Estimating turbulent velocities in the elliptical galaxies NGC 5044 and NGC 5813

0.4 MB·English

by  J. de Plaa

Astronomy&Astrophysicsmanuscriptno.18404 c ESO2012
(cid:13)
January11,2012
Estimating turbulent velocities in the elliptical galaxies
NGC 5044 and NGC 5813
J.dePlaa1,I.Zhuravleva2,N.Werner3,J.S.Kaastra1,4,E.Churazov2,5,R.K.Smith6,
A.J.J.Raassen1,andY.G.Grange1
1 SRONNetherlandsInstituteforSpaceResearch,Sorbonnelaan2,3584CAUtrecht,TheNetherlands
e-mail:[email protected]
2 Max-Planck-Institutfu¨rAstrophysik,Karl-Schwarzschild-Strasse1,85741Garching,Germany
3 KavliInstituteforParticleAstrophysicsandCosmology,StanfordUniversity,382viaPuebloMall,Stanford,CA94305-4060,USA
2 4 AstronomicalInstitute,UtrechtUniversity,POBox80000,3508TAUtrecht,TheNetherlands
1 5 SpaceResearchInstitute(IKI),Profsoyuznaya84/32,Moscow117810,Russia
0 6 Harvard-SmithonianCenterforAstrophysics,60GardenSt.,Cambridge,MA02138,USA
2
Received4November2011/Accepted22December2011
n
a
ABSTRACT
J

9 Context.Theinterstellarandintra-clustermediumingiantellipticalgalaxiesandclustersofgalaxiesisoftenassumedtobeinhy-
drostaticequilibrium.Numericalsimulations,however,showthatabout5–30%ofthepressureinaclusterisprovidedbyturbulence
] inducedby,forexample,thecentralactivegalacticnucleusandmergeractivity.
O
Aims.Weaimtoputconstraintsontheturbulentvelocitiesandtheturbulentpressureintheintra-clustermediumofthegiantelliptical
C galaxiesNGC5044andNGC5813usingXMM-NewtonReflectionGratingSpectrometer(RGS)observations.
. Methods.ThemagnitudeoftheturbulenceisestimatedusingtheFeXVIIlinesat15.01Å,17.05Å,and17.10ÅintheRGSspectra.
h
Atlowturbulentvelocities,thegasbecomesopticallythickinthe15.01Ålineduetoresonantscattering,whilethe17Ålinesremain
p
opticallythin.Bycomparingthe(I +I )/I lineratiofromRGSwithsimulatedlineratiosfordifferentMachnumbers,the
- 17.05 17.10 15.01
o levelofturbulenceisconstrained.Themeasurementis,however,limitedbythesystematicuncertaintyinthelineratioforanoptically
r thinplasma,whichisabout20–30%.
t Results.Wefindthatthe(I +I )/I lineratioinNGC5813issignificantlyhigherthaninNGC5044.Thisdifferencecanbe
s 17.05 17.10 15.01
a explainedbyahigherlevelofturbulenceinNGC5044.Thebestestimatesfortheturbulentvelocitiesusingresonantscatteringand
[ upperlimitsfromthelinewidths,are320< Vturb <720kms−1 forNGC5044and140< Vturb <540kms−1 forNGC5813atthe
90%confidencelimit.
1 Conclusions.The high turbulent velocities and the fraction of the turbulent pressure support of >40% in NGC 5044, assuming
v isotropicturbulence,confirmthatitisahighlydisturbedsystem,probablyduetoanoff-axismerger.Theturbulentpressuresupport
0 inNGC5813ismoremodestat15–45%. The(I +I )/I lineratioinanopticallythinplasma,calculatedusingAtomDB
17.05 17.10 15.01
1 v2.0.1,is2σabovetheratiomeasuredinNGC5044,whichcannotbeexplainedbyresonantscattering.Thisshowsthatthediscrep-
9 anciesbetween theoretical, laboratory, andastrophysical dataon FeXVIIlinesneed tobereduced toimprove theaccuracy of the
1 determinationofturbulentvelocitiesusingresonantscattering.
.
1
Keywords.X-rays:galaxies-Galaxies:ellipticalandlenticular,cD-Galaxies:clusters:intraclustermedium-Scattering-Turbulence
0
-Atomicdata
2
1
:
v 1. Introduction by measuring the width of the emission lines. The Reflection
i Grating Spectrometers (RGS, denHerderetal. 2001) aboard
X
The hot X-ray emitting plasma in giant elliptical galaxies and XMM-Newton have the necessary spectral resolution, but be-
ar clusters of galaxiesis often assumed to be in hydrostatic equi- cause the spectrometers are slitless, the observed lines are
librium. In addition to the thermal pressure, there are, how- blurred due to the extended nature of the source. Despite this
ever,otherprocesseswhichcouldcontributesignificantlytothe complication,attemptshavebeenmadetoputconstraintsonthe
total pressure of the plasma. One of these sources is turbu- turbulentvelocities in groupsand clusters of galaxies from the
lence induced by merger and AGN activity (e.g. Loeb&Mao width of the lines in RGS spectra (Sandersetal. 2011). Upper
1994; Roettigeretal. 1996; Boehringeretal. 1993). Recent es- limitsrangingfrom200kms 1upto1500kms 1werefoundin
− −
timatesfromnumericalsimulationssuggestthatbubblesblown asampleof62clusters,groupsandellipticalgalaxies.
by AGN can driveturbulenceup to velocitiesof about500km Apartfromvelocitybroadeningoflines,therearealsoindi-
s−1(e.g.Bru¨ggenetal.2005;Heinzetal.2010).Inclustermerg- rectmethodsto constrainturbulentmotionsin the hotgas, like
ers, the turbulencecan go up to about 1000km s−1 and higher resonantscattering.Althoughthehotplasmaisingeneralopti-
(e.g.Dolagetal.2005;Lauetal.2009).Typically,turbulenceis callythin,itcanbecomeopticallythickattheenergiesofstrong
thoughttoprovideabout5–30%ofthepressuresupportinclus- resonancelines(Gilfanovetal.1987).Sincethetransitionprob-
ters. abilityofsuchalineislarge,ithasahighcrosssectiontoabsorb
Current X-ray observatories do not have the spectral res- andsubsequentlyre-emitX-rayphotonsthathavethesameen-
olution to measure turbulent velocities in the ICM directly ergy as the line. Along the line of sight toward the core of an
1
J.dePlaaetal.:EstimatingturbulentvelocitiesintheellipticalgalaxiesNGC5044andNGC5813
elliptical galaxy,resonance-linephotonsthat are emitted in the Table 1. Effective exposure times for the XMM-Newton RGS
densecentralregionhavea highprobabilityto bescatteredout observationsafterfiltering.
ofthebeam,becausetheopticaldepthislarge.Duetothecon-
servativenatureofresonantscattering,thescatteredphotonsare
re-distributedtolargerprojecteddistances.Thiseffectcausesan NGC5044 NGC5813
increaseofthesurfacebrightnessattheedgeofthegalaxy(see ObsID Exposure ObsID Exposure
alsothereviewbyChurazovetal.2010).Theopticaldepthde- 0037950101 20.2ks 0302460101 29.9ks
pendson the turbulentbroadeningof resonantlines, especially 0554680101 105.3ks 0554680201 54.9ks
0554680301 52.6ks
forheavyelementslikeiron.Acomparisonofthefluxesofop-
Total 125.5ks Total 137.4ks
ticallythinandopticallythicklinescouldbeusedtoplacecon-
straintsonthelevelofturbulenceingalaxies.
