Embedded Spiral Patterns in the Cool Core of the Massive Cluster of Galaxies Abell 1835

2.2 MB·

by  Shutaro Ueda

DRAFTVERSIONJANUARY25,2017
PreprinttypesetusingLATEXstyleemulateapjv.01/23/15
EMBEDDEDSPIRALPATTERNSINTHECOOLCOREOFTHEMASSIVECLUSTEROFGALAXIESABELL1835
SHUTAROUEDA
InstituteofSpaceandAstronauticalScience(ISAS),JapanAerospaceExplorationAgency(JAXA)
3-1-1Yoshinodai,Chuo
Sagamihara,Kanagawa252-5210,Japan
7 TETSUKITAYAMA
DepartmentofPhysics,TohoUniversity
1
2-2-1Miyama
0
Funabashi,Chiba274-8510,Japan
2

n
TADAYASUDOTANI
a
InstituteofSpaceandAstronauticalScience(ISAS),JapanAerospaceExplorationAgency(JAXA)
J
3-1-1Yoshinodai,Chuo
4 Sagamihara,Kanagawa252-5210,Japanand
2 SOKENDAI(TheGraduateUniversityforAdvancedStudies)
3-1-1Yoshinodai,Chuo
Sagamihara,Kanagawa252-5210,Japan
]
E (Received;Revised;Accepted)
DraftversionJanuary25,2017
H
. ABSTRACT
h
We present the properties of intracluster medium (ICM) in the cool core of the massive cluster of galaxies
p
Abell1835obtainedwiththedatabyChandraX-rayObservatory. Wefinddistinctivespiralpatternswiththe
-
o radiusof70kpc(or18′′)asawholeintheresidualimageofX-raysurfacebrightnessafterthe2-dimensional
r ellipse model of surface brightness is subtracted. The size is smaller by a factor of 2 – 4 than that of other
t
s clusters known to have a similar pattern. The spiral patterns consist of two arms. One of them appears as
a positive,andtheotherdoesasnegativeexcessesintheresidualimage. TheirX-rayspectrashowthattheICM
[ temperaturesinthepositive-andnegative-excessregionsare5.09-+00..1132keVand6.52-+00..1158keV,respectively. In
1 contrast,nosignificantdifferenceisfoundinthe abundanceorpressure, thelatter ofwhichsuggeststhatthe
v ICMinthetworegionsofthespiralpatternsisinpressureequilibriumorclose. Thespatially-resolvedX-ray
7 spectroscopyofthecentralregion(r<40′′)dividedinto92sub-regionsindicatesthatAbell1835isatypical
4 coolcorecluster. Wealsofindthatthespiralpatternsextendfromthecoolcoreouttothehottersurrounding
7 ICM.Theresidualimagerevealssomelumpysub-structureinthecoolcore. Theline-of-sightcomponentof
6 thedisturbancevelocityresponsibleforthesub-structuresisestimatedtobelowerthan600kms- 1. Abell1835
0 maybenowexperiencinganoff-axisminormerger.
.
1 Keywords: galaxies: clusters: individual: (Abell1835) — X-rays: galaxies: clusters — galaxies: clusters:
0 general
7
1
: 1. INTRODUCTION face brightnessof clusters, such as a sub-peak(e.g., Arnaud
v
et al. 2000; Sun et al. 2002), a cold front (e.g., Markevitch
i Clustersofgalaxiesarethelargestgravitationally-bounded
X objects in the Universe. Cold dark matters form a gravita- et al. 2000; Vikhlinin et al. 2001), and an elongated profile
(e.g.,Brieletal. 1992;Allenetal. 2002)(seeMarkevitch&
r tional potential well of a cluster. The hot and optically thin
a plasma called intracluster medium (ICM) fills at the typical Vikhlinin 2007, for a review). On the other hand, relaxed
temperatureof 107- 108K in the well. At the bottomof the clusters,whichhasasymmetricalX-raysurfacebrightnessin
itscore,arethoughtnottohaveexperiencedaviolent(major)
well,agiantellipticalgalaxy,whichisoftencalledthebright-
mergerinthelastseveralGyrs(e.g.,Pooleetal.2006).
estclustergalaxy(BCG),issituatedinmostofclusters. Since
Recent high angular-resolution observations with Chan-
the density of the ICM in the vicinity of the BCG is much
dra and XMM-Newton have discovered embedded spiral
higher than that in the outskirts, its cooling time is shorter
patterns with a size of a few hundreds kpc in the resid-
thanthelifetimeofclusters. Then,acoolcoreisformedand
ual image of X-ray surface brightness of several clusters
itwasspeculatedthatthecooledICMwouldinducemassive
and groups of galaxies after their nominal profile is sub-
starburst in the vicinity of BCG (Fabian 1994). However, a
tracted, e.g., the Perseus cluster (Churazov et al. 2003),
massivestarbursthasnotbeenobservedinmostofclustersto
Abell2052 (Blanton et al. 2011), Abell2029 (Clarke et al.
dateexceptforfew(e.g.,McDonaldetal.2012).
2004; Paterno-Mahler et al. 2013), Abell496 (Ghizzardi
Clusters of galaxies are also a dynamically young system
et al. 2014), Abell3581 (Canning et al. 2013), PKS0745-
in the Universe under the scheme of hierarchical evolution.
191 (Sanders et al. 2014), Abell85 (Ichinohe et al. 2015),
Theyhaveevolvedthroughmergersbetweensmallerclusters.
