeStro¨mgrensphere.Moreover,mod- classifiedwithinthering-likeclass(Sabbadinetal.1980).Itsstel-
elling of star-forming regions showed that in case of multiple larcontenthasbeenthoroughlystudied(Jametetal.2004;Pellerin
ionizing sources, geometrical effects affect the physical proper- 2006;U´beda&Drissen2009).Thedifferentestimatesforitstotal
ties(i.eelectrontemperatureandionizationstructure)ofthesere- stellarmassrangebetween∼1.3×103and∼5.6×103M⊙with
gions(Ercolanoetal.2007;Jamet&Morisset2008).Theinhomo- anagefortheburstof∼ 3.5−4.2Myr andlowmetallicity(i.e.
geneities of the Interstellar Medium (ISM) have been taken into Z ∼ 0.4Z⊙),consistentwithdirectmeasurementsusinglong-slit
account within the HII regions using the filling factor. This de- (12+log(O/H)=8.30,V´ılchezetal.1988).NGC588containstwo
scribesthefractionofthetotalvolumeoftheHIIregionwithhigh Wolf-Rayet(WR)stars.OneofthemwasclassifiedasWNLwhile
densegaswhiletheremainingvolumeisconsideredtobeofnegli- the other as Ofpe/WN9, an intermediate object between Of and
gibledensity(Osterbrock&Flather1959).Recently,detailedmod- WNstars(Masseyetal.1996).Fromthekinematicpointofview,
elsshow thatthedensityvariationsassumingopticallythickhigh TAURUS-2 Fabry-Pe`rot data show how NGC 588 seems a rela-
densitygasclumpsgiverisetoinhomogeneitiesinthetemperature tivelyevolvedsystem,withitsionizedgaskinematicsdominatedby
andionizationparameter (Giammancoetal.2004,2005).Thus,a acollectionoflargestellarwindshells(Mun˜oz-Tun˜onetal.1996).
singlevalueperphysicalmagnitude,usuallyextractedfromaspe- Finally,arelativelyfaint andpoint-likeX-rayemittingsourceas-
cificareaoftheregion,isnotnecessarilyrepresentativeofitsphys- sociatedwiththisregionhasbeendetected(Plucinskyetal.2008).
icalconditions. Inthiswork, wecombine 2D optical spectroscopic observa-
ItisinverynearbyGHIIRs(i.e.atD<∼6Mpc),whereground tionswiththePotsdamMulti-ApertureSpectrophotometer(PMAS,
basedopticaltelescopescanachievehighlinearspatialresolution Rothetal.2005)andinfraredimagingwithSpitzer.Thesewillcon-
(i.e.<∼40 pc arcsec−1), enough to resolve the different elements stitutethefirstpublished resultsfromdataobtained withthenew
(i.e.starclusters,ionizedgas,dust,etc.),playingaroleintheinter- PMAS’s CCD. A modelling of NGC 588 intending to reproduce
actionbetweenthemassivestarsanditssurroundingenvironment. theobserved magnitudes will bepresented inacompanion paper
Also,itistherewherethevariationsofthephysicalandchemical (Pe´rez-Monteroetal.inpreparation).Wedescribetheobservations
propertiesoftheionizedgascanbeproperlysampled. anddataprocessingtocreatethemapsoftherelevantmagnitudesin
From the observational point of view, the study of these re- Section2.ThemainobservationalresultsarepresentedinSection
gionswouldbenefitfromhighqualityspectroscopicdatathatmap 3.Finally,wesummarizeourmainconclusionsinSection4.
in an un-biased way the surface of the GHIIR. Nowadays, the
technique of integral field spectroscopy (IFS), able to record si-
multaneously the spectra of an extended continuous field, offers
thepossibility of performing such amapping. At present, studies 2 OBSERVATIONSANDDATAREDUCTION
of GHIIRsbased onIFSare stillscarce. Anexample isprovided
2.1 Observations
by Garc´ıa-Benitoetal. (2010), who analyzed one of the bright-
estGHIIRsinNGC6946.Also,Lo´pez-Sa´nchezetal.(2010)pre- The IFS data of NGC 588 were obtained on October 9-10, 2009
sentedadetailedstudyofastar-formingregionatthelowerlimit duringthecommissioningrunofthenewPotsdamMulti-Aperture
ofGHIIRsintermsofHαluminosityandsizeinIC10,ourclos- Spectrophotometer (PMAS, Rothetal. 2005) CCD at the 3.5 m
eststarburst.Recently,wepresenteda2Dspectroscopicanalysisof telescopeattheCalarAltoObservatory(Spain).ThenewPMAS4k
thesecondbrightest HIIregioninM33,theTriangulumGalaxy: x4kCCDisreadoutinfourquadrantswhichhaveslightlydiffer-
NGC595(Relan˜oetal.2010).There,weshowedhowtheoptical entgains(Rothetal.2010).DataweretakenusingtheLensArray
extinctionmapandtheabsorbedHαluminosityarespatiallycor- Mode(LARR)configurationwhichismadeoutofa16×16arrayof
relatedwiththe24µmemissionfromSpitzer,andhowtheioniza- microlensescoupledwithfibres(hereafterspaxels).Weusedthe1′′
tionstructure of theregion nicely follows theHα shell morphol- magnificationwhichprovidesafieldofview(FoV)of16′′×16′′.
NGC 588withPMAS 3
the spectra were traced on a continuum-lamp exposure obtained
before each target exposure. We extracted the target spectra by
addingthesignalfromthecentraltracedpixelsantitsfourneigh-
bours.Thespectrawerewavelengthcalibratedusingtheexposures
of HgNe arc lamps obtained immediately after the science expo-
sures.Wechecked theaccuracyof thewavelengthcalibrationus-
ing the [OI]λ5577 A˚ sky line, and found standard deviations of
<0.14A˚,which allowedustodetermine thecentroidof linewith
44" anaccuracyof∼7kms−1at∼5000A˚.