Inprinciple,thereareseveralsuitablelinestostudyresonant
scatteringintheX-rayband(seeChurazovetal.2010,foralist
NGC 5813. Metal abundancesare given with respect to proto-
oftheselinesemittedbycoolandhotclusters).Churazovetal.
solarabundancesbyLodders(2003)anduncertaintiesaregiven
(2004)usetheFe-Kcomplexoflinesat6.7keVtofindalower
atthe68%confidencelevel,unlessstatedotherwise.
limittotheturbulenceinthePerseuscluster.Independently,us-
ing the same data, Gastaldello&Molendi (2004) reached sim-
ilar conclusions. The spectral resolution of the current instru-
2. Dataanalysis
ments at these energies is however low, which limits the ac-
curacy of the measurement. At the moment, the most useful
Inouranalysis,weuseallavailableRGSdataonNGC5044and
linesto study resonantscatteringare the 15.01Å, 17.05Å and
NGC 5813 that have exposures larger than 10 ks. Flares were
17.10 Å lines of FeXVII in the Fe-L complex, which are de- discardedby filtering a lightcurveextractedfromthe outerre-
tectableinrelativelycoolellipticalgalaxieswithRGS(Xuetal. gionsofCCD9.Binswithcountratesabove0.25counts/swere
2002; Werneretal. 2009). The 15.01Å line has the largestos- discarded.Theobservationswereonlymodestlyaffectedbysoft-
cillator strength (f = 2.31 2.73, depending upon the calcu- proton flares. For each observation, less than 20% of the data
−
lation) of the lines and is most sensitive to resonantscattering. wasdiscardedduetoflares.Asummaryoftheobservationsused
The oscillator strengths of the 17.05 Å and 17.10 Å are much withtheireffectiveexposuretimeafterflarefilteringisprovided
lower, f = 0.12and f = 10 8 respectively.Sinceallthreelines inTable1.
−
are emitted by FeXVII ions, the ratio between the 15 Å and
17Ålinesisagoodindicatorforresonantscattering.Inasam-
2.1.RGSanalysis
ple of five elliptical galaxies, Werneretal. (2009) find that the
15.01Ålineissuppressedinfouroutoffivesystems.Sincethe RGS spectra of NGC 5044 and NGC 5813 were extracted us-
significanceoftheindividualdetectionsislow,2–4σpergalaxy, ing the SAS XMM-Newton analysis software version 10.0 (see
theycouldonlyestimateanupperlimitfortheturbulentveloc- Tamuraetal. 2001; dePlaaetal. 2006, for a detailed descrip-
ity in NGC 4636 which is 0.25 times the sound speed at the tion ofthe analysis). We use the RGS modelbackgroundspro-
90%confidencelevel.Inthesestudies,isotropicturbulenceis videdbytheSASsoftwareasabestestimateforthebackground
a∼ssumed. However,these measurementsare sensitive to the ra- spectrum and we subtract it from the extracted source spectra.
dial direction of the turbulence. If the direction of the motions Sincethesuppressionofthe15Ålineduetoresonantscattering
are predominantlyradial or tangential, it would change the re- wouldbelargestintheverycentreofthegroup,we extractthe
sults(Zhuravlevaetal.2011). spectrumfroma1 wideregion(from 0.5 to0.5 withrespect
′ ′ ′
−
Recently, long RGS exposures were obtained for the cool tothecentreofthegroup)inthecross-dispersiondirectionofthe
(kT <1 keV) elliptical galaxies NGC 5044 and NGC 5813. RGSinstrument.
Thesetwogalaxieshaveverydifferentmorphologies.NGC5044 SinceRGSisaslitlessspectrometer,weneedtotaketheline
has bubbles which rise buoyantly, but are moved around in all broadeningdueto thespatialextentofthe sourceintoaccount.
directions due to sloshing motions induced by a recent merger The shape of the observed line profile depends on the surface
(Davidetal. 2009; Gastaldelloetal. 2009). NGC 5813, on the brightnessdistributionof the source in the dispersiondirection
other hand, has bubbles which rise nearly linearly along the ofRGS. Inordertomodelthelinebroadening,aspatialprofile
NE-SW axis and does not show evidence of sloshing or re- is extracted from MOS1 data in a region corresponding to the
cent merger interaction (Randalletal. 2011; Tranetal. 2001; selectedRGSfieldofview.We puttheprofileonawavelength
Emsellemetal. 2007). In the sample of Werneretal. (2009), grid using the relation ∆λ = 0.138 Å ∆θ, where ∆θ is the off-
NGC 5813 shows a hint of a presence of resonant scattering. setangleinarcminuteswithrespecttothecentreofthesource.
The (I +I )/I line ratio is 1.99 0.34in a 30 ks ob- To apply the effect of spatial broadeningto the spectral model
17.05 17.10 15.01
±
servation with respect to an expected ratio of 1.3, which sug- thatiscalculatedduringspectralfitting,weusethisprofileasa
gests that the turbulent velocity is indeed low as is expected convolutionkernel.Themodelspectrumisfirstconvolvedusing
for a mildly disturbed system. We aim to further constrain the thespatialprofile,beforewe convolveitwiththeresponsema-
(I +I )/I line ratio forboth galaxiesusing the deep trix. In the fit, we are able to optimise the width and centroid
17.05 17.10 15.01
RGS spectra and obtain better limits on the turbulent velocity. of the observedline profile by changingtwo parametersof the
This would allow us to study the connectionbetween the mor- convolution kernel. The multiplication factor s is the width of
phologyof the ICM andthe turbulentvelocity in two verydif- theresultinglineprofilewithrespecttotheoriginalprofilefrom
ferentenvironments. MOS1.Thelinecentroidisrepresentedby∆λ,whichistheshift
′
Inthispaper,weuseadistanceof31.2MpcforNGC5044 ofthecentroidinÅngstrom.Thebest-fitresultsoftheseparam-
and 32.2 Mpc for NGC 5813 (Tonryetal. 2001). At these dis- eterswillbeshownin additiontothe otherspectralparameters
tances,1 correspondsto9.1kpcforNGC5044and9.4kpcfor intheresultssectionofthispaper.
′
2
J.dePlaaetal.:EstimatingturbulentvelocitiesintheellipticalgalaxiesNGC5044andNGC5813
Table 2. Wavelengths and oscillator strengths for all strong 0
1
FeXVIIlinesinthe15Å to17Å range. Fe XVII Fe XVII
3C 3G+M2
Transition λ(Å)a fb fc fd 8
3C 2s22p61S –2s22p53d1P 15.015 2.31 2.73 2.49 Fe XVIII
0 1 Fe XVII Fe XVIII Fe XVII
3D 2s22p61S0–2s22p53d3D1 15.262 0.63 0.61 0.64 Å−1 3D O VIII Lβ 3F
3F 2s22p61S0–2s22p53s3P1 16.777 0.11 0.11 0.10 m −2 6
3MG2 22ss2222pp6611SS0––22ss2222pp5533ss13PP1 1177..005947 0.12 5100.138 5100.138 s −1
0 2 − − s
nt 4
u
Notes. (a) Wavelengths in Å according to NIST; o
C
http://physics.nist.gov/PhysRefData/ASD/lines form.html
(b) Oscillator strengths as provided by Shorer (1979); NIST
2
http://physics.nist.gov/PhysRefData/ASD/lines form.html
(c) Calculated oscillator strengths (D. Liedahl/SPEX) (d) AtomDB
v2.0.1(Lochetal.2006)
0
14 15 16 17 18
Wavelength (Å)
Fig.1. RGS spectrum of NGC 5813in the 14–18Å band.The
2.2.Spectralmodelling
line propertiesof the labelledFeXVIItransitionscan be found
inTable2.
