A deep radio and X-ray view of cluster formation at the crossroads of filaments

0.17 MB·English

by  J. Bagchi

Mem.S.A.It.Vol.00,0
(cid:13)c SAIt 2008 Memoriedella
A deep radio and X-ray view of cluster formation
at the crossroads of filaments
J. Bagchi1,R.J.vanWeeren2,S.Raychaudhury3,H.J.A.Ro¨ttgering2,H.T.Intema2,
1
1 F.Miniati4,T.A.Enßlin5,M.Markevitch6,andT.Erben7
0
2
1 Inter-UniversityCentreforAstronomyandAstrophysics(IUCAA),Pune411007,India
n
2 LeidenObservatory,LeidenUniversity,POBox9513,2300RALeiden,TheNetherlands
a
3 SchoolofPhysicsandAstronomy,UniversityofBirmingham,Edgbaston,Birmingham
J
B152TT,UK
4 4 Physics Department, Wolfgang-Pauli-Strasse 27, ETH-Zu¨rich, CH-8093 Zu¨rich,
2
Switzerland

5 Max-Planck-Institut fu¨r Astrophysik, Karl-Schwarzschild-Str.1, PO Box 1317, 85741
]
O Garching,Germany
6 Harvard-SmithsonianCenterforAstrophysics,60GardenStreet,Cambridge,MA02138,
C
USA
. 7 Argelander-Institutfu¨rAstronomie,Universita¨tBonn,AufdemHu¨gel71,D-53121Bonn,
h
Germany
p
- e-mail:[email protected]
o
r
t
s
Abstract. DeepX-raydatafromChandraandXMM-Newton,andGMRTradiodataare
a
[ presentedforZwCl2341.1+0000,anextremelyunusual andcomplex mergingclusterof
galaxies at the intersection of optical filaments. We propose that energetics of multiple