Fibres have different transmissions that may depend on the
wavelength. The continuum-lamp exposures were used to deter-
mine the differences in the fibre-to-fibre transmission and to ob-
tainanormalizedfibre-flatimage,includingthewavelengthdepen-
30"
100 pc dence.Thisstepwascarriedoutbyrunningthefiber flat.pl
scriptfromtheR3Dpackage(Sa´nchez2006).Inordertohomoge-
nizetheresponseofallthefibres,wedividedourwavelengthcal-
ibrated science images by the normalized fibre-flat. To estimate
Figure1.MosaictomapNGC588overplottedonacontinuum-subtracted
the accuracy of the fibre-to-fibre response correction, we fitted a
Hαdirect imagefromNOAOScience Archive (Masseyetal.2007).The
orientationisnorthupandeasttotheleft.TheHαimageisshowninlog- Gaussiantofouremissionlinesdistributedalongthewholespectral
arithmicstretchtobetterenhanceallthemorphologicalfeaturesoftheHII rangeinanextracted,wavelengthcalibratedandflat-fieldcorrected
regionandcoversarangeof3.4dex. arcexposure.Weusedtheratiobetweenthestandarddeviationand
themeanfluxineachlineasaproxyfortheaccuracyofthefibre-to-
fibreresponsecorrection.Forthoselinesinthecentralpartofour
We used the V600 grating and the 2×2 binning mode achieving spectralrange,thiscorrectionwasverygood,withratiosof∼2%.
aneffectivedispersionof 1.59A˚ pix−1,and a∼3.4A˚ fullwidth Intheblueandrededgestheseratiosreachedvaluesof∼12%,due
halfmaximumspectralresolution.WiththenewPMAS’sCCD,the tothecontributionoffibresaffectedbyvignetting.
coveredspectralrangewasfrom3620to6800A˚ formostofthe Inthenextstep,thethreeexposurestakenforthesamepoint-
spaxels.Thispermitsustoobservethemainemissionlinesinthe ing were combined in order to remove cosmic rays, using the
optical from [OII]λλ3726,3729 to [SII]λλ6717,6731. However, imcombineroutineinIRAF.1Fluxcalibrationwasperformedus-
for ∼40 spaxels, always at the edge of the LARR, this spectral ingtheIRAFtasksstandard,sensfuncandcalibrate.We
range was slightly reduced due to the vignetting associated with co-addedthespectraofthecentralfibresofthestandardstarexpo-
the3.5mtelescope. Thisprevents us fromobtaining information suretocreateaonedimensionalspectrumthatwasusedtoobtain
for the [OII]λλ3726,3729, [SII]λλ6717,6731, and Hαemission thesensitivity function. For wavelengths larger than 4000 A˚, the
linesinsomespecificareas(seealsosection2.2). uncertaintyinthefluxcalibrationis∼1%,whileacrossthebluer
We made a mosaic of 6 tiles to map most of the surface of spectralrange(i.e.<4000A˚),theassociatederrorcanreach∼5%.
NGC588.ThedistributionofthedifferenttilesisshowninFig.1 Giventhelargesizeof thenew PMAS’sCCD,thefour cor-
overplottedontheHαemission-lineimagefromNationalOptical nersofeachexposuresufferfromtelescopevignetting(Rothetal.
Astronomy Observatory (NOAO) Science Archive (Masseyetal. 2010).DuetothewaythefibresoftheLARRarearrangedatthe
2007). Contiguous tiles had a 2′.′0 overlapping, to make easier a entranceofthespectrograph,amaximumoftwocolumnsoffibres
commonrelativefluxcalibrationofthedata.Intotal,wecovereda attheeastandwestsidesoftheLARRwereaffectedbythis.The
fieldof30′.′0×44′.′0whichatthedistanceofNGC588corresponds wavelengthrangeaffectedwaslargerforthosefibreslocatedmore
to∼120pc×180pc. towardstheedgeofthespectrographandcouldbeupto∼3898 A˚
We obtained three exposures of 400 s per tile. Atmospheric intheblueendandfrom∼6498A˚ intheredend.Thesepartsof
conditionsduringtheobservationswerenon-photometricandtypi- thespectraweremaskedandthenweusedtheoffsetscommanded
calseeingrangedbetween1′.′2and1′.′6.Theskytransparencydur- to the telescope and the PMAS acquisition images toconstruct a
ingtheobservingnightspresentedvariationsof<∼15%.Allthedata mosaicdatacube.
weretakenatairmasses<∼1.1inordertopreventstrongeffectsdue After creating this datacube, maps for the different ob-
todifferentialatmosphericrefraction. servables were derived following the methodology presented in
In addition to the science frames, continuum and HgNe arc Relan˜oetal. (2010). Basically, we performed a Gaussian fit to
lampexposuresinordertominimizetheeffectsduetoinstrument the emission lines using the IDL-based routine mpfitexpr
flexures.Also,anearbyskybackgroundframewasobtainedduring (Markwardt 2009) and derived the quantities of interest for each
thesecondnightbymovingtheIFUoff-target. individual spaxel. Then, weused thesetogether withtheposition
Finally, exposures of the spectrophotometric standard star of the spaxels within the datacube to create an image suitable to
BD+28D4211wereobtainedinordertocorrectfortheinstrument bemanipulatedwithstandardastronomicalsoftware.Hereafter,we
responseandperformarelativefluxcalibration. willrefertothiswithbothterms:mapandimage.
Evenafterhavingallowedfor2′′ofoverlapbetweentiles,vi-
2.2 Datareductionandmapcreation
The first steps of the data reduction were done through the P3d 1 IRAF is distributed by the National Optical Astronomical Observato-
toolthatisdesignedtobeusedwithfibre-fedintegral-fieldspectro- ries, which are operated by the Association of Universities for Research
graphs (Sandinetal. 2010). After trimmimg, combining the four inAstronomy,Inc.,undercooperativeagreementwiththeNationalScience
quadrantsandsubtractingthebiaslevel,theexpectedlocationsof Foundation.