IRAS09104+4109 (O’Sullivan et al. 2012), and NGC5044
Mergersoften generate irregular structuresin the X-ray sur-
(O’Sullivanetal.2014).Thosespiralpatternsusuallyconsist
[email protected] oftwo arc-likeparts. Oneofthemcomesoutaspositiveex-
2 UEDA ETAL.
cessintheresidualimageandtheotherasnegative.Abrighter mated to be M200 =1.09×1015h- 1M⊙ from a weak-lensing
areainanobservedX-raysurfacebrightnessisusuallyinter- observation (Okabe et al. 2010). The mass reconstruction
preted as a regionwith a higher density of ICM than thatin image from the weak-lensing shear signal shows a standard
darkerparts, andsoprobablyisthepositive-excessregionin centrally-concentratedprofile and no featuresof violentdis-
the residual X-ray image of a cluster. Moreover, the spiral turbance at least within r<10′ (Clowe & Schneider 2002).
armsshowalowertemperatureandentropy(K=kT×n- 2/3) TheseresultsimplythatA1835showsnosignificantevidence
e
in the positive-excess region than in the negative-excess re- ofmajormerger. Basedontheobservationsofthemassfrac-
gion. The metal abundance in the positive-excess region is tion of the sub-structures, Smith & Taylor (2008) suggested
higher than that in the surroundings (Blanton et al. 2011; thatthisclusterwasformedatz≥0.8andhasgrowninmass
Paterno-Mahleretal.2013;Ghizzardietal.2014). However, by≤10%inthelast2Gyrattheclusterframe,whichcorre-
thespatialvariationofpressure(P=kT×n )isstillunclear. spondstoz∼0.4. TheirresultsalsoconfirmedthatA1835is
e
Numericalsimulationssuggestthatanoff-axisminormerger arelaxedcluster.
can generatessuchspiralpatterns(e.g.,Ascasibar& Marke- Throughout the paper, we adopt the abundance table of
vitch2006;ZuHoneetal.2010,2011;Roedigeretal.2011). Anders & Grevesse (1989), the Hubble constant of H0 =
Apossiblescenarioofitsformationisasfollows.Aninfalling 70kms- 1Mpc- 1,andthecosmologicaldensityparametersof
subclustersloshesthegravitationalpotentialwellofthemain ΩM = 0.27 and ΩΛ = 0.73. Then, 1 arcsec corresponds to
cluster. Then,thecool,dense,andmetal-richICMatthecen- 3.97kpc at the redshift z=0.2532. All the errors quoted in
terofthemainclusterislifteduptotheoutskirtsandthehot thispaperarein90%confidencelevel(90%CL)unlessoth-
ICM at the outskirts flows to the center, which creates the erwisespecified.
featurescommonlyobserved.SincetheinflowICMhasarel-
atively high entropy, the mixing transfers heat to the cooler 2. OBSERVATIONSANDDATAREDUCTIONS
ICM. On the other hand, it is considered that the gas slosh- Chandra observations of A1835 with Advanced CCD
ing is one of the heating sources to suppress cooling of the ImagingSpectrometer(ACIS;Garmireetal.2003)wereper-
ICM (Ghizzardietal. 2010;ZuHoneet al.2010)in addition formedfivetimeswiththeassignedObsIDsof495,496,6880,
tothefeedbackofactivegalacticnucleus(AGN)intheBCG 6881,and7370. TheobservationsofObsID of495and496
(McNamara&Nulsen2007). were carried out with the Chandra/ACIS-S and the others
Althoughitisconsideredthatembeddedspiralpatternsare were with the Chandra/ACIS-I. Among them, the data ob-
closelyrelatedtothecoolcore,onlyalimitedamountofstud- tained with the ACIS-S were affected by a mild flare as re-
ieshavebeenmadesofar.Inparticular,noneofthepublished ported by McNamara et al. (2006), and thus we do not use
dataaregoodenoughtoinvestigatethetimeevolutionofspi- theminthispaper.Table1summarizestheobservationlogfor
ralpatterns. Thenumberofthesampleofmassiveclustersis thedatausedinthispaper. Wereprocessedtheeventdataus-
alsosmall. Mergersareusuallyassociatedwithanevolution ingtheChandraInteractiveAnalysisofObservations(CIAO;
of the mass of the clusters. We need more samples of dis- Fruscioneetal.2006)version4.7andthecalibrationdatabase
tantandmassiveclusterstofullyunderstandtheevolutionof (CALDB)version4.6.9.Thetotalnetexposureis193.7ksec.
spirals.
In this paper, we present our result of, in quest for possi-
bleevidenceofpastmergers,the coolcoreofa massiveand Table1
ObservationlogofA1835withChandra
relaxed clusters of galaxies Abell1835 (hereafter A1835) at
theredshiftofz=0.2532withthedataofdeepChandraob-
ObsID Instrument Obs.Date Expo.(ks)
servations. A1835is the mostluminousX-raycluster in the
6880 ACIS-I 2005-12-07 117.9
ROSATBrightClusterSample(BCS;Ebelingetal.1998),and 6881 ACIS-I 2006-08-25 36.3
thusis one of the most massive clusters. It is classified as a 7370 ACIS-I 2006-07-24 39.5
relaxed and massive cooling-flow cluster. Its X-ray surface
brightnessshows a strong central concentration(Allen et al.
1996).Acoolcoreisformedinitscenter. Ontheotherhand,
Bonamenteetal.(2013)andIchikawaetal.(2013)suggested 3. ANALYSESANDRESULTS
thattheICMintheoutskirtsofA1835ismostlyoutofhydro-
We perform the X-ray imaging and spectral analyses to
staticequilibrium.TheglobalpatternofpropertiesoftheICM
studythepropertiesoftheICMinthecentralregionofA1835.
isstudiedbyKirkpatrick&McNamara(2015)andHofmann Wehavechosentheannularregionwiththeradiirof4′<r<
et al. (2016) The BCG of A1835 has an exceptionally high 5′ from the center of the cluster to extract the background
star-formation rate of 100 – 180M yr- 1 (McNamara et al.