For the spectral fitting, we use the SPEX spectral fitting pack-
age (Kaastraetal. 1996)1. We fit the RGS spectra with a
single-temperature model of plasma in Collisional Ionisation 3. Modellingofresonantscattering
Equilibrium(CIE).SinceNGC5044showsevidenceformulti-
temperaturestructure(seee.g.Tamuraetal.2003;Grangeetal. Modelling the resonant scattering effect that is observed in
2011), we also fit its spectrum with a Gaussian Differential RGS spectra consists of two parts. First, the FeXVII line ratio
Emission Measure model (gdem, dePlaaetal. 2006). To mea- (I17.05+I17.10)/I15.01 for an opticallythin plasmashouldbe de-
termined.Since we cannotmeasurethis ratiodirectly fromour
surethethreelinesofFeXVIIat15Åand17Åindependentlyof
observationsduetotheyetunknownopticaldepthoftheplasma
thespectralcode,theselineshavetoberemovedfromthemodel
andmonochromaticlineshavetotaketheirplace.InSPEX,this in the 15 Å line, it is calculatedusing atomic data andspectral
canbeachievedbyignoringtheFeXVIIionduringthecalcula- modelling.Secondly,we calculate how this ratio is affected by
tionof the line strengths.In Table 2, we list allstrongFeXVII resonantscatteringandturbulenceintheICM.
linesinthe 15Åto17Årange.Theselinesaremodelledwith
∼
delta functionsduringfitting and also broadenedby the spatial 3.1.TheFeXVIIlineratiosinopticallythinplasmas
profile derived from MOS1. Delta lines can be used, because
the natural width and the Doppler broadening of the lines are The line intensities of the Ne-like FeXVII lines in the 15–
muchsmallerthanthebroadeningduetothespatialextentofthe 18 Å range (see Table 2) have been subject to a long stand-
ing debate in atomic physics(partof the debate is summarised
source.Wesumthelinesat17.05Åand17.10Å,becausethey
in, for example, Guetal. 2007). For many years, the two
arenotresolvedbyRGS.
maingroupsperforminglaboratoryexperimentsattheNational
In the 15–17 Å range, there are, of course, also weaker
Institute of Standards and Technology (NIST) and Laurence
FeXVIIlinesthatcontributetotheflux,butwhicharenotmod-
Livermore National Laboratory (LLNL) were unable to reach
ellediftheFeXVIIionisignored.Theircontributionturnsoutto
agreementontheirmeasuredFeXVIIlineratios(see,forexam-
belimited.Thesixthbrightestlineinthe15–17Årange,which
ple Beiersdorfer 2003). On the other side, theorists, using dif-
hasawavelengthof15.46ÅandisnotlistedinTable2,hasan ferentmethodsandcodes,alsoobtainedratiosthatwereneither
intensity which is at peak emission only 3.5% of the intensity consistentwithothertheoreticalcalculationsnorwiththeexper-
ofthelineat15.01Å.Becauseofthesmallcontributionofthese imentaldata.Onlyrecently,Gillaspyetal.(2011)appeartohave
weaklinesandthecontributionofstrongerFeXVIIlinesoutside convergedtoexperimentalvaluesthatagreewiththeclosestthe-
the15–17Årange,wefixthetemperatureandFeabundanceto oretical predictionswithin 10%. However, given the history of
thebest-fitvaluethatweobtainfromafitwheretheFeXVIIion thisdebate,thisislikelynotthefinalwordonthisissue.
isincluded. There are several publicly available spectral codes that at-
Fig. 1showsthefluxedRGSspectrumofNGC5813inthe temptto modelthe line strengthsofthe FeXVIIlines in astro-
physicalplasmaseitherbasedontheoreticalcalculations,exper-
14–18Åband.Themodelfitdoesnottakeintoaccountresonant
scattering effects. The figure shows that the line at 15 Å (3C) imentaldata,orboth.Inthiswork,wecomparetheresultsfrom
AtomDB2,whichisintegratedinthewidelyusedAPECmodel,
is overestimated by the model. However, also the other lines
with the SPEX code, which is based on MEKAL (Meweetal.
from FeXVIII and OVIII are not well fitted. These discrepan-
1995)andupdatedwiththetheoreticalFeXVIIcalculationsby
cies are mostlikely dueto uncertaintiesin the atomic data and
Doron&Behar(2002).The(I +I )/I ratiosasafunc-
theionisationbalance,whichcanleadtosystematicerrorsupto 17.05 17.10 15.01
tion of the temperature for the different codes are plotted in
30%inindividuallinestrengths.Clearly,thisisanissueforthe
Fig.2.Ascanbeexpectedfromtheinconsistenciesbetweenthe
FeXVIIlinesaswell.Therefore,wediscusstheuncertaintiesin
experimentalatomic data and theoretical predictions, the spec-
theatomicdataofFeXVIIextensivelyinthispaper.
tralcodesdonotagree.
1 Seehttp://www.sron.nl/spexformoreinformation 2 Website:http://www.atomdb.org
3
J.dePlaaetal.:EstimatingturbulentvelocitiesintheellipticalgalaxiesNGC5044andNGC5813
Doron&Behar(2002)aremoreconsistentwiththeexperimen-
talvalues,whiletheyshouldnotbe,becausetheionisationand
recombinationprocessesincludedinthismodeldonottakeplace
in the experimental setup. Electron beam ion trap (EBIT) ex-
periments predominantlycontain ions from the same kind, i.e.
FeXVII.AsDoron&Behar(2002)showedinthe3-ionmodel,
the presence of the neighbouring ions (FeXVI and FeXVIII),
asinastrophysicalsources,increasesthepopulationofthelev-
els of the 16.78 Å, 17.05 Å, and 17.10 Å lines, thus enhanc-
ing the (I + I + I )/I ratio with respect to lab-
16.78 17.05 17.10 15.01
oratory measurements. Although this effect is genuine and ex-
pectedinastrophysicalplasmas,theirpredictedvaluesusingthe
single-ionmodelarenotconsistentwiththelaboratoryvaluesof
Gillaspyetal.(2011).
Recently, AtomDB v2.0.1 was released (Fosteretal. 2010)
containing major updates of the atomic data and new calcula-
tions of the line formationprocesses. The AtomDB developers
chosetousemorerecentcalculations(Badnell2006a,b)3 based
onthecodebyBadnell(1986),whichshouldbemoreconsistent
Fig.2. Measured and theoretical values for the line ratio with the experimentaldata andalso includesionisationand re-
(I17.05 + I17.10)/I15.01 as a function of temperature. The lines combinationprocesses,likeinDoron&Behar(2002).Asshown
indicate the values predicted by the currently available spec- by the dashed line in Fig. 2, the predicted ratio from AtomDB
tral codes: AtomDB v1.3.1 (grey), SPEX (black) updated with v2.0.1 has increased with respect to the other codes. The ratio
Doron&Behar(2002),andAtomDBv2.0.1(dashed).Theblack dropsfromabout2.5at0.2keVto1.4at1.0keV.
triangle indicates the value found through laboratory measure- Comparingthepredictedratioswithlaboratoryexperiments
ments (Beiersdorferetal. 2004). The greytriangles show mea- or astrophysical observations is not straightforward, because it
surementsofotherellipticalsperformedbyWerneretal.(2009) isdifficulttodeterminetowhatextentresonantscatteringisact-
andWerneretal.(2006)(M87).ForM87,thetemperaturedeter- ingintheexperimentalsetuporintheastrophysicalsource.The
miningthelineratioisassumedtobethecoolesttemperaturein tokamak measurements reported by Beiersdorferetal. (2004)
the two-temperaturemodel.The black dots show our measure- are believed not to be affected by resonant scattering. The
mentsforNGC5044andNGC5813. weighted average of their measured data points is shown in
Fig.2.Thevaluesareconsistentwithinerrorswiththeupdated
spectral codes, but only marginally consistent with MEKAL
The codes based on older atomic data, AtomDB v1.3.1 and AtomDB v1.3.1. We also compare these ratios with the
and MEKAL (not shown in Fig. 2), both predict a line ratio values measured in a small sample of giant elliptical galaxies
of about 1.0 with a relatively small dependence on tempera- (Werneretal. 2009, 2006), which are shown in grey in Fig. 2.