4 A. Monreal-Iberoet al.
sityoveralocallyfittedcontinuum.AswasshowninRelan˜oetal.
Table1.Observedandextinction-corrected emissionlinefluxes,normal-
izedtoI(Hβ)=1fromtheintegratedspectrumofNGC588.f(λ)isthered- (2010),bothsplotandmpfitexprgivesimilarresultsforthe
deningfunctionnormalisedtoHβfromFluksetal.(1994). high signal-to-noise integrated spectrum. We derived the redden-
ing coefficient, c(Hβ), from the F(Hδ)/F(Hβ), F(Hγ)/F(Hβ) and
F(Hα)/F(Hβ) line ratios. We performed a linear fit (by minimiz-
EmissionLine λobs f(λ) I(λ)/I(Hβ) I(λ)/I(Hβ)corr
ingthechi-squareerrorstatistic)tothedifferencebetweenthethe-
3727[OII] 3725.52 0.255 1.922±0.048 2.317±0.091 oretical and observed Balmer decrements vs. the reddening law
3868[NeIII] 3866.01 0.227 0.389±0.013 0.461±0.020 (Fluksetal. 1994), while simultaneously solving for the effects
3889H8+HeI 3886.21 0.223 0.423±0.014 0.501±0.021
3970Hǫ+[NeIII] 3966.03 0.208 0.152±0.011 0.179±0.013 of underlying Balmer absorption with equivalent width, EWabs.
4101Hδ 4099.02 0.180 0.199±0.013 0.274±0.016 We assumed that the EW is the same for all Balmer lines
abs
4340Hγ 4338.19 0.133 0.397±0.010 0.451±0.020
(e.g.Kobulnickyetal.1999).ThetheoreticalBalmerlineintensi-
4861Hβ 4859.46 0.000 1.000±0.016 1.000±0.021
4959[OIII] 4957.02 -0.027 1.500±0.021 1.524±0.046 tieswereobtainedfromStorey&Hummer(1995)assumingCase
50508776[OHeIIII] 55080742..9400 --00..024117 40..410504±±00..004094 40..412480±±00..103017 B,Te=104K,ne=100cm−3(typicalvaluesfoundinHIIregions,
6563Hα 6559.34 -0.314 3.508±0.034 2.938±0.114 Osterbrock&Ferland2006).
6584[NII] 6579.40 -0.316 0.246±0.004 0.210±0.008 WederivedameanextinctionfortheregionofAV = 0.548.
6678HeI 6674.29 -0.329 0.036±0.002 0.031±0.002 GiventhattheGalacticextinctioninthelineofsighttoNGC588
6717[SII] 6712.40 -0.332 0.210±0.004 0.178±0.007
6731[SII] 6726.76 -0.335 0.147±0.003 0.124±0.005 is AGVal = 0.146 (Schlegeletal. 1998) most of it is intrinsic to
NGC588 andagrees withintheuncertaintieswithprevious mea-
surementonitsbrightestparts(A =0.49−0.81,Melnick1979;
V
Viallefond&Goss1986;Melnicketal.1987).
Table2.Maindiagnosticemissionlineratiosandintegratedphysicalprop- Table 1presents both the measured and extinction-corrected
erties ofNGC588. Allthe physical properties were estimated usingour fluxes. Errors were estimated using the formula presented in
extinctioncorrectedintegratedfluxes.
Castellanosetal.(2002a).
EW
Parameter Valuecorr σline=σcont×N1/2(1+ N∆λ)1/2 (1)
log([NII]λ6584/Hα) -1.146±0.077 whereσ isthestandarddeviationinacontinuumclosetothe
log([SII]λλ6717,6731/Hα) -0.988±0.078 cont
lineofinterest,N isthenumberofpixelssamplingtheline,EW
log([OIII]λ5007/Hβ) 0.646±0.050 isitsequivalent widthand ∆λ isthe dispersion in A˚ pix−1. The
log([OIII]λλ4959,5007/[OII]λ3727) 0.406±0.069
quoteduncertaintiesofthereddeningcorrectedlinefluxestakeinto
logR23 0.915±0.053
N2O3 -1.792±0.127 accountthemeasurementandreddeningerrors.
[SII]λ6717/[SII]λ6731 1.435±0.143 Toinvestigateifthenebularpropertiesderivedfromlong-slit
log([NII]λ6584/[OII]λλ3726,3729) -1.043±0.077 arerepresentativeofthewholeHIIregion,wecomparedourflux
log([NII]λ6584/[SII]λλ6717,6731) -0.271±0.078 measurements from the integrated spectrum of NGC 588 to the
values presented by Jametetal. (2005) and V´ılchezetal. (1988)
c(Hβ) 0.26±0.03
ne(cm−3) <100 whousedlongslitatpositionangle(P.A.)∼ −45◦ includingthe
qeff (cms−1) 3.9×107 mainionizing clusterinNGC588 (e.g.seeFig.1in Jametetal.
Te(K) 11140±180(∗) 2005).Mostofourmeasurementspresentadifferencewithrespect
12+log(O/H) 8.16±0.02 toHβwhencomparingwithpreviouslyreportedmeasurementsthat
canrangebetween∼8%(e.g. [OIII]λ5007andHeIλ6678)upto
(∗) Valuederivedfromthe[OIII]λ4363/λ5007lineratiobyJametetal. ∼64% in the case of H8+HeIλ3889. This indicates that it is not
trivialhowthelineratiosmeasuredatthebrightestknotstracethose
(2005).
fortheHIIregionasawhole.Thisissuewillbeexploredinmore
detailinSec.3.6.
gnetting prevents us from deriving information for some lines in
Table2containstheprincipaldiagnosticemission-lineratios
thecentralcolumnsofourmosaic.Inthosemapswhere(atleast)
measuredfromtheintegratedspectrumaswellasthephysicalpa-
one line affected by vignetting was involved, those spaxels were
rametersderivedfromthem.Giventheblue-shiftforM33andthe
maskedandinterpolatedusingfixpixwithinIRAF,forpresenta-
presenceintheskysubstractedspectrumofstrongresidualsforthe
tionpurposes.