⊙ data, which correspondsto over 1Mpc in distance from the
2006)andthusisa rareobjectlike the Phoenixcluster(Mc-
center. The cluster centeris definedas the brightestpixelin
Donaldetal.2012). However,mostoftheICMinthecenter
theX-raycountmapafterallthedatahavebeenmergedand
is still hot (Peterson et al. 2001). The AGN in the BCG is
is located at (RA, Dec) = (14h01m01.794, +02d52m41.55)
not luminous in X-rays at all (Russell et al. 2013), whereas
in J2000. We made the exposure-correctedX-ray image of
moderatelybrightatradiowavelengths(Korngutetal.2011). A1835in the energyrangeof 0.4- 7.0keV after subtracting
AlthoughthesignalofSunyaev-Zel’dovicheffect(Sunyaev&
thebackgroundasdisplayedinFigure1. Noevidenceofma-
Zeldovich1972,1980;Kitayama2014,forareview)wasob-
jor mergeris foundin these imagesof X-ray surface bright-
servedanddetectedbytheMUSTANG(Korngutetal.2011),
ness. The X-ray profile appears to be elliptical and shows a
thesignalinthecentralregion(r<18′′)wasstronglycontam- cool core in the cluster center, which suggests A1835 to be
inatedbytheAGNemission. Govonietal.(2009)suggested
oneofrelaxedandcool-coreclusters.
that a diffuse radio emission (i.e., a radio mini-halo) resides
with a size of several hundredskpc. The total mass is esti- 3.1. Searchandanalysesofthespirals
EMBEDDEDSPIRAL PATTERNSIN THECOOL CORE OF ABELL1835 3
3’ or 715 kpc
Figure1. X-rayimageofA1835intheenergyrangeof0.4–7.0keVwiththebackgroundsubtractedandexposurecorrected. Leftpanel: large-scaleimage
ofA1835. Theyellowbarindicatesthescaleof3′or715kpc. ThisimageshowsadistinctivecoolcoreinthecenterofA1835,whichappearstobeelliptical.
NoevidenceofmajormergerisfoundinthisX-raysurfacebrightness. Rightpanel: zoomed-upX-rayimageofthecentralregionofA1835withinr<40′′or
159kpcfromtheleftpanel.
Even if a cluster is classified as a relaxed and cool-core ofthespiralpatternasthedistancefromthepeakpositionto
cluster, some subtle and possibly complex features may ex- thefarthesttailofthecontourthatcorrespondsto10%ofthe
ist,whichareoftenembeddedunderaglobalimageprofileof peakresidualbrightnessshownin Figure2. Thus, the spiral
theclusterandcanbeeasilyoverlooked.Detailedimageanal- patternsasawholeextendtoacircularareawiththeradiusof
ysiswitharesidualimageofitscore,aftertheglobalprofile ∼18′′ (or∼70kpc)in the residualimageof A1835. More-
issubtractedfrom,maywellhighlightthosefeatures(seeIn- over,thesizeofthespiralwithapositive-excessisconsistent
troductionforsomeexamples). Inthissubsection,wesearch withthatofthespiralwithanegative-excess.
forsuchembeddedfeaturesusingaresidualimage. Next,weextractX-rayspectraofthepositive-andnegative-
First,weapproximatetheX-rayimageprofileofthecluster excess regions of the spiral patterns from the regions as
withthe2-dimensionalellipsemodel. Todeterminetheratio markedinFigure3.WefitthemusingXSPECversion12.9.0o
of major-axis to minor-axis (f) and angle (θ) of the ellipse (Arnaud1996).Inthespectralfitting,theGalacticabsorption
model, we optimize f and θ in the ranges of 0< f ≤1 and (N ) is fixed to 2.04×1020cm- 2 (Kalberla et al. 2005) and
H
0≤θ<180,respectively,tominimizethesumofRMS/mean, the ICM emission is assumed to be reproduced by a single
where“mean”istheaveragephotoncountsineachisophote temperature model of a thin-thermal plasma (APEC; Smith
ringand “RMS” is the root-mean-squaredifferencebetween et al. 2001). Figure 4 shows the fitting results of the X-ray
themodelandthephotoncountsinthesamering.Asaresult, spectra of the positive (left) and the negative (right) excess
we obtain f =0.857andθ=170.9,respectively. Ourdefini- regionsfor each ObsID; the spectra in black, red, and green
tionissuchthatθ=0and90meanthedirectionsofsouthand areextractedfromthedataofObsID=6880,6881,and7370,
west, respectively. Themeansurfacebrightnessateachspot respectively.TheICMspectraintheregionsofthespiralpat-
is thus given by the mean photon counts in the correspond- ternsarewelldescribedbythesingletemperaturemodelwith
ing isophotering in the ellipse model. The derivedratio (f) χ2/dofof778/706and812/716forthepositive-andnegative-
is consistent with that derivedby Schmidtet al. (2001)with excessregions,respectively.
the 30ks exposure data of an early observation of Chandra The ICM temperatures of the positive- and negative- ex-
and those of typical relaxed clusters reported by Kawahara cess regionsare foundto be 5.09+0.12keV and 6.52+0.18keV,
(20T1h0e)n., we subtract the obtained model surface brightness respectively. Therespectiveabund- 0a.1n3ces(Z)are 0.3-70+-.001..50043Z⊙
from the original X-ray image. The resultant image is and 0.39-+00..0044Z⊙. The electron density, pressure, and en-
smoothedwith a 2′′ Gaussian andshownin theleft panelof tropy are computed assuming that the ICM is distributed
uniformly over the line-of-sight of length L. Assuming
Figure2. Wefindadistinctivepositive-excessregion,which
a factor of density correction of n /n = 1.17+0.02×
extends from north to south counterclockwise, as well as a e H
Z (with Anders & Grevesse (1989), the primordial He-
negative-excessregionattheoppositesideoftheclustercen-
lium abundance Y = 0.25), the electron density is esti-
ter in a nearly symmetric shape, i.e., the two regions form p
spiralpatterns. Weplottwowhiteellipsesintheleftpanelof mated to be ne = 0.0169±0.0003cm- 3(L/1Mpc)- 1/2 and
Figure2, whichshowthepositionandsize ofX-raycavities n =0.0120±0.0003cm- 3(L/1Mpc)- 1/2 for the former and
e
reportedbyMcNamaraetal.(2006),readofffromFig. 9and latter, respectively, where L denotes the depth of line-of-
Table 2 of McNamara et al. (2006). The right panel of Fig- sight.Then,theelectronpressureandentropyofthepositive-
ure 2 shows the distribution of the X-ray surface brightness excessregionare 0.086±0.003keVcm- 3(L/1Mpc)- 1/2 and
ofthe spiralpatternsasa contourmapoverlaidonthe origi- 77.3+2.1keVcm2(L/1Mpc)1/3, and those of the negative-
nal X-ray image of Figure 1 rightpanel. We define the size exce-s2s.2region are 0.078+0.003keVcm- 3(L/1Mpc)- 1/2 and
- 0.002
4 UEDA ETAL.
0
0.
