Bulk motion measurements in clusters of galaxies with ATHENA-like missions

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by  J. Nevalainen

Astron. Nachr./AN,No.,789–793(2012)/DOIpleasesetDOI!
Bulk motion measurements in clusters of galaxies with ATHENA-like
missions
J.Nevalainen1,2,⋆
1TartuObservatory,61602To˜ravere,Estonia
2DepartmentofPhysics,UniversityofHelsinki,Finland
3
Received ,accepted
1
0 Publishedonlinelater
2
n Keywords galaxies:clusters:general –instrumentation:spectrographs –intergalacticmedium–techniques: spectro-
a scopic–X-rays:galaxies:clusters
J
Thehierarchicalformationofclustersofgalaxiesbyaccretionofmaterialreleasesgravitationalenergywhichdissipates
9 intotheintraclustergas.Theprocessheatsthematerialandgeneratesgasturbulenceandbulkmotionsandthuskinetic
pressure.Mappingthevelocityfieldsofthemovingsubunits wouldenableanewdiagnosticstoolforclusterformation
] studiesandunbiasedX-raymassestimates.Therequiredspatiallyresolvedhighresolutionspectroscopyisnotcurrently
O
available.IdemonstrateherethefeasibilityofdetectingandmappingthevelocitiesofthebulkmotionsusingtheDoppler
C shiftoftheFeXXVKαlinewiththeproposedATHENAsatellite.
.
h
(cid:13)c 2012WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
p
-
o
r 1 Introduction (Russell et al. 2012). However, in most lines of sight the
st merger features are hidden by the projection. This allows
a During its life cycle, a clusterof galaxiesexperiencescol- thepossibilityofmeasuringtheradialbulkmotionsviathe
[
lisionsandmergerswithotherclustersandsubunits.Even- Dopplershiftoftheemissionlines.

1 tuallytherelaxationprocessestakeplaceandtheintraclus- Currently the constrains on radial bulk motions are
v ter material approacheshydrostatic and virial equilibrium. ratherpoordueto thelackofspatially resolvedhighspec-
2 Consequently,therewillbebulkmotionsintheclusterma- tral resolution instruments. The proposed ATHENA mis-
5
terial at different distance scales and velocity levels, de- sion would have carried such an instrument, an X-ray Mi-
8
1 pendingonthe magnitudeofthe eventandatwhichphase crocalorimeterSpectrometerXMS. In this paperI will ex-
. weobservethecluster. amine the expected quality of bulk motion measurements
1
0 Mappingthe velocityfields of the bulk motionswould with a mission approximating the capabilities of the pro-
3 open a new tool for studying the dynamics of the clus- posedATHENAinstruments,i.e.anATHENA-likemission.
1 ters. Comparingthevelocitymapswiththose predictedby
: cosmologicalsimulationswould be usefulforthe study of
v 2 Bulk motionsinclusters
i the formationof the largescale structureand providecon-
X
straints on cosmological parameters. Combining with the
r measuredgalaxydensityfield,thebulkvelocitymapscould 2.1 Rangeofvelocities
a
beusedtotestwhetherthefundamentalpictureofgravita-
Itiswidelyacceptedthatclustersofgalaxiesformbymerg-
tionalcollapseiscorrect(Branchinietal.2001;daCostaet
ingofsmallerstructuresofmatteraccretedalonglargescale
al.1998;Dore´etal.,2003).
structurefilaments.Thevelocitiesoftheaccretionflowsare
The bulk motions produce kinetic pressure which can assumedto reacha levelof 1000kms−1 (e.g.Frenket al.
reachalevelof 10%levelofthatofthethermalgasevenin
1999).A collisionof two clustersorprotoclustersof simi-
mostrelaxedclusters(Lauetal.2009).Ifnotaccountedfor,
larmassproducesastrongmerger.Therelatedbulkmotion
thekineticpressuremaybiasthehydrostaticX-raymasses- velocitiescan reach several1000 km s−1 as in the case of
timates low by ∼10%at r500. Thus,it would be important theBulletcluster(e.g.Markevitchetal.2002).Simulations
tomeasurethiscomponentinclustersinordertoobtainun-
of Nagai et al. (2002;2003) indicate that ongoing minor
biasedmassestimatesforthecosmologicalapplications. mergersmayproducebulkvelocitiesupto ∼1000kms−1.