ture, dropping from about 1.2 at 0.2 keV to 0.9 at 1.0 keV. If a data point is above the calculated curve, then it is an indi-
However, comparisons between stellar flare observations per- cationofresonantscattering,becausethe15Ålinecanbesup-
formedbytheXMM-NewtonandChandragratingspectrometers pressed through photons effectively scattering out of the beam
(e.g. Meweetal. 2001; Nessetal. 2002), laboratory measure- when pointing to the core of the galaxy. The only data point
ments(e.g.Beiersdorferetal.2004),andtheoreticalcalculations taken next to a galaxy core is ”NGC 4636 out”. In principle,
(e.g.Doron&Behar2002)suggestthatthisratioisprobablyun- onlyforthispointthe15Å linecouldbeenhancedbyphotons
derestimated.Doron&Behar(2002)foundthatdi-electronicre- effectively scattered into the beam and lowering the ratio, but
combination (DR) and resonant excitation (RE) processes are since the spectrum in this regionis dominatedby the emission
dominant in the 0.4–0.6 keV temperature range, while these nearthe coreofthe galaxywherethereis noenhancementyet,
processes were only modelled using scaling laws in MEKAL. thiseffectisverysmall.Otherwise,findingaratiobelowthecal-
Becauseofthisfinding,SPEXwasupdatedatthattimetochange culatedvaluesisveryunlikely.
thepredictedFeXVIIlinestrengthstothenewvaluesbasedon The astrophysical data from elliptical galaxies presented
thesetheoreticalcalculations.Theresulting(I17.05+I17.10)/I15.01 here appear to favour the ratio found using SPEX above the
ratioisshownasablacklineinFig.2.Thisratioisconsiderably AtomDB v2.0.1 value if we only take the statistical uncertain-
higher than the results from the MEKAL and AtomDB v1.3.1 ties into account. This allows us to favour the numerical value
codes.Itdropsfromaratioofabout1.9at0.2keVto1.25at1.0 obtainedbySPEX,butitdoesnotmeanthatthephysicalback-
keV. groundoftheDoron&Behar(2002)modeliscorrect.Theolder
The results obtained by Doron&Behar (2002) are, how- MEKALandAtomDBv1.3.1underestimatetheratioconsider-
ever, not without problems. Although they were right to con- ably,whichcouldbeexplainedbytheloweraccuracyofthees-
sider recombinationand ionisationin their model,neithertheir timationsusedtomodelthelineformationprocesses.Although
distortedwave calculationsnorthe ionisationbalancethat they the SPEX predictions appear to be more consistent with mea-
adopted(Mazzottaetal.1998)arestate-of-the-artfortheselines surements,asystematicuncertaintyinthelineratioofabout20–
or ions. For example, the single-ion model of Doron&Behar 30% remains when we consider the difference between SPEX
(2002) predictsa value for the (I16.78 + I17.05 + I17.10)/I15.01 ra- andAtomDBv2.0.1(Fig.2)andtheuncertaintyintheoscillator
tio that is always <2 for temperatures below 1 keV. But ac- strengthofthe15.015Åline(Table2).Thisisthesamelevelof
cordingtonewexperimentalandtheoreticaldata(Gillaspyetal. uncertaintyasdiscussedinBeiersdorfer(2003).
2011), this ratio should be >2 at these temperatures. The re-
sultsfromthe 3-ionmodel(FeXVI,FeXVII,andFeXVIII)of 3 http://amdpp.phys.strath.ac.uk/tamoc/DATA/
4
J.dePlaaetal.:EstimatingturbulentvelocitiesintheellipticalgalaxiesNGC5044andNGC5813
Table3.ChandraobservationsofNGC5044andNGC5813. Table4.Opticaldepths(τ)intheFeXVIIlinesat15.01Åand
17.05ÅinthegalaxiesNGC5044andNGC5813calculatedfor
Machnumbers0,0.25,0.5and0.75.
NGC5044 NGC5813
ObsID 798 9399 5907 9517
Exposuretime 21ks 84ks 49ks 100ks NGC5044 NGC5813
Totalexposure 105ks 149ks
Mach 15.01Å 17.05Å 15.01Å 17.05Å
0 3.8 0.2 6.3 0.4
0.25 1.6 0.09 2.6 0.1
0.50 0.8 0.05 1.4 0.08
0.75 0.6 0.03 0.9 0.05
3.2.ModellingresonantscatteringintheICM
The modelling of resonant scattering in the 15 Å line in giant 3.2.1. MonteCarlosimulations
ellipticalsisperformedthroughMonteCarlosimulations.Asan
IntheMonteCarlosimulations,wecalculatetheopticaldepthτ
inputtothesimulations,weusedeprojecteddensity,temperature
inthecentreofthelineusing
andFeabundanceprofilesobtainedwithChandra.TheChandra
observations of NGC 5044 and NGC 5813 that we use, are
τ= nσ dl, (3)
listedinTable3.Theinitialdataprocessingisperformedusing i 0
Z
thestandardmethoddescribedinVikhlininetal.(2005)andus-
ingthelatestcalibrationcorrections.Subsequently,wedeproject where l is the distance along the photon propagationdirection,
theobservedspectrumto measurethetemperature,densityand ni is the number density of ions in the ground state of a given
abundanceprofilesoftheplasma.Thedeprojectiontechniqueis transitionandσ0 isthecrosssectionforscatteringatthecentre
describedindetailbyChurazovetal.(2003)andChurazovetal. ofaresonantline.Thiscross-sectionisgivenby
(2008).Inbrief,wefindthesetof3Dspectrainsphericalshells
√πhr cf
thatfitthe observedspectrain concentricannulibestafterpro- σ = e , (4)
jection. Assuming that the volume emissivity in the outermost 0 ∆ED
shell at all energies declines as a power law with radius, we
wherehisthePlanckconstant,r istheclassicalelectronradius,
e
model the observed spectra as a linear combination of spectra
f theabsorptionoscillatorstrengthofa givenatomictransition
insphericalshellsplusthecontributionoftheoutermostlayers.
and∆E theDopplerwidthoftheline,whichisdefinedas
D
Thematrixthatdescribestheprojectionoftheshellsintoannuli
with the contributionofouterlayersis invertedand thespectra 2kT V2 1/2
intheshellsarecalculatedbyapplyingtheinvertedmatrixtothe ∆E = E e + turb . (5)
D 0Am c2 c2 
oinbsXeSrvPeEdCspweicthtraa.nTAhPeErCesumltoindgels(pSemctirthaeintaelv.e2r0y0s1h)e.ll are fitted Here,E istherpestenergyoftheline,T istheelectrontempera-
0 e
The radial profiles of the deprojected gas parameters in ture,Aistheatomicmassoftheion,m istheprotonmassandc
p
NGC 5044 and NGC 5813 are shown in Fig. 3. One can isthespeedoflight.Usingtheadiabaticsoundspeedc ,onecan
s
see that assuming a fixed abundance of 0.5 times Solar expressthelevelofturbulenceasaMachnumber M = V /c
turb s
(Anders&Grevesse 1989) does not change the profiles of the (see,e.g.,Churazovetal.2004),i.e.
densityandtemperaturesignificantly.Theblackcurvesshowthe
fits of the densityand temperaturethatwe used asan inputfor ∆E = E 2kTe (1+1.4AM2) 1/2. (6)
our Monte Carlo simulations. The number electron density in D 0"Am c2 #
p
NGC 5044 is described as a β model with a normalisation of
n = 0.06cm 3,acoreradiuso−fr = 3kpcandaβ = 0.4.The The calculated optical depths in the considered lines of both
0 − c
temperatureprofile(inkeV)isparametrisedas galaxiesareshowninTable4forMachnumbersrangingfrom0
to0.75.