HgIλ4358sky-line,itwasnotpossibletomeasurethe[OIII]λ4363
nebular line and thus determine the electron temperature (T ) by
e
means of the [OIII] λ4363/λ5007 line ratio. Another possibility
3 RESULTS would have been using [SII]λ4067 and/or [NII]λ5755 together
with [SII]λλ6717,6731 and/or [NII]λ6584. To search for these
3.1 Integratedproperties
lines,inadditiontothetotalspectrum, wecreatedaspectrum by
Here we analyze the integrated spectrum for NGC 588, obtained co-addingthespectrainanapertureof5×9spaxelscentredatthe
afterco-addingthesignalof∼1000spaxels.Theskysubstracted peakofemissionfortheionizedgas.Inthisway,weincreasedthe
spectrumisshowninFig.2,whereweusedtwodifferentnormal- signal-to-noise ratio – and thus, our detection limit – since only
ization factors in order to better display all the observed emis- thebrightestspectrawereincluded.However,thesefeatureswere
sion lines. The positions of the detected nebular emission lines detectedneitherinthetotalspectrumnorintheoneinvolvingthe
were marked with labels. In addition, several sky line residuals brightestspaxels.Thus,weassumedaT of11140K,asderivedby
e
are clearly visible. Flux for the main emission lines was mea- Jametetal.(2005)fromthe[OIII]λ4363/λ5007lineratiotoesti-
suredusing splotwithinIRAF,whichintegratesthelineinten- matetheelectrondensity(n )andusedthetasktemden,basedon
e
NGC 588withPMAS 5
Integrated spectrum
[NeIII]+HeI+H8 [NeIII]+Hε HeI
3
[OII] Hδ Hγ Hβ [OIII] HeI Hα+[NII] [SII]
a.u.) 2
x (
u
Fl
1

0.4
u.) 0.3
a.
x (
u 0.2
Fl
0.1
4000 4500 5000 5500 6000 6500
λ(Å)
Figure2.IntegratedspectrumofNGC588obtainedbyco-addingthesignalofallthespaxelsinthefieldofviewwithtwodifferentnormalizationstobetter
visualizethedifferentobservedemissionlines.Fluxesareinarbitraryunits.Theskyspectrumwascreatedbycombiningthespectracorrespondingtothe
vigneting-freefibresofthebackgroundframe.
thefivelprogram(Shaw&Dufour1995)includedintheIRAF per or the lower branch. Instead, the metallicity of NGC 588
packagenebular.Thederivedn wasconsistentwithbeingbe- falls in the knee region of the Z − R23 relation, where un-
e
lowthelowdensitylimit. certainties can be as high as 0.7 dex. Alternatively, one can
AnotherquantityquotedinTable2istheionizationparameter, usetheN2O3=log(([NII]λ6584/Hα)/([OIII]λ5007/Hβ)).Wede-
definedas: rived a metallicity of 12 + log(O/H) = 8.15 and 8.18 us-
ing the parametrization proposed by Pettini&Pagel (2004) and
Q(H )
qeff = 4πR2on (2) Pe´rez-Montero&Contini (2009), respectively. Thus for the pur-
s e pose of this section, wewill consider as thecharacteristic metal-
whereQ(Ho)isthenumberofionizingphotonspersecondemitted licity of the region, the mean of those derived from the N2 and
bythestars,RsistheStro¨mgrenradiusoftheHIIregionandneis N2O3parameters: 8.16±0.02. Thisvalue agreewithintheuncer-
theelectrondensity.Weestimatedthenumberofionizingphotons taintieswiththe12+log(O/H) = 8.17 metallicityreportedby
usingtheexpressionprovidedbyKennicutt(1998): Jametetal. (2005), is slightly lower than the expected value as-
sumingthatthisregionfollowsthemetallicitygrandientforM33
Q(Ho)(s−1)=7.31×1011L(Hα)(ergs−1) (3) (8.28±0.08, Rosolowsky&Simon 2008) and ∼0.15 dex lower
and the extinction corrected Hα luminosity reported by thanthevaluereportedbyV´ılchezetal.(1988).
Relan˜o&Kennicutt(2009).Sincethesulfurlineratioisconsistent
withbeingbelowthelowdensitylimitregime,weassumedaface
valueofn = 20cm−3.Also,weutilizedaStro¨mgrenradiusof
e 3.2 Structureoftheionizedgasandthestellarcomponent
80pcwhichisanapproximatedvalueinferredfromtheHαimage.
Table2alsoincludesseveralmetallicitysensitivelineratios. In Figure 3, we present the Hβ and [OIII]λ5007 flux maps for
ThemostwidelyusedisprobablytheR23=([OII]λλ3726,3729+ NGC 588 derived from our PMAS data. Contours reproducing
[OIII]λλ4959,5007)/Hβ index(Pageletal.1979).However,the archivecontinuumimagesintheredandbluespectralbandshave
Z −R23 relation is two valued and thus, independent metallic- been overplotted for reference (see caption of Fig. 3 for details).