In case of recent,strongcollisionswhich happenclose The released gravitationalenergy is dissipated into the in-
totheplaneofthesky,themergershockscanbeobservedby traclustermaterialwhicheventuallyapproachesthehydro-
theX-raymorphology,asintherarecasesofA520(Marke- static equilibrium.However,simulationsindicatethateven
vitch et al. 2005), A754 (Macario et al. 2011) and A2146 inthemostrelaxedclustersthereareresidualbulkmotions
⋆ Correspondingauthor:e-mail:jukka.h.nevalainen@helsinki.fi presentthroughouttheclustervolumeat∼100kms−1level
(e.g.Lauetal.,2009;Nagaietal.2003).
(cid:13)c 2012WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
790 J.Nevalainen:BulkmotionswithATHENA
2.2 Dopplershift instrumentsonboardXMM-NewtonandChandra,andspa-
tial resolution of 10 arcsec, XMS would enable spatially
The radial componentsof the bulk motions of 100 - 1000
resolved high resolution spectroscopy. This would yield a
km s−1 correspond to 2-20 eV shift of the Fe XXV and
breakthrough in mapping the spectral properties for ex-
XXVI Kα emission line centroid energies (∼6 keV). It is
tendedsourceslike clusters. Also, the bandpassextendsto
challengingtousethecurrentlymostpowerfulX-rayinstru-
12keVwhichallowsthemeasurementoftheFeXXVKα
ments at 6 keV (XMM-Newton/EPIC, Chandra/ACIS and
emissionline,prominentinclustersofgalaxies.Thedown-
SUZAKU/XIS CCDs) forthese measurementsdue to lim- sideofXMSistherelativelysmallFOV(2.3x2.3arcmin2)
itations in the energyresolution (∼100eV). The Gaussian
whichrendersthevelocitymappingofawholeclusterchal-
centroid can still be determined with precision better than
lenging.
100eV,assumingthattheinstrumentgainisaccuratelycal-
ibrated. This requirement can be relaxed if one considers
3.2 WideFieldImagerWFI
relatimemotionsbetweenthemainclusterandthemoving
part.InthefollowingIdiscussonlythestatisticalprecision
EventhoughtheenergyresolutionofWFI(150eVat6keV)
of the line centroid determination, and not the calibration
is notsufficientto resolve the componentsof Fe Kα com-
issuesnortheuncertaintiesofthecosmicredshiftmeasure-
plex,itcanstilldetermineverypreciselythecentroidofthe
ments.
Gaussiandistributionofthetotallineemission(seeFig.1)
due to high photon statistics. This is due to the very large
2.3 Observationalconstraints effective area (∼0.5 m2) of the X-ray telescopes onboard
ATHENA. Thus also WFI is a powerful tool for mapping
Suzaku/XISinstrumentshavebeenusedtoplaceupperlim- theclustervelocities.
itsforthebulkmotionvelocitiesat∼1000kms−1 levelin
several clusters: A2319 (Sugawara et al. 2009), Centaurus
(Ota et al., 2007), AWM7 (Sato et al. 2008). A first sig-
nificantdetectionofa bulkmotionhasbeenachievedwith
SuzakuforthemergingsubclumpinA2256(Tamuraetal.
2011).Theradialvelocitydifferencebetweenthemainclus-
terandthesubclumpis1500±300±300kms−1(wherethe
twosetsofuncertaintiesrefertostatisticalandsystematical
ones,respectively).
3 ATHENA-likemissions
ATHENA (Advanced Telescope for High ENergy Astro-
physics) was one of three L-class (large) missions being
considered by the European Space Agency in the Cosmic
Vision 2015-2025 plan. In May 2012 the Jupiter mission Fig.1 Data from a 100ksWFI simulation of the A2256
JupiterIcyMoonsExplorer(JUICE)waschosenforlaunch. maincluster(redcrosses)andthe subclump(bluecrosses)
However,ESAhascommittedtocontinuesupportingtech- together with the input models (solid curves). The verti-
nologydevelopmentsforafuturelargeX-rayfacility.Atthe caldashedlineshighlightthecentroidsoftheFeXXVKα
time of writing this paper (Oct 2012) there was an under- lines.
standingthatESAwouldshortlyappointasmallteamfrom
thecommunitytoprovideinputbasedonthetheATHENA
studyteamactivities.