1 mergersandaccretionflowshasresultedinwide-spreadshocks,accelerationofcosmicray
v particles and amplification of weak magnetic fields. This results in Mpc-scale peripheral
9 radiorelicsandhalolikenon-thermalemissionobservednearthemergingcenter.
2
6 Keywords.Galaxies:clusters:general–Galaxies:clusters:individual:ZwCl2341.1+0000
4 – X-rays: galaxies: clusters – Radio continuum: galaxies – Shock waves – Cosmology:
1. Large-scalestructureofUniverse
0
1
1 1. Introduction tersarestillgrowingbymatteraccretionatthe
:
v junction of large scale filaments of galaxies,
i Galaxy clusters grow by mergers of smaller mediatedbythegravityofdark-mattercompo-
X
subclusters and galaxy groups as predicted nent.KeypropertiesfortestingmodelsofLSS
r byhierarchicalmodelsoflarge-scalestructure formationinclude the total energybudgetand
a
(LSS) formation.For the most massive merg- the detailed temperature distribution within a
ersa significantamountofenergyis released; cluster,whicharebothstronglyaffectedbythe
upto1063−1064 erg,accordingto thesemod- cluster’smergerhistory.Moreover,thephysics
els. All massive clusters have undergonesev- of shock waves in the tenuous intra-cluster
eralmergersintheirhistoryandpresentlyclus-
Bagchietal.:ZwCl2341.1+0000:deepradioandX-rayview 1
medium (ICM) and the effect of cosmic-rays distributed roughly along a north-south axis,
(CR; relativistic particles) on galaxy clusters spanning &4 Mpc. A network of several fila-
are all fundamental for our understanding of mentsofgalaxiesareseen branchingofffrom
LSSformation. the mainstructuretowardstheeastandnorth-
Diffuse radio sources with steep spectra eastforminga‘cosmic-web’(Fig.1&Fig.2).
(α . −0.5), not directly associated with indi-
vidual galaxies, are observed in a number of
2. RadioandX-rayObservationsof
clusters ( cf. review by Ferrariet al. (2008)).
ZwCl2341.1+0000
Largescale(l&500kpc)diffuseradiosources
in clusters are commonly divided into ‘radio ForprobingtheBremsstrahlungemissionfrom
halos’ and ‘radio relics’. Radio halos have the diffuse,hot ICM of this remarkablelarge-
smoothmorphologies,areextendedwithsizes scale structure, deep X-ray observations with
&1 Mpc, unpolarized, and are found at the Chandra and XMM-Newton were obtained.
centers of clusters, co-spatial with the X-ray The X-ray emission is detected for about 2.5
emitting hot ICM gas. Giant radio relics are Mpc in the N - S direction and an extension
foundontheclusterperiphery,withsizesupto towards the east is also visible, similar to the
several Mpc, sometimes showing arc or ring- patternofgalaxydistribution.InFig.2wealso
like structures, and are highly polarized (p∼ show the GMRT 610 MHz radio contourssu-
10-50% at 1.4 GHz). They are probably sig- perposedontheChandraACIS-IX-rayimage.
natures of electrons accelerated at large-scale In addition to N - S X-ray emission roughly
shocks. The vastmajority of clusters with ex- followingthefilamentofgalaxiesvisibleinthe
tendeddiffuseradiosourcesaremassive,X-ray optical image, embedded within the extended
luminous and are merging systems. Although diffuseemission,wecanidentifyseveralother
stillunderdebate,shocksandturbulence,both prominentX-rayemittingsystems,whichmost
causedbymergers,arethoughttoberesponsi- likely representhotgasassociated with merg-
blefor(re)acceleratingelectronstohighlyrel- inggroupsandclusters(Fig.1).
ativistic energies,andsynchrotronradiationis The overallcluster temperaturein a r=4′
producedinthepresenceofmagneticfieldsin region (roughly1 Mpc or close to r ), mea-
500
the ICM. Thereforedeep radio and X-ray ob- sured from the Chandra ACIS-I observation,
servationsofmergingclustersmayprovideex- is 4.4±0.6keV. ThecorrespondingX-raylu-
tremely important information on the dynam- minosity within the same region in the 0.1–
icsofLSSformation. 2.4 keV range is L =2.4×1044 erg s−1, and
X
ZwCl2341.1+0000,discoveredbyBagchi the bolometric X-ray luminosity being L =
bol
et al. (2002), is an extremely interesting and 4.6 × 1044 erg s−1(h = 0.71, Ω = 0.27,
M
complexsystem of galaxies, composedoutof Ω = 0.73). From the XMM-Newton com-
Λ
several different subclusters arranged along a binedPNandMOSobservations,fromthecen-
north-south filamentary axis extending upto tral r = 4′ region, we obtain an average tem-
at least ∼4 Mpc. It is probably a very large peratureof 4.7±0.5 keV. Thusboth Chandra
and massive cluster coming together at the andXMM-Newtonmeasuredtemperaturesare
junction of supercluster filaments. A 2.5 Mpc consistentwitheachother.
scale diffuse radio emission was detected in Deep GMRT radio maps were published
the main filamentary structure (Bagchi et al. by us recently (van Weeren et al. 2009) at
2002;Giovanninietal. 2010)whichmakesit 610,240and150MHz.Thesemaps(specially
uniqueandahighlyvaluablesourceforcosmo- 610 MHz map shown here in Fig. 2) success-