2
0
0.
0
3:
5
on2:
ti
a0
clin40.
e
D
0
0.
2
2:
5
03.0 01.5 14:01:00.0
Right ascension
Figure2. Spiralpatterns inthecentral regionofA1835. Leftpanel: X-rayresidualimageofthecentralregionofA1835, smoothedwitha2′′ Gaussian.
Pronouncedspiralpatternsarefound.Thepositive-excessregionextendsfromnorthtosouthandthenegative-excessregionisattheoppositesidetothecenter.
Thewhitecircleofr<20′′isoverlaidasareference.ThecyancrossshowsthepositionoftheBCGofA1835.Thetwowhiteellipsesshowthepositionandsize
ofX-raycavitiesreportedbyMcNamaraetal.(2006). Rightpanel: sameasFigure1butthecontoursofthepositive(inwhite)andnegative(inblack)excess
regionsareoverlaid.Whitecontoursshowtheexcesslevelsof100%,90%,···,20%,10%ofthepeakvalueofthepositivebrightnessresiduals.Blackcontours
arethesameasthewhitecontoursbutforthenegativeresiduals.Boththespiralpatternsareembeddedinthecoolcoreandarenotdistinguishableintheoriginal
X-rayimage.
Table2
Best-fitparametersofX-rayspectrainthespiralpatterns.
Region Positive-excess Negative-excess
Temperature(keV) 5.09+- 00..1123 6.52-+00..1158
Abundance(Z⊙) 0.37+- 00..0043 0.39-+00..0044
Redshift 0.251+- 00..000034 0.249-+00..000044
Density(cm- 3(L/1Mpc)- 1/2) 0.0169±0.0003 0.0120±0.0003
χ2/dof 778/706 812/716
Pressure(keVcm- 3(L/1Mpc)- 1/2) 0.086±0.003 0.078-+00..000023
Figure3. TheregionsfromwhichtheX-rayspectraofthepositive(left)and Entropy(keVcm2(L/1Mpc)1/3) 77.3+- 22..12 124.2-+33..38
negative(right)excessregionsareextracted. X-rayspectraareaccumulated
frominsidetheregioninwhitelineexcludingthesmallerregioningreenline
withareddiagonallineineachpanel.
We first apply the contour-binning algorithm (hereafter
ContBin) presented by Sanders (2006) to determine how to
124.2+3.8keVcm2(L/1Mpc)1/3,respectively. Notethatthese
- 3.3 dividetheregionintomanysubregionstoextractX-rayspec-
are not real electron density, pressure, or entropy, as we did
tra from. This algorithm yields a set of sub-regions so that
notperformanydeprojectiontoestimatea3-dimensionaldis-
eachregionhasroughlythesamelevelofsignal-to-noisera-
tribution. Themeasuredredshiftsareconsistentamongthese
tio(S/N).Wedividethewholeregion(rightpanelofFigure1)
fitting results and cluster’s one. We find no significant bulk
into92sub-regionswithmorethan∼2000countsineachre-
motionsin the directionof line-of-sightwith the upperlimit
gion after subtracting the background. The S/N, where the
forthevelocityofICMmotionintheredshiftdifference(∆z)
Poisson noise N includes the background, of each region is
of 0.002, which corresponds to 600kms- 1. We summarize within42.2–57.6inthe0.4–7.0keVband. Then,wemake
thebest-fitresultsinTable2. X-rayspectrumineachregion.Byfittingeachspectrumwith
Figure5showsthepropertiesoftheICMintheregionsof theAPECmodel,weobtainthespatialdistributionoftheICM
the spiral patterns. Although the temperature, density, and temperature, abundance, and density. The pressure and en-
entropy differ between the two regions, the pressures of the tropyineachregionarethencalculated.
ICMbetweenthemareverysimilar. Itsuggeststhattheyare We show the ICM temperature, density, pressure, and en-
inpressureequilibriumorclose. tropymapsinFigure6,7,8,and9,respectively. Thestatisti-
calerrorsintheinner/outerregionsinthetemperaturemapare
3.2. Spectralanalysesofsurroundings
7%/15%, respectively. Those for the density, pressure, and
WehavemeasuredtheICMpropertiesintheregionsofthe entropy are 5%/5%, 9%/16%, and 8%/15%, respectively,
spiralpatternsintheprevioussubsection. Inthissubsection, fortheinner/outerregions.
weperformspatially-resolveddetailedX-rayspectroscopyof Thetemperaturemap(Figure6)showsthattheICMinthe
the entire cluster-center region (r<40′′ or r<160kpc) in- center is the coolest at 3.5keV in the center and becomes
cludingthespiralpatterns. gradually hotter at a larger radius. In the outermost region
EMBEDDEDSPIRAL PATTERNSIN THECOOL CORE OF ABELL1835 5
V−1 0.1 V−1 0.1
e e
k k
nts s −1 nts s −1
u u
o o
d c 0.01 d c 0.01
e e
z z
ali ali
m m
nor nor 1.6
1.2 1.4
atio 1 atio 1.2
r r 1
0.8 0.8
1 2 5 0.5 1 2 5
Energy (keV) Energy (keV)
Figure4. X-rayspectraextractedfromthepositive-excessregion(left)andfromthenegative-excessregion(right)foreachObsIDareshownwiththebest-fit
APECmodels. Theratiosofthedatatothemodelareplottedinthebottompanels. Thespectrainblack,red,andgreenareextractedfromthedataofObsID
=6880,6881,and7370,respectively.AlloftheX-rayspectraarewellreproducedwiththeAPECmodel.