InthispaperIusedtheresponsesandbackgroundesti- 4 Improvement withATHENA-likemissions
matesfortheX-rayMicrocalorimeterspectrometer(XMS)
andawidefieldimager(WFI)asreportedintheATHENA I examine here the capability of the proposed ATHENA
assessment report used by the ESA Space Science Advi- instruments for measuring the expected bulk motions
soryCommittee(theATHENAYellowBook,Barconsetal., in clusters. I used the MEKAL model (Kaastra 1992)
2012). in the XSPEC package to model the cluster emission
(bremsstrahlung continuum + collisional excitation lines)
adopting the metal abundances of Grevesse & Sauval
3.1 X-rayMicrocalorimeterSpectrometerXMS
(1998).
TherequirementfortheenergyresolutionofXMSis3eV Iusedthecurrentestimatesoftheinstrumentresponses
at 6 keV. With its 100-1000 times higher effective area at (Barconsetal.,2012)tosimulatetheclusterspectra,includ-
0.5 keV, compared to those of the current high resolution ingtheinstrumentbackgroundand90%resolvedcosmicX-
(cid:13)c 2012WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2012) 791
Table 1 Velocityprecisionforaclusterwithz=0.1and considerarangeofluminositiesforthemovingregionsand
kT=5keVwith100ksobservation theirvelocitiesanddirectionsofmotion.
In this work I limited the complexity of the above ap-
XMS WFI
distancea ∆rb σV ∆r σV proach by using an exposure time of 100ks for a cluster
(r ) (r ) (kms−1) (r ) (kms−1) with kT= 5 keV andz=0.1,assumingthatallofthe emis-
500 500 500
0 0.2 1 0.1 100 sioninthelineofsightofastudiedregionoriginatesfrom
0 0.05 10 – – the moving subunit. I further assumed that the luminosity
0.25 0.2 200 0.2 200 of the moving subunit at a given distance from the cluster
0.5 0.2 400 0.3 800 centre can be estimated with a a β - modelforthe surface
Notes.(a)Distancefromtheclustercentre.(b)Spatial brightnesswithβ=2/3andrcore=0.1r500.Thismaybean
resolution. underestimateoftheactualsignalsincethelikelyenhance-
mentoftheemissionduetothesubunitisnotaccountedfor.
raybackground.ThenIfittedthesimulateddatainthe5.5– IadoptedabolometricluminosityLbol(r500)=7×1044erg
7.5 keV band(excludingthe channelswhere the Fe XXVI s−1 withinr500 fromL-TrelationofPrattetal.(2009).Us-
Kα emission is significant) with a model consisting of a ingr500-TrelationofVikhlininetal.(2006)Iadoptedr500
=10arcmin.
power-law component for the continuum and a Gaussian
line for the Fe XXV Kα emission. The 1 σ statistical un-
certaintyoftheGaussiancentroidthenyieldedtheestimate
forthestatisticalprecisionofthevelocitymeasurement,as
summarisedinTable1.
4.1 A2256andthesubclump
In order to obtain a real-life example of the performance
of XMS and WFI formeasuringthe velocitiesof a nearby
minormerger,IusedtheSuzakuresultsfortheA2256and
thesubclump(Tamuraetal.2011)tosimulatespectrausing
anexposuretimeof100ks.
A fit to the simulated XMS data (see Fig. 2) yielded
a statistical uncertainty of the redshift (σ ∼10−6) corre-
z
spondingto a velocityprecision at a levelof ∼ 1 km s−1, Fig.2 Datafroma100ksXMSsimulationoftheA2256
i.e. a 1500σ detectionfortheclumpmotion.Measurement subclump (blue crosses) together with the input model,
isveryprecisebecausemanylinefeaturesareresolvedand shiftedby100kms−1(redline).