ity tracers are needed to determine which of the two branches OurPMASdatacover thewholesouthern part of theregion plus
is appropiate for NGC 588. One possibility would be the most of the northern one. This GHIIR is dominated by emission
N2 = log([NII]λ6584/Hα) line ratio. According to the empiri- fromabrokenelongatedring-likestructurewithmajorandminor
calparametrizationproposedbyPe´rez-Montero&Contini(2009), axesof∼40′′and∼25′′(i.e.∼160pcand∼100pc),respectively
we derived a metallicity of 12 + log(O/H) = 8.16. This re- andatP.A.∼10◦ and∼100◦.Inaddition,thereisabridgeofion-
sult does not point in a conclusive manner to either the up- izedgasemissionjoiningthetwohalvesofthering-likestructure
6 A. Monreal-Iberoet al.
Flux(Hβ) Flux([OIII]λ5007)
15 +2.30 15 +3.30
10 10
5 +1.69 5 +2.47
+ +
0 0
) )
c c
e e
s −5 +1.09 s −5 +1.64
c c
r r
a a
( (
∆δ −10 ∆δ −10
−15 −15
+0.48 +0.81
−20 −20
−25 −25
−0.12 −0.02
5 0 −5 −10 −15 −20 5 0 −5 −10 −15 −20
∆α (arcsec) ∆α (arcsec)
Figure3.Left:MapoftheobservedHβfluxderivedfromourPMASdata.Eachspaxelhasa1′′×1′′size.Alogarithmicstretchcovering2.4dexwasusedto
betterenhanceallthemorphologicalfeatures.Unitsarearbitrary.ContourscorrespondtotheRbanddirectimagefromNOAOScienceArchive(Masseyetal.
2007)andshowthelocationoftheionizingstarswithintheregion.Theorientation isnorthupandeasttotheleft.Thepeakinthiscontinuumimage,at
coordinates RA(J2000):1h32m45.7s,Dec.(J2000):+30d38m55.1s,markstheoriginofourcoordinate systemandwillappearasacrossinthefollowing
figuresforreference.Right:Similarmapfortheobserved[OIII]λ5007flux.Thelogarithmicstretchcovers3.3dexandcontoursrepresenttheHST-WFPC2
imagewiththeF336W filter(program5384;P.I.W.William),convolvedwitha0′.′5Gaussianfilter.
from∼[1′.′0,−7′.′0]to∼[−7′.′0,−3′.′0]2 atP.A.∼ −70◦.Themor- 10′′×8′′.However,itwasnotdetected,andthusimpossibletobe
phologies of the Hβ and the [OIII]λ5007 maps are very similar fittedforindividualspaxels.
but show some rather subtle differences which indicate the com- The reddening map was created assuming E(B − V) =
plexionizationstructure,whichwillbeexploredinmoredetailin A /3.1(Rieke&Lebofsky1985)andisdisplayedintheleftpanel
V
section3.6.Themainionizing cluster,asdepicted bythearchive ofFig.4withthe24µmimagefromSpitzeroverplottedwithcon-
continuumimages,isnotatthecentreoftheringlikestructure,but tours.Thismapshowshowirregulartheextinctiondistributionis
at ∼2′.′0fromthepeak of emission inHβ.Finally, therearealso withlowvaluesofreddening(∼ 0.00−0.25).Thisstronglycon-
several secondary peaks of emission, most of them in the north- trastswiththefindingsofJametetal.(2004)who,usinglong-slit,
ernhalfoftheregion,whichareassociatedwithverymassive(i.e. reportedanalmostconstantextinctionofE(B−V)=0.11±0.02
30−45M⊙)individualionizingstars(Jametetal.2004). andreinforcestheneedof2Dunbiasedspectralmappingtochar-
acterizethephysicalpropertiesofGHIIRs.Theopticalreddening
mappresentsthreemaximathatspatiallycorrelateverywellwith
3.3 Extinctiondistributionanddust themaximaofdustemissionintheSpitzer24µmand8µmbands
(seeFig. 4). Thisisconsistent with theidea of extinction caused
ThedistributionoftheextinctionwasderivedbymeansoftheHα
by absorption of dust associated with the GHIIR. Other dust-gas
andHβemissionlinemaps.WeassumedanintrinsicBalmeremis-
configurationswouldhavecausedadifferentsetofmaps.Forex-
sionlineratioofHα/Hβ=2.86(Osterbrock&Ferland2006)fora
ample,ifdust werebehindtheregion,therewouldnothavebeen
caseB aproximation andTe = 11150 K and usedthe extinction a counterpart in the E(B −V) map to the peaks in the map at
curveofFluksetal.(1994).Weincludeda1A˚ correctiontotake
24 µm. In the right panel of Fig. 4 we present the absorbed Hα
intoaccounttheHβabsorptionlineduetheunderlyingstellarpop-
luminosity-definedasthedifferencebetweenthetotalextinction-
ulation. This absorption feature was clearlyvisible ina co-added
correctedHαluminosityandtheHαluminositycorrectedforthe
spectrumextractedinanrectangularareawithlowsurfacebright- foregroundGalacticextinction,E(B−V)=0.044,(Schlegeletal.
nessintheemissionlineslocatedat ∼ [−12′.′0,−17′.′0]ofabout
1998)-obtainedusingourderivedreddeningmapwiththe8µm
contoursoverlaid.Acomparisonofthesetwopanelsshowshowthe
24µmemissionpresentsamorecompactdistributiontowardsthe
2 Hereafter,thereportedpositionswillrefertherelativecoordinatestothe centreoftheregionandcorrelatesbetterwiththeabsorbedHαlu-
mainionizingcluster. minositymapthanthe8µmemission.ThiswasseeninotherHII
NGC 588withPMAS 7
Figure4.Left:ReddeningmapforNGC588obtainedassuminganintrinsicHα/Hβ=2.86andacorrectionof1A˚ inabsorptionforHβ.Theextinctionlaw
ofFluksetal.(1994)andE(B−V)=AV/3.1(Rieke&Lebofsky1985)wereutilized.ThemaphasbeenconvolvedwithaGaussianfilterwithσ=1′′to
bettertracetheextinctionstructure.The24µmemissionwasoverplottedwithcontours.Theintensitycontoursareat2,5,10,20,40,60,80,95percentof
themaximumintensitywithintheregion.A1%contourlevelcorrespondstoa3σvalue.Right:AbsorbedHαluminosityofNGC588with8µmemission
contoursoverplotted.Thecontourlevelscorrespondtothesamepercentageasforthe24µmcase.Here,a1%contourlevelcorrespondstoa1σvalue.