(35′′ < r < 40′′), the lowest and highest temperatures are isoneoftheirsamplesanditsunsharp-maskedcountimageis
7.7keV and 11.9keV, respectively. We find that the lower shownin Fig. E.1 in Hofmannet al. (2016). Since they did
temperature region is elongated toward the southeast direc- notfocusonthecentralregion(lessthan100kpc),thespatial
tion, which roughly traces the positive-excess region of the resolutionof their unsharp-maskedcountimage is low. The
spiralpatterns(rightpanelofFigure6).Mostofthenegative- feature such as the spirals we found is not apparent in their
excessregionofthespiralpatternshasarelativelyhighertem- results.
peraturethanthatof thepositive-excessregion. Thisis con-
sistent with the picture provided by the spectral analyses of
4.1. Propertiesofspiralpatterns
thespirals. Itisnoteworthythatthespiralpatternsappearin
therelativelylowertemperatureregions. We have estimated the size of the spiral patterns to be
Theabundancemapiseffectivelyflatanditsglobalpatterns 70kpc in radius (Figure 2). This size is a factor of 2 – 4
isconsistentwiththatreportedbyKirkpatrick&McNamara smallerthanthoseofotherknownclusters,e.g.,about200kpc
(2015). forA496(Ghizzardietal.2014),300kpcforA2029(Clarke
Thedensitymap(Figure7)traceswelltheshapeofX-ray etal.2004),and240kpcforA2052(Blantonetal.2011). To
surfacebrightnessofA1835,i.e.,itiselongatedtothesouth- comparethesizeofthespiralpatternsinmoreobjectiveman-
eastwiththepeakattheclustercenter. ner,were-analyzethe650ksecdataofA2052observedwith
The pressure map (Figure8) shows thatthe ICM pressure Chandra,whichare the same asthose used inBlanton etal.
smoothly decreases toward the outer region with no signif- (2011). Then, we measure the size of the spiral patterns in
icant discontinuities in the central region. In the region of A2052usingthesamemethodasdescribedinSection3.1to
the spiral patterns, the pressure profile showsno discontinu- be170kpc.ForA496,fromFig.13ofGhizzardietal.(2014),
ity,whichisconsistentwiththatobtainedbytheX-rayspec- wemeasurethesizeofspiraltobe150kpcbyusingthesame
tral analyses of spirals. These results imply that the ICM is methodas in Section 3.1. These results furtherconfirm that
in pressure equilibrium or close at least in the region of the the size of the spiral patterns of A1835 is smaller. We will
spiralpatterns. performmoresystematicinvestigationsofthe spiralpatterns
Theentropymap(Figure9)showstheentropyoftheICM foralargersampleofclustersinourfuturepublication.
islowestatthecenter. Italsoshowsthatthespatialdistribu- We also measure the core radius of A1835 to be 31±
tion of the lower entropy ICM is elongatedto the southeast, 0.4kpc using our a single β model. It is within the range
beingsimilarto thatof thepositive-excessregionof the spi- of the core radii of the other known clusters with spiral pat-
ralpatterns. Notethatthisresultisconsistentwiththedirect terns;thecoreradiioftheabove-mentionedthreeclustersof
spectralanalysesofthespiralpatternspresentedintheprevi- A496, A2029, and A2052 are 15kpc (Laganá et al. 2008),
oussubsection,wherethespectrumofthenegative-excessre- 38kpc (Lewiset al.2003),and18kpc(Blantonetal. 2011),
gion,whichislocatedinthewesternside,hasshownahigher respectively. We findnoobviouscorrelationbetweensize of
entropythan thatof the positive-excessregionin the eastern spiralsandthatofcoreradius. Infact, thesize ofspiralpat-
side. ternsofA1835ismuchclosertothecoreradiusthantheother
clusters;theyextendfromwellinsidethecoolcoreouttothe
hotter surrounding ICM. This may indicate that the stirring
4. DISCUSSION
motionproducingthespiralstransportsafractionofthecool
Wehavefoundtheembeddedspiralpatternsintheresidual gasouttolargerradiiandthehotgasintosmallerradii.
imageofX-raysurfacebrightnessofthecoreofA1835. We We have also analyzed the X-ray spectra extracted from
thenderivedthespectralpropertiesoftheICMinthemandin the positive- and negative-excess regions of the spiral pat-
theirsurroundings. Inthissection,wediscusstheresultsand terns(Figure3)andhavemeasuredthespatialdistributionof
theirimplications. theICMparameters(Table2andFigure5). Wefindthatthe
In previous works, by using the data of 33 clusters, Hof- differenceandsimilarityoftheICMparametersbetweenthe
mannetal.(2016)studiedthelarge-scale(typicallyoverhun- tworegionsarequalitativelyingoodagreementwiththoseof
dredskpc)perturbationsofX-raysurfacebrightness. A1835 otherknownclusters(Clarkeetal.2004;Blantonetal.2011;
6 UEDA ETAL.
theICMdensityintheformerishigherthanthatinthelatter.
7.0
Whereas the pressure has no difference between the two re-
e6.5 gions. Thespectralresultsoftheirsurroundingareahasalso
r
u confirmedtheuniformpressureacrosstheregionofthespiral
at6.0 patterns. Therefore, the ICM in the region of the spiral pat-
r
e ternsisinpressureequilibriumorclose. Forthetemperature
p5.5
m map, Hofmann et al. (2016) studied the large-scale distribu-
e5.0 tioninA1835.Theyreportedthatthetemperatureoftheouter
T
regionexceeds10keVanditsdistributioniselongatedtoward
4.5
Bright Dark thenorthwestandsoutheast.
For the abundance, we have found no difference between
thepositive-andnegative-excessregions. By combiningthe
0.45
resultsofspectralanalysesofthespiralsandoftheirsurround-
e ings,wefindalmostuniformdistributionofmetalsacrossthe
c
n0.40 regionwitharadiusofatleastr<40′′inA1835.Thisisdif-
a
d ferentfromthatreportedinthepreviousstudiesofthespiral
n patterns. Forexample,inthecaseofA496,theabundanceof
u0.35
b the positive-excess region is larger by a factor of ∼1.5 than
A
thatofthenegative-excessregion(Ghizzardietal.2014).