Assuming spherically symmetric models of galaxies, we
r 3 then calculate the plasma line emissivities using APEC
1+1.75
Te(r)=0.73 r(cid:18)173(cid:19) , (1) (aSremittahkeetnalf.ro2m00t1h)e. ALtionmeDeBne4rg(iLeoschanedtalo.sc2i0ll0a6to)ransdtreNngISthTs
1+ Databases5. The ionisation balance (collisional equilibrium) is
(cid:18)17(cid:19) taken from Bryansetal. (2009). Multiple resonant scatterings
are calculated using a Monte Carlo approach, which treats an
where r is in kpc. The number electron density in NGC 5813 individual act of resonant scattering in full detail (details of
is also described as a β model with a normalisation of n0 = these simulations are described in e.g. Churazovetal. 2004;
0.15 cm−3, a core radius−of rc = 0.5 kpc and a β = 0.37. The Werneretal.2009;Zhuravlevaetal.2010,2011).InFig.4,we
temperatureisdescribedas show the resulting ratio of the intensities in the FeXVII lines,
(I + I )/I , for both elliptical galaxies and for Mach
17.05 17.10 15.01
numbersbetween0and0.75.Onecanseetheeffectofresonant
r 2 0.07 scattering: in the core the line intensity is suppressed, while in
0.55 1+ r 5kpc
Te(r)=0.75"1+(cid:18)5r(cid:19)2#−0.09 r ≥<5kpc (2) th4eshuttrpr:o//uwnwdwin.agtormegdibo.norsgt/heintensityoflineincreases.
 " (cid:18)1(cid:19) # 5 http://www.nist.gov/pml/data/asd.cfm
5
J.dePlaaetal.:EstimatingturbulentvelocitiesintheellipticalgalaxiesNGC5044andNGC5813
Fig.3. Deprojected profiles of number electron density, temperature and Fe abundance in NGC5044 (left panel) and NGC5813
(rightpanel).Greypoints:deprojectedprofileswithfreeabundance,blackpoints:deprojectedprofileswithfixedabundance.Black
curves:fitsofdensityandtemperatureusedinourcalculations.
Fig.4.FeXVIIlineratiosforNGC5044(left) andNGC5813(right).Thelinesgivethepredicted(I +I )/I lineratio
17.05 17.10 15.01
fordifferentMachnumbers(0,0.25,0.5,and0.75)fromMonteCarlosimulationsassumingisotropicturbulence.Note thatthese
profileshavebeenre-scaledtotheoptically-thin(I +I )/I lineratiofromDoron&Behar(2002).Thedatapointineach
17.05 17.10 15.01
plotisthemeasuredlineratioderivedfromRGS.
4. Results best-fitparametersforasingle-temperaturemodelandthegdem
model are listed in Table 5. Due to resonantscattering and the
We fit RGS 1 and RGS 2 first order spectra extracted from a systematic uncertaintiesin the atomicdata, the fits are notper-
region0.5 aroundthecentreofthegroupwithasingletemper- fect, but acceptable for our purpose. The multi-temperature fit
′
ature model. In addition, we also fit a multi-temperature gdem forNGC5044iscomparableinqualitytothesingle-temperature
modeltothespectrumofNGC5044.Unfortunately,thespectral fitandyieldsparametervaluesthatareconsistentwiththesingle-
resolutiondeterioratestoo quicklyto derive meaningfulresults temperatureresults.Thewidthofthelinesduetothespatialex-
fromthe outer spatialregionsin the cross-dispersiondirection. tentofthesourcearewellfitted bythe spatialprofileextracted
The rotation anglesof the RGS extractionregionswith respect fromEPICMOS,becausethescalefactor s(seeSection2.1)is
tothegroupemissionisvisualisedinGrangeetal.(2011).The consistent with being 1 for both groups, which means that the
6
J.dePlaaetal.:EstimatingturbulentvelocitiesintheellipticalgalaxiesNGC5044andNGC5813
Table 5. RGS best-fit model results for NGC 5044 and NGC Table6.EstimatesfortheturbulentvelocitiesinNGC5044and
5813 for an extraction region of 0.5 around the centre of the NGC5813.
′
source. For NGC 5044 we also show the results from a multi-
NGC5044 NGC5813
temperaturegdemmodel.
M V a Pred.b M V a Pred.b
turb turb
0.00 0 1.76 0.00 0 2.94
Parameters NGC5044 NGC5044gdem NGC5813 0.25 115 1.53 0.25 104 2.13
Y(1070m 3) 8.9 0.3 8.8 0.3 1.79 0.12 0.50 230 1.42 0.50 208 1.75
− ± ± ± 0.75 345 1.37 0.75 311 1.61
kT (keV) 0.784 0.003 0.795 0.007 0.647 0.003
± ± ± 1.27 1.30
σ 0.043 0.012 ∞ ∞ ∞ ∞
± Observed 1.25 0.08 Observed 1.87 0.08
O 0.40 0.03 0.42 0.03 0.49 0.05
± ±
Ne 0.81±0.09 0.72±0.11 0.32±0.07 LowerLimit >320kms−1 LowerLimit >140kms−1
Fe 0.56±0.02 0.59±0.03 0.70±0.05 UpperLimitc <770kms−1 UpperLimitc <540kms−1
± ± ±
s 1.01 0.03 1.01 0.03 0.99 0.03
∆λ (mÅ) -1.1±3.1 0.2±3.2 -1.2±2.7 Notes. (a) Equivalent turbulent velocity estimate in km s−1 using the
′ ± ± ± Machnumberandtheadiabaticsoundspeed,whichis460kms 1 for
C-stat/d.o.f. 959/518 955/517 1303/781 −
NGC5044and415kms 1forNGC5813.(b)Emissionweightedaver-
−
ageofthepredicted(I +I )/I ratioovertheRGSextraction
17.05 17.10 15.01
region. (c) 90% upper limits on the turbulence derived from the line
widthinRGS.
width is the same as the width of the originalprofile. Also the
wavelength,whichdependsonthepositionofthesource,ises-
timatedproperly.Thewavelengthshift∆λ isfreeinthefitand
′ increase due to resonant scattering in the core of an elliptical
thebestfitvalueisconsistentwithbeing0.
galaxy, we cannot explain the low value for NGC 5044 if the
AnotherreasonfortherelativelyhighC-statvaluesthatwe
AtomDBv2.0.1ratioisassumed.Thevalueof1.25is,however,
obtaincouldbethepresenceofmulti-temperaturestructure.The
consistentwiththepredictedratiousingDoron&Behar(2002).
RGS data of NGC 5813, however, do not show significant ev-
The elliptical galaxy NGC 4649 in the sample of Werneretal.
idence for that (Grangeetal. 2011). In NGC 5044, there are
(2009)andM87(Werneretal.2006)alsoshowaratioof 1.25–
indications for multi-temperature structure, but the lower cut- ∼
1.30, but with a larger uncertainty. It should be noted that one
offtemperatureisat0.4keV(Grangeetal.2011),whichmeans
of the points, NGC 4636 out, was derived from a RGS spec-
thatthereisnoindicationfortemperaturesbelow0.4keV.Fig.2
trum just outside the galaxy centre where the optical depth of
shows that the expected line ratios flatten out above tempera-
turesof 0.5keV,whichjustaddsanuncertaintyof.10%tothe the15Ålineisexpectedtobesmallerthaninthecentre.
optically∼-thinline ratio, which is small comparedto the uncer- Since the (I17.05 + I17.10)/I15.01 ratio for an optically-
taintiesintheatomicdata.Sinceplasmabelowtemperaturesof thin plasma is uncertain from the atomic data side and the
Doron&Behar (2002) results are problematic (Section 3.1),
0.5keVisextremelyrareincool-coreclustersandgiantellip-
∼ticals (Sanders&Fabian2011) and we donotsee evidencefor we are left without a good reference ratio to base our turbu-
OVII emission lines indicating temperatures below 0.4 keV, lence estimates on. The best we can do is to assume that the
∼ hot gas in NGC 5044 is subject to such high levels of turbu-
wewillusetheresultsfromthesingle-temperaturemodel.