regions like NGC 604 (Relan˜o&Kennicutt 2009) and NGC 595 themainionizingclusters.Here,weapplythesamemethodology
(Relan˜oetal.2010).The24µmemissionhasprovedtobeagood toNGC588.
traceroftherecentstarformationforalargerangeofHαluminosi- There are two known WR stars in NGC 588 that have been
ties, ranging from moderate HII regions to dusty powerful star- widely studied in the past. The first one, named UIT-011 by
bursts(e.g.Riekeetal.2009;Calzettietal.2010).Thespatialcor- Masseyetal.(1996)-MC3byDrissenetal.(2008)-wasdetected
relationbetweentheHαluminosityand24µmmapsindicatesthe for the first time using narrow band imaging by Conti&Massey
infrared band as astar-formation tracer also inlow dust environ- (1981) and was spectroscopically confirmed by Massey&Conti
ments,suchasNGC588.Duetothelowspatialresolutionofthe (1983)lateron.ItwasclassifiedasWNLwithM = −7.9mag.
V
70µmand160µmSpitzerbandswewerenotabletocomparethe The second one, named UIT-008 is a transition Of/WN9 star.
emission at these wavelengths withour reddening map. Herschel Both stars were modelled using multi-band photometry with the
data,whichwillbeavailablesoontothescientificcommunity,will HSTby U´beda&Drissen (2009). They derived effective temper-
permitapropercomparisontobemadeinthefuture. atures of 57000 K and 32000 K and bolometric luminosities of
log(L/L⊙) = 6.48 and 5.97, for UIT-011 and UIT-008 respec-
tively.
3.4 WRstellarpopulation
Fig.5presentsthecontinuumsubstractedbluebumpmapaf-
Relan˜oetal.(2010)presentedanovelandsimpletechniquetode- ter convolving with a 1′′-Gaussian. We identify two main peaks
tectWRstarsinaswiftwayandcompareditsresultstoclassical, of emission whose positions agree well withthose previously re-
moretime-consuming techniques. Usingthesamesetofobserva- portedforUIT-011andUIT-008(Drissenetal.2008).Noaditional
tions (i.e. the datacube), one can identify the WR candidates by WRinNGC588wasfound.Theextractedspectraforthetwostars
simulatingtheactionofnarrowfiltersandcreatingcontinuumsub- ispresentedinFig.6andshowclearlyboththeblueandredbump.
tractedmapsattheemissionbumpat4700A˚ (thebluebump)andat ThecasesofNGC595andNGC588,inHIIregions,aswell
5700A˚ (theredbump),characteristicofWRstellaremission,and asexistingonesforstarburstgalaxies(e.g.theAnntenae,IIZw70,
localizingthepeaksofemission,afterwards.Then,thecandidates Mrk996,NGC5253,IC10,Bastianetal.2006;Kehrigetal.2008;
canbeconfirmedbyextractingthespectraoftheassociatedspax- Jamesetal.2009;Monreal-Iberoetal.2010;Lo´pez-Sa´nchezetal.
els.Followingthismethodology,weconfirmedpreviouscataloged 2010)illustratetheeffectivenessofIFSinthefindingandcharac-
WRstarsinNGC595anddiscoveredanewonefurtherawayfrom terization of the WR population. At this stage, the possibility of
8 A. Monreal-Iberoet al.
465 − 475 nm
15 +0.08
70
60 UIT−011
a.u.) 50
10 Flux ( 3400
20
10
5 +0.01 4000 4500 5000 5500 6000
λ(Å)
UIT−008
UIT−011
+
0 30 UIT−008
u.) 25
sec) −5 −0.06 Flux (a. 1250
c 10
r
a 5
∆δ (−10 4000 4500 5λ0(Å00) 5500 6000
Figure6.ExtractedspectrafortwodetectedpeakofemissioninthePMAS
−15 continuumsubtracted4650−4750A˚.Verticalarrowsmarkthepositionof
−0.13 theexpectedblueandredbumps.
−20
v (km s−1)
15 Hβ −110.00
−25
−0.20
5 0 −5 −10 −15 −20 10
∆α (arcsec)
5 −130.00
Figure5.4650−4750A˚ mapderivedfromthePMASdataafterconvolv- +
0
ingwithaGaussianofσ=1′.′0.Continuumwassubtractedbyaveragingthe
spectralrangesof4490−4540A˚ and4755−4805A˚.Alogarithmicstretch )
c
covering∼0.3dexwasusedtobetterenhancethepeaksofemission.Labels se −5 −150.00
withthedetectedWRareincludedandwhitecontoursdelineatethespaxels c
r
a
utilizedtocreatetheextractedspectra.Contourscorrespondtothecontin- (
uumsubtractedRbroad-banddirectimagefromNOAOScienceArchive ∆δ −10
(Masseyetal.2007).Theorientation isnorthupandeasttotheleft.The
mainionizingcluster,atcoordinatesRA(J2000):1h32m45.7s,Dec.(J2000):
+30d38m55.1s,markstheoriginofourcoordinatessystem. −15
−170.00
usingthistechniqueroutinelyshouldbetakenintoaccount.Inpar- −20
ticular,itwouldsuitperfectlyinthecaseofWRfindingingalaxies
atlargerdistancesand,moreimportant,withlargegradientsintheir
−25
velocityfields.Here,thetraditionaltechniqueofsearchforcandi- −190.00
datesviaimagingfirst,andspectroscopicconfirmationafterwards, 5 0 −5 −10 −15 −20
might well miss some of the WR populations since the blue/red ∆α (arcsec)
bumpmightmoveoutsidethespectralrangeofthenarrowfilter.On
thecontrary,themethodologypresentedherecanbeeasilymodi-
fiedandimplementedtodefinewhatcanbecalledsynthetictunable
Figure7.VelocityfieldderivedfromtheHβemissionline.Contourscor-
filtersthattakeintoaccountthemovementsofthegalaxyandthus
respond to the continuum subtracted Hα direct image from NOAO Sci-
preventingtheselosses.AnadditionaladvantageofusinganInte-
enceArchive(Masseyetal.2007).Theorientationisnorthupandeastto
gralFieldUnit,especiallyifithasalargefieldofviewisthedetec-
theleft.Themainionizingcluster,atcoordinatesRA(J2000):1h32m45.7s,
tionofrunawayWRstarsejectedbythecentralstarclusterofthe
Dec.(J2000):+30d38m55.1s,markstheoriginofourcoordinatessystem.