0.30 We consider two possible scenarios for the origin of the
Bright Dark spiral patterns: an energetic past AGN activity, including
jets, andagassloshinginducedbyanoff-axisminormerger
(e.g.,Markevitch&Vikhlinin2007).McNamaraetal.(2006)
0.018
pointedoutthepresenceofX-raycavitiesinthecoreandsug-
gestedthattheenergeticAGNoutbursthadoccurred40Myr
y0.016
t ago. They also suggested that the past jet activity had sup-
i
s
n presseda coolingofmostoftheICM. Suchanenergeticac-
De0.014 tivitycaninfluencethesurroundingsclosetothecenterasin
thecaseofthePerseuscluster(Fabianetal.2011). However,
0.012
McNamaraetal. (2006)estimatedthesizeoftheX-raycav-
itytobe∼20kpc,whichissignificantlysmallerthanthatof
Bright Dark
thespiralpatternsthatwefound(leftpanelofFigure2). Itis
thereforemorelikelythattheoriginisthesameasintheother
clusters, i.e., the spiralpatternsarecreatedby the gasslosh-
ing. Ifso,sinceitisconsideredthatanoff-axisminormerger
y120 inducesthegassloshing(e.g.,Markevitch&Vikhlinin2007).
p
o Thepresenceofthespiralpatternsisapieceoftheevidence
tr100 ofapastminormergerinA1835. Weshouldnotethatasub-
n
E structureexistsandmustbedisturbingthegasinthecoolcore
80 inA1835,eventhoughtheX-raysurfacebrightnessindicates
thatitisrelaxed.
Bright Dark If the origin of the spiral patterns is gas sloshing induced
byaminormerger,therelativelysmallsizeofthespiralpat-
0.095 ternsmayreflectatimeevolutionofspirals. Accordingtothe
numericalsimulations(e.g.,ZuHoneetal. 2010),the size of
0.090
the spiralsdependson the length of the periodafter a minor
e
ur0.085 merger began. In the early stage of a minor merger, a vol-
s ume over which the gas is disturbed is relatively small and
es0.080 the effectof disturbanceis limited in the core of the cluster.
r
P0.075 However,theconditionsofinitialmergerparameterssuchas
animpactparameteralsoaffecttheformandsizeofspirals.
0.070 For the pressure profile in the cluster center, observations
Bright Dark
oftheSunyaev-Zel’dovich(SZ)effectwouldenableustodi-
rectlymeasurethepressureoftheICM.Althoughtheobser-
Figure5. Best-fitparametersofspectralanalysesinthepositive-(leftsidein vations of the SZ effect for A1835 were performed by Ko-
eachpanel)andnegative-(rightside)excessregions.Thepanelsfromthetop rngutetal.(2011),thesignalinthecentralpart(r<18′′)was
tobottomshowthetemperatureinunitsofkeV,abundanceinsolar,densityin
stronglycontaminatedbytheAGNemission. Thesizeofthe
cm- 3(L/1Mpc)- 1/2,entropyinkeVcm2(L/1Mpc)1/3,andpressureinkeV
contaminatedregionis consistentwith that of the spiral pat-
cm- 3(L/1Mpc)- 1/2,respectively.
terns. Now, observations with ALMA, which can achieve a
highangularresolutionof5′′ andhighsensitivityforthe SZ
effect (Kitayama et al. 2016), will likely enable us to make
Paterno-Mahler et al. 2013; Ghizzardi et al. 2014), i.e., the an accurate measurement of the pressure of the ICM in the
temperatureandentropyoftheICMinthepositive-excessre- regionofthespiralpatterns.
gion are lower than those in the negative-excessregion, and
EMBEDDEDSPIRAL PATTERNSIN THECOOL CORE OF ABELL1835 7
12 12
11 11
9.9 10
8.9 9
7.9 8
6.9 7
6 6
5 5
4 4
3 3
Figure6. TemperaturemapsobtainedbyX-rayspectralanalyseswiththeContBin-definedregionsinunitsofkeV.Theleftpanelshowstherawtemperature
map. Thestatisticalerroroftheinner/outerregionsinthemapare7%/15%,respectively. Therightpanelshowsthesmoothedimageoftheleftpanelwith7′′
Gaussian,togetherwiththecontoursoverlaidofthespiralpatternsasintherightpanelofFigure2.
0.02560 0.02560
0.01192 0.01192
0.00551 0.00551
0.00254 0.00254
0.00116 0.00116
0.00053 0.00053
0.00023 0.00023
0.00009 0.00009
0.00003 0.00003
0.00000 0.00000
Figure7. SameasinFigure6butforthedensitymapsinunitsofcm- 3(L/1Mpc)- 1/2.Thestatisticalerroris5%forboththeinnerandouterregions.Theright
panelshowsthesameimageastheleftpanelbutthatissmoothedwith7′′GaussianandthecontoursofthespiralpatternsshownintherightpanelofFigure2
areoverlaid.
4.2. Spiralpatternsandmergers which is within the rangeof the mass of groupsof galaxies.
In that case, it satisfies the conditionsproposed by Smith &
Following the simulations of A1835 (e.g., Ascasibar &
Taylor (2008). Bonamente et al. (2013) and Ichikawa et al.
Markevitch2006;ZuHoneetal.2010;Roedigeretal.2011),
(2013)suggestedthattheICMintheoutskirtsisoutofhydro-
weconsideranorbitalplaneofthefirstpassageoftheminor
staticequilibrium.Recentminormergerthatwehaveguessed
merger. Theyarguethatthelow-temperatureregion(i.e.,the
A1835hasexperiencedmayhaveinducedit.
positive-excessregion)appearsatthepositionoppositetothe
The velocity difference of the ICM between the positive-
directionofthetrajectory.Sincethepositive-excessregionof
and negative-excessregions is smaller than 600kms- 1. We
A1835extendsfromthenorthtosouthcounterclockwise,we
conjecturethattheinfallingsubclustertraveledfromnorthto havefoundnobulkmotionsalongthe line-of-sight. Sanders
southwest. etal. (2010)reportedthatthe turbulencevelocityatthe cen-
Asdiscussedabove(Section4.1),thecomparativelysmall terissmallerthan274kms- 1. SincetheICM doesnotslosh
size of the spiral patterns of A1835 may be an indication fast,thesetworesultssupportthehypothesisthattheorbitof
that the length of the period after the beginning of the mi- theminormergerwasintheplaneofthesky. Tomakeanac-
nor merger is relatively short. On the other hand, Smith & curatemeasurementofthevelocityoftheICMisbeyondthe
Taylor(2008)suggestedthatgrowthinmassofA1835inthe capabilityofcurrentlyavailableX-rayobservatories,butwill
last 2Gyr is smaller than 10%, based on the observations bepossiblewithfutureX-raymicro-calorimetermissions.