Todeterminethe(I +I )/I lineratio,weusethe lence that the gas is effectively optically thin and estimate the
17.05 17.10 15.01
turbulence in NGC 5813 relative to that ratio. Since the shape
best-fitmodelshowninTable5andignoreFeXVIIinthespec-
of the (I + I )/I ratio as a function of temperature
tralcalculation.ThisremovesthestrongestFeXVIIlines,which 17.05 17.10 15.01
from Doron&Behar (2002) is very similar to the shape of the
arelistedinTable2,fromthemodel.Wethereforereplacethese
AtomDB v2.0.1 curve (see Fig. 2), and because it is also con-
fivelineswithfourdeltafunctions.Sincewecannotresolvethe
sistentwiththeratiofromNGC5044,weassumeforthispaper
17.05Å and17.10Å lines,theyaremodelledbya singledelta
thatDoron&Behar(2002)indeeddescribetheFeXVIIratiosin
function at the average wavelength of the two lines. Then, we
anoptically-thinplasmacorrectlyandthatthegasinNGC5044
fit the spectrum again with the temperature and Fe abundance
isopticallythin.Atthesametime,weareawareoftheproblems
fixed to their best-fit values, because these two parameters are
withtheDoron&Behar(2002)result.Therefore,wediscussthe
likely to introduce bias due to the lack of FeXVII lines in the
systematicuncertaintiesextensively.
spectral model (See Sect. 2.2). Other parameters, like the nor-
malisation,theoxygenandneonabundances,thedeltalines,and
thespatialbroadeningparametersareallowedtooptimise.From
4.1.Turbulentvelocityestimates
thesefits,weobtaina(I +I )/I lineratioof1.25 0.08
17.05 17.10 15.01
±
for NGC 5044 and 1.87 0.08 for NGC 5813. Also the ratio The(I +I )/I lineratiosfromthefitscanbedirectly
17.05 17.10 15.01
±
from the multi-temperaturefit of NGC 5044 yields 1.25 0.08, comparedtothepredictedratiosfromourMonteCarlosimula-
±
which shows that the line ratio does not depend on the multi- tions.TheobservedratiosandtheMonteCarloresultsareplotted
temperaturestructure.Theratiosmeasuredinthesetwogalaxies inFig.4.Notethatthesimulationresultshavebeenre-scaledto
are significantly different from each other, which suggests that berelativetotheoptically-thinlineratiopredictedusingSPEX.
theresonantscatteringeffectinNGC5813ismuchstrongerthan ThehorizontalerrorbaronourdatapointindicatestheRGSex-
inNGC5044. tractionregion.Fromthesimulations,itis evidentthatthe pre-
The comparison of the observed ratios with the line ratios dicted ratio varies substantially within this region. In order to
from an optically-thin plasma calculated by the different spec- make a fair comparison, we choose to calculate the emission-
tralcodesyieldsaveryinterestingresult.InFig.2,theobserved weighted average of the predicted line ratio within the 0.5–
′
−
ratioforNGC5044appearstobebelowthepredictedratiofrom 0.5 fieldofview.Weusethecountratesobservedinthecross-
′
AtomDB v2.0.1 at a 2σ confidence level considering only dispersiondirectionofRGSinthisintervalasweightingfactors
∼
statistical errors. Since the (I + I )/I ratio can only forthecalculation.TheresultingratiosarelistedinTable6.
17.05 17.10 15.01
7
J.dePlaaetal.:EstimatingturbulentvelocitiesintheellipticalgalaxiesNGC5044andNGC5813
Usingtheemissionweightedratiosfromthesimulationand AnimportantquestiontounderstandthestructureoftheICM
theobservedratiosinTable6,wecanconstrainthelevelofthe ingroupsandclustersofgalaxiesiswhatfractionofthepressure
turbulentvelocityintheinner0.5 (r.5kpc)ofthegalaxy.For intheICMisduetoturbulence.Thisfractionisequivalenttothe
′
NGC 5044, this is more difficult than for NGC 5813, because ratiooftheturbulentenergy(ǫ )tothethermalenergy(ǫ ),
turb therm
of our assumption that the gas in NGC 5044 is optically thin. which can be easily calculated from the Mach number of the
Attheselowopticaldepths,theresonantscatteringmethodloses turbulence(Werneretal.2009):
sensitivityinthevelocityestimate.Thereforewecanonlyderive
ǫ γ
a lower limit for the turbulence. If we only consider statistical turb = M2, (7)
errors, the 90%confidence lower limit is 320 km s 1 for NGC ǫtherm 2
−
5044.ForNGC5813,the15Ålineissignificantlysuppressedby whereγistheadiabaticindexforanidealgas(γ = 5/3).From
resonantscattering with respectto the SPEX prediction,which
the lower limit derived for NGC 5044, we derive that the tur-
allowsabetterestimateoftheturbulence.Toobtaintheveloci-
bulentenergycan rangefrom40%of the thermalenergyup to
tiesinkms 1,weinterpolatelinearlybetweenthesimulatedval-
− a factor of 2 times the thermal energy. The turbulent pressure
uesinTable6.Sincetheerrorsduetothelinearinterpolationare
clearly is a component which should be considered for NGC
smallerthanthesystematicerrorsintheatomicdata,wedonot
5044. The turbulent energy fraction for NGC 5813 is signif-
gainaccuracybyperformingmoreMonteCarlosimulationsfor
icantly lower, between 15% and 45% if we take into account
otherMachnumbers.OurbestestimateforV inNGC5813,
turb thesystematicuncertaintyintheinitiallineratio.Sinceweonly
withouttakingintoaccountanysystematicerror,is175 22km
probethe inner 0.5 of both galaxieswith RGS and the optical
s 1,whichmeansastatisticalprecisionofabout13%. ± ′
− depthin the 15 Å line decreasesrapidlyabover > 10 kpc, the
From the discussion in Sect. 3, however, we know that we valuesderivedaboveare valid for the innerregionswithin r .
also needto takeinto accountthe systematicuncertaintyin the 5–10kpc.
optically-thinlineratios.Forthecalculationabove,weusedthe
line ratio predicted by SPEX, which appears to be a reason-
4.2.ChandraimagesofNGC5044andNGC5813
able estimate. But, the typical difference between SPEX and
AtomDB v2.0.1 is about 15%, which also adds to the uncer- Sincewefindasignificantdifferencebetweenthemeasuredtur-
tainty. Using the AtomDB v2.0.1 ratio would not give a phys- bulentvelocitiesin NGC 5044and NGC 5813,it is interesting
icalresultforNGC5044,butwecancalculateitforNGC5813. tolookfortheoriginsoftheturbulencebylookingatthespatial
Assuming an optically-thin ratio of 1.513, the turbulent veloc- structure of the groups. They both show X-ray cavities blown
itywouldbe310kms−1.Thedifferencebetweenthevelocities bythe centralAGN in theircore,whichis a possiblesourceof
based onthe AtomDBv2.0.1and the SPEX ratiois a factorof turbulence. In order to visualise these cavities, we used the 4
two,whichmeansthatderivingtheturbulentvelocityinthisway Chandra ACIS observations listed in Table 3 to create an im-
isverysensitivetotheassumedoptically-thinlineratio. agewheretheaverageICMemissionissubtracted(seealsoe.g.