region(Drayetal.2005).Inparticular,IFS-basedinstrumentswith
relativelylargefieldofviewlikePPak/PMAS(Kelzetal.2006),or
MUSE (Baconetal. 2010) are(or will be soon) under operation. ityfieldmapsfromthestrongestemissionlines.InFig.7weshow
Theyopenthepossibilityofcarryingoutsurveysoflargesamples themapcorrespondingtoHβ.Norelevantdifferenceswerefound
ofgalaxieswherethiskindofsimpletechniquescouldbeparticu- fromthemapderivedusing[OIII]λ5007.Thevelocityfieldhasa
larlyuseful. complexstructurewithvaluesrangingbetween−190kms−1and
−110kms−1.Thenorth-westpartoftheregionseemstobemore
redshiftedthanthesouth-east,whichistheregionwithhigherHβ
3.5 Kinematicsoftheionizedgas
surfacebrightness(seeFig.3).Inthesurroundingsofthelocation
Thesuperiorspectralresolutionofthepresentobservations,twice ofthestellarcluster, markedinFig.7asablackcross, thereisa
thatoftheobservationsofNGC595,allowedustoderivetheveloc- pronouncedvelocitygradient:thenorth-westparthasvelocitiesof
NGC 588withPMAS 9
−120kms−1whilethesouth-easthasvelocitiesof−170kms−1. 3.6.2 LineratiosintheBPTdiagnosticdiagrams
Thevelocityseparationof ∼25kms−1 between thesezones and
Diagnostic diagrams, where different areas of a given diagram
the location of the stellar cluster, corresponding to a velocity of
∼30kms−1 inthegalaxyplane(inclinationofM33,i = 56deg areoccupied by gasexcited via different mechanisms, have been
widelyused tostudy theionization conditions of theISM.Inthe
vandenBergh2000),andthesymmetryofthevelocityfieldsug-
opticalspectralrange,themostpopularareprobablytheBPTdia-
gesttheexistenceofashellexpandingintheinterioroftheregion.
Theshell expansion velocity, ∼30km s−1, isslightlylower than grams, firstproposed by Baldwinetal. (1981) andlater reviewed
by Veilleux&Osterbrock (1987). Their wide use to study the
thevaluesobservedinhighluminosityHIIregionsofasetofspiral
galaxies(v ∼ 40−90kms−1,Relan˜oetal.2005)andalsoin ionization conditions in star-forming and starburst galaxies (e.g.
exp
NGC604(v ∼40kms−1,Yangetal.1996).However,basedon Alonso-Herreroetal. 2010) is due to the fact that they involve
exp
emission lines that are relatively strong and line ratios that have
akinematicstudyofNGC588andNGC604,Mun˜oz-Tun˜onetal.
(almost)nodependenceontheextinction.
(1996)suggestthatNGC588ismoreevolvedthanNGC604and
With the present 2D unbiased mapping, one can determine
therefore,wewouldexpectlowervelocitiesfortheshellsinthefirst
theselineratioslocally.Inthisway,itispossibletoevaluatetheir
regionthaninNGC604,consistentwiththeresultfoundhere.
dependenceonthepositionwithintheGHIIRandrelativesurface
Inorder tocheckwhether thewindscomingfromthestellar
brightnessoftheareaunderstudy.Moreover,onecanmakeacom-
population withinthe HII regioncould produce theexpansion of
parisonbetweenintegratedandlocalvalues.
theobservedshell,wehavemadeacrudeestimationofthekinetic
energy involved in the shell and compared with the input kinetic We present the maps for the three available line ra-
energyfromthestars.UsingtheHαluminosityoftheregionfrom tios involved in the BPT diagrams - namely [NII]λ6584/Hα,
Relan˜o&Kennicutt(2009),wepredictanemissionmeasure(EM) [SII]λλ6717,6731/Hα, and [OIII]λ5007/Hβ - in Fig. 8. These
of4000(pccm−6)foranHIIregionradiusof140pc,correspond- maps show that the ionization structure in NGC 588 is complex.
ingtotheapertureradiususedtoobtainedtheHαluminosity.The The [NII]λ6584/Hα and [SII]λλ6717,6731/Hα maps present a
EMisthenusedtoderivea< n > of5cm−3 andintegrat- rathersimilarstructure.Inbothcases,theminimumislocatednei-
e rms
ingovertheHIIregionvolumewederiveatotalionizedmassof theratthepeakofemissioninHβ(i.e.ionizedgas)norattheone
∼6×105M⊙.Assuming,asanupperlimitthatthewholemasshas forthecontinuum(i.e.stars)butinthemiddlepointbetweenthem.
beensweptupbytheshellweobtainakineticenergyfortheshell Then,lineratiosincreaseoutwards,followingtheringstructureof
of3.8×1051erg.Starburst99models(Leithereretal.1999)using theregion.