of the mass fraction associated with the sub-structure. The Govonietal.(2009)reportedthepresenceofaradiomini-
simulationsdemonstratedthat a subclusterwith a mass ratio haloinA1835.Theradioemissionisconcentratedinthecen-
of 5% can slosh the gas in the gravitational potential well ter and showsno obviousasymmetry. We do notfind direct
of the main cluster after the first contact and can then cre- correspondencebetweenthemorphologyofthespiralpattern
ate a temperature structure of spiral patterns (e.g., ZuHone in the X-ray residual brightness and that of the radio mini-
et al. 2010). For A1835, the mass ratio of 5% corresponds halo.
to ∼ 5×1013M for M of A1835 (Okabe et al. 2010),
⊙ 200 5. SUMMARY
8 UEDA ETAL.
Figure8. SameasinFigure6butforthepressuremapsinunitsofkeVcm- 3(L/1Mpc)- 1/2,whichiscalculatedbyP=kT×ne.Thestatisticalerrorofinner
regioninthemapis9%andthatinouterregionis16%.
Figure9. SameasinFigure6butfortheentropymapsinunitsofkeVcm2(L/1Mpc)1/3,whichiscalculatedbyK=kT×n-e2/3.Thestatisticalerrorofinner
regioninthemapis8%andthatinouterregionis15%.
Wefindtheembeddedspiralpatternsinthecoolcoreinthe imply that A1835 is a typical cool-core cluster. The spiral
residualimageofX-raysurfacebrightnessaftertheaveraged patternsextendfromthecoolcoreouttothehottersurround-
profileissubtractedinthemassiveclustersofgalaxiesA1835 ingICM.Thismayindicatethatthestirringmotionproducing
using the data of Chandra X-ray Observatory. We measure the spirals transports a fraction of the cool gas out to larger
the size of the spiral patternsto be 70kpc, which is a factor radiiandthe hotgasinto smaller radii. The spatial distribu-
of 2 – 4 smaller than that of other clusters known to show tionofmetalsseemstobeuniformatleastintheregions.The
thesimilar features. Thespiralpatternsconsistoftwo arms, pressuredistributionshowsagradualdecreasetowardtheout-
whichappearasthepositive-andnegative-excessintheresid- skirtsandimpliesthattheICMinA1835isinpressureequi-
ualimage.WeanalyzetheX-rayspectraextractedfromthose librium at least in the central region, even though the spiral
tworegionsseparately.TheICMtemperaturesinthepositive- patternsexistinthecore,whichsuggeststhatthecoreisdis-
andnegative-excessregionsarefoundtobe5.09+0.12keVand turbed.
- 0.13
6.52+0.18keV,respectively. Nosignificantdifferenceisfound Two scenarios are consideredas the mechanism of distur-
in th-e0.1I5CM abundance in the two regions, nor in the pres- banceofthe coolcore. Oneis an energeticpastAGN activ-
sure,whichindicatesthattheICMisinpressureequilibrium ityincludingjetsandtheotherisgassloshinginducedbyan
orclose. We measurethevelocitydifferenceoftheICMbe- off-axisminormerger. Althoughwe can notdeterminecon-
tweenthepositive-andnegative-excessregionalongline-of- clusivelywhich is moreplausible becauseof the insufficient
sighttobesmallerthan600kms- 1. X-raydata,theICMpropertiesintheregionofthespiralpat-
terns are similar to those in other clusters that show embed-
Applying the algorithm of ContBin, we define the sub-
dedspiralpatternsintheirX-raysurfacebrightness. Ineither
regionssothatanyofthemhasS/N>42inthe0.4–7.0keV
case,thespiralpatternsindicatethatasub-structureexistsin
band(i.e.,∼2000photonsineachregion)inthecentralregion
ofr<40′′inA1835afterthebackgroundissubtracted.Com- thecoolcoreevenif theX-raysurfacebrightnessappearsto
be relaxed. The velocity of disturbance in the line-of-sight
pilingtheX-rayspectrafromthe92sub-regions,wemakethe
temperature,density,pressure,andentropymaps(Figures6– islowerthan600kms- 1 forA1835,whichisconsistentwith
9). The radial profiles of temperature, density, and entropy that of the Perseus cluster, as directly measured by Hitomi
(HitomiCollaborationetal.2016).
EMBEDDEDSPIRAL PATTERNSIN THECOOL CORE OF ABELL1835 9
We are gratefulto the anonymousreferee for helpfulsug- HitomiCollaboration,Aharonian,F.,Akamatsu,H.,etal.2016,Nature,535,
gestions and comments. We also thank Masaaki Sakano for 117
Hofmann,F.,Sanders,J.S.,Nandra,K.,Clerc,N.,&Gaspari,M.2016,
carefulreadingof the manuscriptand usefulcomments, and
A&A,585,A130
Yuto Ichinohe for helpful discussions. The scientific results Ichikawa,K.,Matsushita,K.,Okabe,N.,etal.2013,ApJ,766,90
of thispaperare based in partonthe data obtainedfromthe Ichinohe,Y.,Werner,N.,Simionescu,A.,etal.2015,MNRAS,448,2971
Kalberla,P.M.W.,Burton,W.B.,Hartmann,D.,etal.2005,A&A,440,775
ChandraData Archive: ObsIDs6880,6881, and7370. This Kawahara,H.2010,ApJ,719,1926
research is supportedby Japan Society for the Promotionof Kirkpatrick,C.C.,&McNamara,B.R.2015,MNRAS,452,4361
Kitayama,T.2014,ProgressofTheoreticalandExperimentalPhysics,2014,
Science (JSPS) KAKENHI Grant Number 15K17610 (SU),
060000
25400236(TK),and24105007(TD). Kitayama,T.,Ueda,S.,Takakuwa,S.,etal.2016,PASJ,68,88
Korngut,P.M.,Dicker,S.R.,Reese,E.D.,etal.2011,ApJ,734,10
Laganá,T.F.,LimaNeto,G.B.,Andrade-Santos,F.,&Cypriano,E.S.