Although the optically-thin line ratio is sensitive to the Davidetal.2009;Randalletal.2011,forNGC5044andNGC
plasma temperature, we do not expect that multi-temperature 5813 respectively). In all observations, the source was centred
structure in the gas adds to the systematic uncertainty. Fig. 2 on ACIS-S3 of the ACIS-S detector. For both sources, the two
showsthatthelineratiohasitslargestdeclinebetween0.2and individualeventfiles from the standard pipeline productswere
0.5 keV. From 0.5 to 1.0 keV, the ratio flattens. The tempera- mergedtocreateasingleeventfilecontainingthemaximumpos-
turesthatwe measureforNGC5044andNGC5813are inthe sibleexposure.Then,weextractedradialsurfacebrightnesspro-
range 0.6–0.8keV, where the ratio is fairly constant. The ratio files within a 2.5′ radius from the central AGN in the 0.5 – 7
could be biased if there were temperatures present below 0.5 keV band. The maximum extraction radius of 2.5′ falls within
keV. However, we do not detect significant OVII emission in the CCD 7 boundariesin all observations. The derived surface
the spectrum, which would indicate the presence of those low brightnessprofileswerefittedusinganempiricalmodelconsist-
temperatures.ItisthereforesafetoassumethattheFeXVIIline ingoftwoKingprofilesandaconstant.Thefullexpressionfor
ratios are notbiased due to a significantcontributionfrom low thismodel f(r)is:
temperaturesand that the systematic uncertaintyin the derived
N N
velocitiesduetomulti-temperaturestructureisnegligible. f(r)= 1 + 2 +C (8)
r2 β1 r2 β2
Another method of estimating the turbulence from high- 1+ 1+
resolutionspectraisbymeasuringthelinewidth.Althoughthe  r2   r2 
linebroadeningduetothespatialextentofthesourcemakesan  c1  c2
withnormalisationsN andN ,coreradiir andr ,andexpo-
accuratedeterminationoftheturbulentvelocityimpossible,itis 1 2 c1 c2
nentsβ andβ forbothKingprofiles.AconstantC wasadded
possible to obtain an upper limit of the turbulentvelocity with 1 2
to accountfor the backgroundemission. The expected number
RGS.Inthefit,webothallowtheturbulentvelocityintheCIE
ofcountsfromthebest-fitdouble-Kingmodelwassubsequently
modelandthescalefactorsthatcontrolsthewidthofthespatial
subtracted from the observed number of counts in the original
distributionto be free.Since the spatialdistributionis different
imageforeverypixel.Theresidualimagesthatweobtainedwere
frombeingGaussian,itdoesnotcorrelatesignificantlywiththe
smoothedwithaGaussianusingawidth(σ)of0.98 .Theresult
Gaussian velocitybroadening,which allowsus to obtainupper ′′
forbothgalaxiesisshowninFig.5.
limits with a good accuracy. During the error calculation, also
the scale factor s is allowed to optimise, which means that un-
certaintyisincludedintheupperlimit.Theresultsareshownon
5. Discussion
thelastrowofTable6.ForNGC5044,the90%upperlimitcon-
strainstheturbulentvelocitytotherangebetween320<V < Using deep XMM-Newton RGS data of NGC 5044 and NGC
turb
770kms 1. TheupperlimitforNGC5813of< 540kms 1 is 5813,wehavefoundasignificantdifferencebetweenthe(I +
− − 17.05
consistentwiththeestimatedvaluefromresonantscattering. I )/I line ratios in these galaxy cores. Our result shows
17.10 15.01
8
J.dePlaaetal.:EstimatingturbulentvelocitiesintheellipticalgalaxiesNGC5044andNGC5813
Fig.5.ChandraACISimagesofNGC5044(left)andNGC5813(right)aftersubtractionoftheaverageICMemission.
that the FeXVII line at 15 Å in NGC 5813 is significantly af- RGSobservationsofellipticalgalaxieslikeNGC5044also
fected by resonantscattering, confirmingthe 2σ hint foundby helptoconstraintheoptically-thinline ratio.Althoughwecur-
Werneretal.(2009)thatisbasedonashorter30ksRGSexpo- rently lack the statistics to definitively rule out certain model
sure. The (I + I )/I line ratio in NGC 5044, on the calculations, a larger sample of deep observationsof these ob-
17.05 17.10 15.01
otherhand,isfoundtobeconsistentwiththelineratiopredicted jects will help to constrain the upper limits for this ratio. As
bytheCIEmodelinSPEX,whichisinagreementwiththecon- shown in Section 3.1, laboratory measurements and theoreti-
clusionofTamuraetal.(2003),basedona20ksRGSexposure, calcalculationscouldeventuallyimproveourknowledgeabout
that resonance scattering effects in NGC 5044 are small. The the optically-thin line ratio as soon as they yield consistent re-
difference between the line ratios in these galaxies is best ex- sults. Thereare signsthattheoreticaland laboratoryresultsare
plained by resonant scattering in the 15 Å line of NGC 5813, converging(Gillaspyetal. 2011), but this still needs confirma-
whichisstrongduetotherelativelymodestturbulentvelocities tion. Consideringthe remaininguncertaintiesin individualline
( 0.4M) in the core of this object. Other effects that could af- strengths,it isclearly necessaryto investin theoreticaland ex-
f∼ectthemeasuredlineratio,likemulti-temperaturestructure,do perimentalresearchtoimprovetheatomicdatabeforenewmis-
not appear to play a role. Interestingly, we do not find strong sions like Astro-H are launched (Takahashietal. 2010). This
evidenceformulti-temperaturestructureinNGC5044,contrary work shows that accurate atomic data is important to perform
to the results found by Tamuraetal. (2003) and Grangeetal. robustmeasurementsofastrophysicalsources.
(2011). This difference originates from our relatively narrow
cross-dispersionselection that we need to detectresonantscat-
tering. We extract the RGS spectrum from a 1′ wide region in 5.2.Lineprofilesandvelocitybroadening
the cross-dispersiondirection,while Tamuraetal. (2003) use a
widthof2 andGrangeetal.(2011)awidthof5.Theoriginof
′ ′ Upperlimitsonthe turbulentvelocitywere estimatedforNGC
themulti-temperaturesignalreportedinthesepreviousworksis
5044 and NGC 5813 before by Sandersetal. (2011) based on
therefore most likely the temperaturegradient in the inner few
the oldest data sets with unfiltered exposures of 43 and 73 ks,
arcminutesofthesource.
respectively. They find upper limits of 1500 km s 1 for NGC
−
5044 and 1400 km s 1 for NGC 5813 at the 90% confidence
−
level.Ourupperlimitsaresignificantlysmaller.ForNGC5044,
5.1.Systematicsintheoptically-thinlineratio
we find an upper limit based on line broadening of <770 km
s 1, which is a factor of 1.95 improvement with respect to
The systematic uncertainty in the unscattered optically-thin −
∼
(I + I )/I ratio is the most important limitation for Sandersetal. (2011). This is consistent with the expected im-
17.05 17.10 15.01
measuring turbulent velocities in clusters and elliptical galax- provement( √3)duetotheincreasedstatistics.ForNGC5813,
∼
ies. For NGC 5813, we find a difference of a factor of two in the improvementis much larger than expected based on statis-
the derivedturbulentvelocitywhenthe optically-thinline ratio tics.Theupperlimitdropsfrom<1400kms−1 to<540kms−1
is increased by only 15%. This clearly adds a large systematic (factor 2.5),whichismuchbetterthantheimprovementofthe
∼
uncertaintytothederivedturbulence.Theresultcanonlybeim- statistics(factor √2).Wenotethatthemethodofcorrectingfor
provedwhen the uncertaintyin the optically-thinline ratio de- thespatialbroadeningisdifferentinSandersetal.(2011),there-
creases, which is difficult due to the problems with the atomic foretheresultsmaynotbefullycomparable.Althoughourupper
data of FeXVII. The estimated uncertainty on individual line limitforNGC5813islowerthanexpected,itisstillconsistent
strengthsproducedbyspectralcodesisabout20–30%. withtheindependentmeasurementusingresonantscattering.