eachcentroidgivesweighttoχ2.DividingtheXMSemis-
sion of the subclump into boxes with width of 30 arcsec
(i.e. 5×5 pixel map for the full XMS FOV) yielded a sta-
tistical precision levelof ∼10 km s−1. This level of detail 4.2.1 XMS
wouldprovideabreakthroughinmodellingthedynamicsof A single XMS pointing covers only a fraction of the full
themergingsubclumps.SincethefluxofA2256subclump clustervolume:atz=0.1ther500 radiusfora clusterwith
iscomparabletothatofthecentralregionofsimilarsizein kT = 5 keV is ∼1 Mpc (Vikhlininetal., 2006)and covers
A2256(Tamuraetal.,2011),theabovecalculationsyieldan anareaof ∼300arcmin2,i.e.∼50higherthanthe FOVof
approximateestimateoftheexpectedvelocitymappingpre- XMS. Thus it is not feasible to cover the full r = r500 re-
cisionlevelinthebrightnearbyclustercentreswithXMS. gion in nearby clusters with XMS. Limiting the mapping
Aspectralfittothedatafroma100ksWFIsimulation intothecentralr=0.25r500regionwouldrequire∼10point-
ofA2256mainclusterandsubclump(seeFig.1)yieldeda ingswhichmightbefeasibleforafewclusters.SingleXMS
statisticalprecisionforFeXXVKαshiftcorrespondingto pointingsatlargerradiimightstillbeuseful,andIthuscal-
velocity precision of 60 km s−1, i.e. a detection at ∼25 σ culateestimatesfortheseobservationsinthefollowing.
level. I simulated XMS spectra with the above generic clus-
ter(seeSection4.2)withinthefullFOVi.e.∼(0.2r500)2.
Fitting the simulated spectra yielded a statistical precision
4.2 Mappingthewholecluster
of∼200kms−1 atr=0.25r500.Atr=0.5r500 ∼90%ofthe
In order to derive general conclusions about the perfor- total signal in the 5.5–7.5 keV band is due to background
manceofATHENA-likemissionsforthevelocitymapping emission which degradesthe statistical precisionof veloc-
inclusters,oneshouldsimulatespectrawithagridofrepre- ityto∼400kms−1 andatr500 thedataaresonoisydueto
sentativevaluesforclustertemperatures,metalabundances thebackgroundthatreliablevelocitymeasurementscannot
andredshiftsandrealisticexposuretimes.Also,oneshould beobtained.
www.an-journal.org (cid:13)c 2012WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim
792 J.Nevalainen:BulkmotionswithATHENA
Iftheaboveclusterwaslocatedatz=1.0,asinglecentral of galaxies within the central 0.25 r500. Also the residual
XMSpointingwouldcovertheclusteroutto∼0.5r500.The motions due to past mergers can be mapped in these cen-
receivedfluxwouldbesimilarasthatofthez=0.1clusterat tral regions. Single velocity measurements at the distance
0.5r500,whenusingthefullXMSFOV.Thus,withacentral of0.5r500foraclusteratz=0.1atthelevelof∼400kms−1
100ks XMS pointing it is feasible to obtain a single bulk canbeachievedwith100ksoff-axispointingsusingXMS.
motion measurement at z = 1.0 assuming the full cluster Athigherdistancesthemappingismorelimitedduetothe
within0.5r500 ismovingwitharadialvelocitycomponent background but the average motion within the central 0.5
higherthan400kms−1. r500uptoz=1.0canbemeasuredwithacentral100kspoint-
ing with XMS, if the radial velocity component is bigger
than∼400kms−1.
4.2.2 WFI
These measurements would open a new tool to study
Fitting the simulated data using the generic cluster de- clusterdynamics.Thiswouldbecomplementarytothecur-
scribed in Section 4.2 showed that in the centre the bulk rent analyses of intracluster shocks and sloshing and thus
velocity can be mapped with angular resolution of ∼0.1 giveabetterhandleonthe3-dimensionaldynamicsofclus-
r500 with statistical precision of ∼ 100 km s−1. At r=0.25 ters.Thiswouldyieldabreakthroughinmodellingtheclus-
r500 the lower cluster flux requires a larger extraction re- terphysicsanditsimplicationstocosmology.
gion to achieve similar statistical quality as in the centre.
ThekineticSunyaevZeldovic(kSZ)effectasmeasured
Using an extractionbox with size of (∼0.2 r500)2 yields a by WMAP (Kashlinsky et al., 2008; Osborne et al., 2012)
velocityprecisionof∼200kms−1.Atadistanceof0.5r500 has been used to constrain bulk motions of clusters. How-
fromthecentre,thebackgrounddominates(asinthecaseof
everthese resultshave been controversialdue to relatively
XMS)andthevelocityprecisiondegradesto∼800kms−1
weak kSZ signal compared to the thermal one. Thus, one
levelwhenusing anextractionboxsize of ∼0.3r500). Re- mustcombinealargesampleofclustersandassumeasim-
peating the exercisewith a veryhotcluster(kT = 10keV)
ilar motion for all the clusters in the sample. Mak et al.
improves the continuum signal but the Fe XXV emission
(2011) calculate that a sample of 400 clusters would al-
decreasesandthusthevelocityprecisiondoesnotimprove low Planck to detect a coherent 500 km s−1 bulk motion.
significantly.