rangesofvaluesforthestellarmassof1−6×103 M⊙ andage The [OIII]λ5007/Hβ map is roughly similar to the
of 3.5−4.2 Myr, aSalpeter initialmassfunction, and metallici- [NII]λ6584/Hα and [SII]λλ6717,6731/Hα maps but its value
tiesofZ=0.004and0.008givearangeofkineticenergyinputof varies in the opposite sense. Moreover, there are two main
5.9−64.2×1051 erg,atleasttwiceashighastheupperlimitof differences. The peak of [OIII]λ5007/Hβ (maximum for this
the kinetic energy of the shell. This crude calculation shows that line ratio) is broader than the peak for [NII]λ6584/Hα and
thewindsfromthestellarclusterwithinNGC588areabletopro- [SII]λλ6717,6731/Hα(minimumfortheseones). Morerelevant,
ducetheobservedshellintheHIIregionandcreatetheobserved there is an area of ∼ 5′′ × 13′′ centred at ∼ [7′.′0,−8′.′0] of
Hβ morphology withholesandfilamentsshowninFig.3.More- elevated [OIII]λ5007/Hβ values that do not show specially low
over,thelowerenergyassociatedwiththeshell,30kms−1,com- [NII]λ6584/Hαand[SII]λλ6717,6731/Hα.
paredwithvaluescloserto100kms−1 foryounger, lessevolved Inordertobetterassesshowtheionizationconditionschange
regions(Relan˜oetal.2005),isconsistentwithwiththefindingsby in different parts of NGC 588, we divided our data in three lu-
Mun˜oz-Tun˜onetal.(1996). minosity bins, which sample the low, medium and high surface
brightness areas of this GHIIR. In Fig. 9, we present the posi-
tionofeachindividual spaxelintheBPTdiagnosticdiagramsto-
gether with the borders that separate HII region-like ionization
fromionizationbyothermechanismsaccordingtoseveralauthors
(Veilleux&Osterbrock1987;Kewleyetal.2001;Kauffmannetal.
3.6 Characterizationoftheionizedgas 2003;Stasin´skaetal.2006).Asexpected,alllineratiosarewithin
thetypicalvaluesexpectedforanHIIregion-likeionization.
3.6.1 Densitystructure
However, therearedifferencesbetweenthediagramsassoci-
Electron density can be determined from the ratio between two ated with the different luminosity bins. Firstly, the range of ob-
lines of the same ion emitted by different levels with similar ex- servedvaluesvariesfrom∼0.3to∼0.6dexfor[OIII]λ5007/Hβ,
citationenergies.Weusedthe[SII]λ6717/[SII]λ6731ratioinour from ∼0.6 to ∼1.1 dex for [NII]λ6584/Hα and from ∼0.5 to
analysis.AsisshowninthelowerrightmapofFig.8,nostructure ∼1.1 dex for [SII]λλ6717,6731/Hα, with larger ranges in those
forthen wasfound.Wemeasuredamean(±standarddeviation) areas with lower surface brightness Secondly, as indicated by
e
[SII]λ6717/[SII]λ6731valueof1.20(±0.17),whichisconsistent the position of the red triangles, higher [NII]λ6584/Hα and
withthevaluederivedfromtheintegratedspectrum(seeTable2). [SII]λλ6717,6731/Hαratiosandlower[OIII]λ5007/Hβratiosare
For the assumed temperature, this implies a n of ∼250 cm−3 detectedintheareasoflowersurfacebrightness.Thisimpliesthat
e
and agrees within the uncertainties with the values reported by thedegreeofionizationgetssmallerwithincreasingdistancefrom
V´ılchezetal. (1988) and Jametetal. (2005). This value for the theionizingsource.
electron density corresponds to the density of the clumps within HowdotheseresultscomparewithourfindingsforNGC595?
theregionanddiffersfromther.m.s.electrondensityderivedinthe Ingeneral,thetendenciesinthedifferencesbetweentheintegrated
previoussectionusingtheHαsurfacebrightnessoftheregion.The valuesandthosefortheindividualspaxelsinNGC588aresimilar
ratioofbothdensityestimatesisameasureofthevolumefraction tothosefound inNGC595. However, therangeof observed val-
occupiedbydenseclumps. uesinNGC588issmaller.Thisisseenin[SII]λλ6717,6731/Hα
10 A. Monreal-Iberoet al.
log([NII]/Hα) log([SII]/Hα)
15 −0.60 15 −0.40
10 10
5 −0.95 5 −0.73
+ +
0 0
) )
c c
e e
s −5 −1.30 s −5 −1.05
c c
r r
a a
( (
∆δ −10 ∆δ −10
−15 −15
−1.65 −1.38
−20 −20
−25 −25
−2.00 −1.70
5 0 −5 −10 −15 −20 5 0 −5 −10 −15 −20
∆α (arcsec) ∆α (arcsec)
log([OIII]/Hβ) [SII]λ6717/[SII]λ6731
15 +0.90 15 +2.20
10 10
5 +0.77 5 +1.78
+ +
0 0
) )
c c
e e
s −5 +0.65 s −5 +1.35
c c
r r
a a
( (
∆δ −10 ∆δ −10
−15 −15
+0.52 +0.93
−20 −20
−25 −25
+0.40 +0.50
5 0 −5 −10 −15 −20 5 0 −5 −10 −15 −20
∆α (arcsec) ∆α (arcsec)
Figure8.Mapoftheobserved[NII]λ6584/Hα(upperleft),[SII]λλ6717,6731/Hα(upperright),[OIII]λ5007/Hβ(bottomleft)and[SII]λ6717/[SII]λ6731
(bottomright)lineratios.ContourscorrespondtothecontinuumsubtractedHαdirectimagefromNOAOScienceArchive(Masseyetal.2007).Theorientation
is northupandeast totheleft. Themain ionizing cluster, atcoordinates RA(J2000): 1h32m45.7s,Dec.(J2000): +30d38m55.1s, marks theorigin ofour
coordinatesystem.