9
J.dePlaaetal.:EstimatingturbulentvelocitiesintheellipticalgalaxiesNGC5044andNGC5813
We must also realise that the velocity broadeningmeasure- Anothersourceof turbulencewould be cold frontsinduced
mentsofellipticalgalaxieswithRGSaresensitivetosystematic by an off axis merger. Chandra, XMM-Newton, and optical
effects. The line profile of a line in RGS doesnot only depend data show evidence that NGC 5044 had a recent merger with
on the velocity broadening, but also on the line strength as a a small sub group (Mendeletal. 2008; Gastaldelloetal. 2009;
functionofradiusintheellipticalgalaxy.Sincethelinestrength Davidetal.2009).TheChandraimagesofNGC5044andNGC
of a certain line depends on the temperature, density, element 5813inFig.5indeedshowastrikingdifference.Whilethebub-
abundance, and resonant scattering properties, the line profile blesinNGC5813appeartoriseinastraightlinealongtheNE–
observedwithRGScanbedifferentforeachline.Inordertoin- SWaxis,thebubblesinNGC5044riseinmanydirections.Most
cludethebiasduetothedifferentlinewidthsintheupperlimit, likely, the recentmerger in NGC 5044 has induceda sloshing,
weallowthewidthofthebroadeningprofilederivedfromMOS or swirling, motion in its ICM, causing the bubbles to rise in
(s)tooptimise.Thederivedupperlimitsontheturbulencethus a rotating medium (Davidetal. 2009). Along the edges of the
givearoughestimateofwhatlevelofturbulencewouldstillbe coldfrontsassociatedwiththemerger,instabilitiescauseturbu-
consistentwithintheaverageofthelinewidthsofthelinesinthe lent motionsthat may be detected using resonant scattering. A
spectrum. potentialproblemwiththisinterpretationisthefactthatnumer-
ical simulations, like e.g. Ascasibar&Markevitch (2006) and
In addition, resonantscattering would also change the pro-
Roedigeretal. (2011) show that for a sloshing cluster the ve-
file of the 15 Å line. The core of the line would be suppressed
locityfield inthe centreofthehotgashasa muchsmalleram-
and because resonant scattering is a conservative process, the
plitudethanthevelocitiesintheoutskirts.Sinceweareprobing
line wingsshould be enhanced.Unfortunately,line blendspre-
mainly the inner 10 kpc of the galaxy, we should not see the
ventusfromstudyingthewingsofthe15Ålineinmoredetail. ∼
high velocities associated with the merger. On the other hand,
Since thestrongestresonantscatteringeffectis thesuppression
these simulations were made for larger systems and different
ofthecoreoftheline,ourmethodofusingdeltalinesconvolved
merger geometries. We cannot exclude the possibility that in
with the spatial line profile give us a good estimate of the line
NGC 5044 the gas in the centre is disturbed by the sloshing
ratios.TheotherICMpropertiesthatdeterminethelineprofile,
motions. NGC 5813 does not appear to be disrupted by a re-
like temperature, density, and abundance do not influence our
cent merger (Randalletal. 2011). Most likely differential gas
estimateofthe(I +I )/I ratio,becausetheselinesare
17.05 17.10 15.01 motions, including turbulence, due to the gas sloshing are re-
allemittedbyFeXVIIandtheyshouldhaveasimilarprofilein
sponsible for the observed small optical depth of the FeXVII
the observedtemperature range, apart from resonant scattering
lineat15Å.
effects.
ThederivedvaluesfortheturbulentenergyfractioninNGC
5813 (15–45%), assuming isotropic turbulence, are compara-
5.3.Originofturbulence ble to estimates from numerical simulations (e.g. Dolagetal.
2005; Nagaietal. 2007), which range between 5–30% of the
The RGS spectra of NGC 5044and NGC 5813were extracted thermalenergy.Theturbulentvelocitiesaround100–200kms 1
−
fromthe brightinnerregions(r . 5kpc)ofbothgalaxies.The couldverywellbe inducedby the bubblesblownbythe AGN.
opticaldepth in the 15 Å line declinesrapidly beyonda radius Heinzetal.(2010)showthatbubblescangenerateturbulenceas
of 10 kpc. It is therefore reasonable to assume that most of highas 500kms 1 inclustersofgalaxies.Duetothereduced
−
the∼resonantscatteringsignaloriginatesfromtheinner.10kpc scale of∼an elliptical galaxy with respect to a cluster, an AGN
ofthegalaxy.Strictlyspeaking,anydifferentialgasmotioncan generatedvelocity of 100–200km s 1 seems reasonable.NGC
−
contribute to the line broadening and affect the optical depth. 5044,ontheotherhand,showsaveryhighturbulentenergyfrac-
Therefore,our estimate of the turbulence and the turbulenten- tion>40%.TheICMofNGC5044appearstobeverydisturbed.
ergyimplicitlyincludesalltypesofdifferentmotions,provided Duetothe lowmassofNGC5044,a mergeris themostlikely
that they happen on spatial scales where the accumulated opti- explanationforthehighlevelofturbulence.
cal depth is &1. If one assumes normal turbulence (e.g. with a
Kolmogorovspectrum)mostoftheenergyisfoundatthelargest
scales. Only eddies with a characteristic size comparable to or 5.4.Futureprospects
smaller than the size of the optically thick core affect the am-
plitudeoftheresonantscatteringsignificantly(Zhuravlevaetal.
Estimating turbulentvelocities through resonant scattering and
2011).
line broadening, like presented in this paper, is at the limit
Both NGC 5044 and NGC 5813 show AGN activity in the of the current instrument capabilities. Although fitting high-
form of bubbles, therefore the considerable difference in the resolutionspectrafromnewmicro-calorimeterinstruments,like
measuredturbulencemaybedifficulttoexplainbyAGNinduced SXSaboardAstro-H(Takahashietal.2010),willbeachallenge
turbulencealone.TheAGNactivityinNGC5813hasproduced for the current atomic databases and spectral codes, the excel-
bubbles and relatively strong shocks (Randalletal. 2011), but lent spectral resolution of SXS will enable us to measure tur-
apparentlyonlymodestdifferentialgasmotionsandturbulence bulent velocities directly from the line widths. Since the spec-
alongthelineofsightcomparedtoNGC5044.Theinner10kpc tralbandofSXSrangesfrom0.3–12keV,itwillalsoallowthe
ofNGC5044,ontheotherhandalso showsevidenceofrecent studyofresonantscatteringin otherstrongresonancelines, for
107yrAGNactivity(Gastaldelloetal.2009),butnoevidence example in the Fe-K region around 6 keV (see the review by
∼
ofstrongshockslikeinNGC5813.Therefore,theAGNactivity Churazovetal.2010).Thecombinedeffortofimprovingatomic
in NGC5813appearsto bestrongerthanin NGC 5044,which data and flying new instruments like SXS aboard Astro-H will
isoppositetotheconclusionthatonewouldreachbasedonthe enable a more comprehensive study of turbulence in elliptical
limits on the turbulence that we obtained. AGN activity alone galaxies and clusters of galaxies. The high spectral resolution
cannot explain the difference in turbulence that we detect be- will not only allow measurementsof the amplitude of the mo-
tweenNGC5044andNGC5813. tions,butalsotheiranisotropyandspatialscales.
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