2008,A&A,485,633
Lewis,A.D.,Buote,D.A.,&Stocke,J.T.2003,ApJ,586,135
Markevitch,M.,&Vikhlinin,A.2007,Phys.Rep.,443,1
REFERENCES Markevitch,M.,Ponman,T.J.,Nulsen,P.E.J.,etal.2000,ApJ,541,542
McDonald,M.,Bayliss,M.,Benson,B.A.,etal.2012,Nature,488,349
McNamara,B.R.,&Nulsen,P.E.J.2007,ARA&A,45,117
McNamara,B.R.,Rafferty,D.A.,Bîrzan,L.,etal.2006,ApJ,648,164
Allen,S.W.,Fabian,A.C.,Edge,A.C.,etal.1996,MNRAS,283,263 Okabe,N.,Takada,M.,Umetsu,K.,Futamase,T.,&Smith,G.P.2010,
Allen,S.W.,Schmidt,R.W.,&Fabian,A.C.2002,MNRAS,335,256 PASJ,62,811
Anders,E.,&Grevesse,N.1989,Geochim.Cosmochim.Acta,53,197 O’Sullivan,E.,David,L.P.,&Vrtilek,J.M.2014,MNRAS,437,730
Arnaud,K.A.1996,inAstronomicalSocietyofthePacificConference O’Sullivan,E.,Giacintucci,S.,Babul,A.,etal.2012,MNRAS,424,2971
Series,Vol.101,AstronomicalDataAnalysisSoftwareandSystemsV, Paterno-Mahler,R.,Blanton,E.L.,Randall,S.W.,&Clarke,T.E.2013,
ed.G.H.Jacoby&J.Barnes,17 ApJ,773,114
Arnaud,M.,Maurogordato,S.,Slezak,E.,&Rho,J.2000,A&A,355,461 Peterson,J.R.,Paerels,F.B.S.,Kaastra,J.S.,etal.2001,A&A,365,L104
Ascasibar,Y.,&Markevitch,M.2006,ApJ,650,102 Poole,G.B.,Fardal,M.A.,Babul,A.,etal.2006,MNRAS,373,881
Blanton,E.L.,Randall,S.W.,Clarke,T.E.,etal.2011,ApJ,737,99 Roediger,E.,Brüggen,M.,Simionescu,A.,etal.2011,MNRAS,413,2057
Bonamente,M.,Landry,D.,Maughan,B.,etal.2013,MNRAS,428,2812 Russell,H.R.,McNamara,B.R.,Edge,A.C.,etal.2013,MNRAS,432,
Briel,U.G.,Henry,J.P.,&Boehringer,H.1992,A&A,259,L31 530
Canning,R.E.A.,Sun,M.,Sanders,J.S.,etal.2013,MNRAS,435,1108 Sanders,J.S.2006,MNRAS,371,829
Churazov,E.,Forman,W.,Jones,C.,&Böhringer,H.2003,ApJ,590,225 Sanders,J.S.,Fabian,A.C.,Hlavacek-Larrondo,J.,etal.2014,MNRAS,
Clarke,T.E.,Blanton,E.L.,&Sarazin,C.L.2004,ApJ,616,178 444,1497
Clowe,D.,&Schneider,P.2002,A&A,395,385 Sanders,J.S.,Fabian,A.C.,Smith,R.K.,&Peterson,J.R.2010,MNRAS,
Ebeling,H.,Edge,A.C.,Bohringer,H.,etal.1998,MNRAS,301,881 402,L11
Fabian,A.C.1994,ARA&A,32,277 Schmidt,R.W.,Allen,S.W.,&Fabian,A.C.2001,MNRAS,327,1057
Fabian,A.C.,Sanders,J.S.,Allen,S.W.,etal.2011,MNRAS,418,2154 Smith,G.P.,&Taylor,J.E.2008,ApJL,682,L73
Fruscione,A.,McDowell,J.C.,Allen,G.E.,etal.2006,inProc.SPIE,Vol. Smith,R.K.,Brickhouse,N.S.,Liedahl,D.A.,&Raymond,J.C.2001,
6270,SocietyofPhoto-OpticalInstrumentationEngineers(SPIE) ApJL,556,L91
ConferenceSeries,62701V
Sun,M.,Murray,S.S.,Markevitch,M.,&Vikhlinin,A.2002,ApJ,565,867
Garmire,G.P.,Bautz,M.W.,Ford,P.G.,Nousek,J.A.,&Ricker,Jr.,G.R.
Sunyaev,R.A.,&Zeldovich,I.B.1980,ARA&A,18,537
2003,inSocietyofPhoto-OpticalInstrumentationEngineers(SPIE)
Sunyaev,R.A.,&Zeldovich,Y.B.1972,CommentsonAstrophysicsand
ConferenceSeries,Vol.4851,SocietyofPhoto-OpticalInstrumentation
SpacePhysics,4,173
Engineers(SPIE)ConferenceSeries,ed.J.E.Truemper&H.D.
Vikhlinin,A.,Markevitch,M.,&Murray,S.S.2001,ApJ,551,160
Tananbaum,28–44
ZuHone,J.A.,Markevitch,M.,&Johnson,R.E.2010,ApJ,717,908
Ghizzardi,S.,DeGrandi,S.,&Molendi,S.2014,A&A,570,A117
ZuHone,J.A.,Markevitch,M.,&Lee,D.2011,ApJ,743,16
Ghizzardi,S.,Rossetti,M.,&Molendi,S.2010,A&A,516,A32
Govoni,F.,Murgia,M.,Markevitch,M.,etal.2009,A&A,499,371