Clearly ATHENA would make improvement here, being
abletomeasurethevelocitiesinindividualclusterswithbet-
5 Conclusions and discussion terprecisionandspatialresolution.
Thesimulationsshowedthatforanearby(z ≤0.1)cluster
References
withkT=5keV,usinga100ksexposureusingtheinstru-
mentsproposedforATHENA,onecould
Barcons,X.,Barret,D.,Decourchelle,A.etal.,arXiv:1207.2745
Branchini,E.,Freudling,W.,DaCostaL.Netal.:2001,MNRAS
– measure bulk motions in cluster centres at ∼ 0.1 r500 326,1191
spatial scale with statistical velocity precision level of
da Costa, L.N.,Nusser, A.,Freudling, W.et al.: 1998, MNRAS,
∼10(100)kms−1usingXMS(WFI) 299,425
– map cluster velocities in ∼ 10 regions with WFI with Dor,O.,Knox,L.,&Peel,A.:2003,ApJ,585L
a singlecentralpointinguptor= 0.25r500 with∼200 Frenk,C.S.,White,S.D.M.,Bode,P.etal.:1999,ApJ,525,554
kms−1 statisticalprecisionwithangularresolutionde- Grevesse,N.,&Sauval,A.,1998,SSRv,85,161
creasingfrom ∼ 0.1r500 to ∼ 0.2r500 with increasing Kaastra,J.1992,AnX-RaySpectralCodeforOpticallyThinPlas-
mas(InternalSRON-LeidenReport,updatedversion2.0)
radius
– obtain ∼200 km s−1 precision with XMS up to 0.25 Kashlinsky, A., Atrio-Barandela, F., Kocevski, D., et al., 2008,
ApJL,686,49
r500, buttocoverthe fullclusterwithinthis radiusone Lau,E.T.,Kravtsov,A.V.&Nagai,D.:2009,ApJ,705,1129
needs∼10pointings Macario, G., Markevitch, M., Giacintucci, S. et al.: 2011, ApJ,
– obtainavelocityprecisionof∼400kms−1 withasin- 728,82
gleXSMoff-axispointingat0.5r500usingtheemission Mak,D.,Pierpaoli,E.&Osborne,S.,2011,ApJ,736,116
fromthefullFOV Markevitch,M.,Gonzalez,A.H.,David,L.etal.:2002,ApJ,567,
– obtaina velocityprecisionof ∼800kms−1 withWFI L27
Markevitch, M., Govoni, F., Brunetti, G. et al., 2005, ApJ, 627,
at0.5r500withasinglecentralpointing
733
– not obtain meaningful velocity measurements at r500 Nagai,D.,Kravtsov,A.V.&Kosowsky,A.:2003,ApJ,587,524
duetodominatingbackground
Osborne, S., Mak, D., Church, S. et al., 2012, submitted
arXiv:1011.2781v2
Theaboveestimatesforthelevelofstatisticalprecision
Ota,N.,Fukazawa,Y.,Fabian,A.C.,etal.:2007,PASJ,59,351
ofvelocitymeasurementswithXMSandWFIindicatethat
Pratt,G.W.,Croston,J.H.,Arnaud,M.,etal.:2009,A&A,498,
anATHENA-likemissionwouldenableforthefirsttimethe
361
mapping of radial componentsof the bulk motions due to Russell,H.R.,McNamara,B.R.,Sanders,J.S.etal.:2012,MN-
recentminorandmajormergersinnearby(z≤0.1)clusters RAS,423,236
(cid:13)c 2012WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim www.an-journal.org
Astron.Nachr./AN(2012) 793
Sato,K.,Matsushita,K.,Ishisaki,Y.etal.:2008,PASJ,60,333
Sugawara, C., Takizawa, M. & Nakazawa, K.: 2009, PASJ, 61,
1293
Sun,M.,Murray,S.S.,Markevitch,M.etal.:2002,ApJ565,867
Tamura,T.,Hayashida,K.,Ueda,S.etal.:2011,PASJ,63S1009
Vikhlinin,A.,Kravtsov,A.,Forman,W.etal.,2006,ApJ,640,691
www.an-journal.org (cid:13)c 2012WILEY-VCHVerlagGmbH&Co.KGaA,Weinheim