logicalstudies.Thesystemislocatedatared- fully resolved the compact sources from the
shift of z = 0.27, based on SDSS DR7 spec- extended diffuse emission of extremely low
troscopic redshifts of several galaxies in the surface brightness (only 100 - 200 µJy/beam
vicinity.Agalaxyisodensitymapderivedfrom at 610 MHz), which was found to be located
SDSS imaging data shows an elongated clus- mainlyintwoMpc-scaleradiostructurestothe
ter of galaxies, including several subclusters northandsouthofthecluster,cappingthetwo
2 Bagchietal.:ZwCl2341.1+0000:deepradioandX-rayview
Fig.1.Leftpanel:TheopticalR-bandimageofmergingsystemZwCl2341.1+0000obtainedfromWFIat
theESO2.2mtelescope.TheoverlaidcontoursshowthethermalX-rayemissionintheenergyrange0.5–
3keVobservedwithXMM-Newton.Rightpanel:Grey-scaleversionofthesameX-rayemissionobserved
with XMM-Newton. Both images cover about 4 Mpc on a side at redshift z=0.27 (Raychaudhury et al.
(2011)).
endsofX-rayemittinghotgasandthecentral that is occurringpreferentiallyalongthe NW-
optical filament. We interpretthis structure as SEaxis–i.e.,alongthemajorfilamentclearly
beingaperipheraldoubleradiorelic,wherethe seen in the distribution of galaxies. An ad-
particlesareacceleratedbythediffusiveshock ditional arm of the X-ray emission extends
acceleration (DSA or Fermi-I) mechanism in from the middle of the cluster towards NE,
outwardpropagatingshockwavesgeneratedin roughly coinciding with another filamentary
major cluster merger activity observed at the galaxystructure.ZwCl2341.1+0000maythus
center (cf. Bykov et al. (2008)). The merger representbyfarthemostcomplexmergercon-
axis is clearly along the galaxy filament, with figurationfoundamonggalaxyclusters.Dueto
radiorelicsplacedtangentialtothisaxis,asex- its highly unrelaxed dynamical state revealed
pected in a merger-shockgeometry.However, by our X-ray data, this system may provide
to the limits of surface-brightness sensitivity a rare glimpse into the early steps of assem-
achieved(∼30µJynoisefora5′′beamat610 bly of a galaxy cluster at the junction of fil-
MHz,∼12hon-sourcetime),nocentralradio- aments. In addition, rare detection of diffuse
halowas detectedinourGMRT observations. radioemission fromsuch a system will throw
Possiblereasonisthatsomeoftheshortbase- much light on the dynamicsof cluster forma-
linesinthe GMRTobservationswereaffected tion.Theoptical,radioandX-rayobservations
by RFI and had to be flagged, which allowed presented in this paper firmly establishes the
imaging of spatial scales of < 4′ only (at 610 linkofthismergertotheaxisofasupercluster
MHz).Averydeep327MHzGMRTobserva- filament along which the merger takes place.
tionisinpreparationtoresolvethisissue. This work represents a substantial advance in
the field, because it probes several important
componentsofthecosmicenvironment:inter-
3. Discussion galacticgas,magneticfields,andcosmic-rays.
Largescaleradioemissionindicatesthatmag-
The X-ray images clearly reveal a highly un-
neticfieldsofappreciablestrengtharepresent
relaxedclusterexperiencingacomplexmerger
Bagchietal.:ZwCl2341.1+0000:deepradioandX-rayview 3
notonlyinthe ICM butalso inthediffusein-
tergalacticmedium,i.e.,inthegasthatwillbe
shocked as it accretes onto collapsing struc-
tures-theprecursorsofvirializedgalaxyclus-
ters.Thesearchformagneticfieldsintheinter-
galacticmediumisoffundamentalimportance
for cosmology(Bonafedeet al. 2010;Dolag
2006).
In a recent deep 1.4 GHz VLA observa-
tion,Giovanninietal. (2010)presentevidence
of2Mpc-scalepolarizedradioemission,mid-
way between the peripheral radio relics. The
radioemission clearlyfollowsthedistribution
of optical galaxies and X-ray emission which
might suggest that it is similar to radio ha-
los found in dynamically unrelaxed clusters
(Ferrarietal. 2008).Thepolarizationvectors
(p∼10 - 20%) are very regular, indicating an Fig.2. A radio, optical and X-ray overlay map.
The solid contours represent the radio emission at
organized magnetic field pattern. This feature
610MHzfromtheGMRTdata[vanWeerenetal.
might result from the accretion flow shocks
(2009)].Broadbeamof15′′FWHMisusedtoshow
in filaments compressing the weak magnetic
betterthediffuseradioemission.Theradiocontours
fields of the infalling IGM, which is consis-
aredrawnat[1,2,4,8,16,32,...]×0.224mJy/beam.
tent with a model proposed by Bagchi et al.
Dashed contours show the galaxy isodensity con-
(2002). An observation of large scale diffuse toursfromSDSS,corresponding roughlytoalimit
radioemissionandmagneticfieldsinfilamen- of M = −18.1 (i.e., M∗ +2.4). The dashed con-
V
tarygalacticenvironmentalsoprovidesafore- toursare drawnat [2, 3, 4, 5,...]galaxiesarcmin−2
tasteoffuturedetections,asamajorgoalofup- with a redshift cut. Grey-scle image is smoothed,
cominglowfrequencyradiotelescopeslikethe point source subtracted X-ray emission map from
LOFARandLWA istoprobethenon-thermal Chandrainthe0.5-3.0keVenergyband.
processesinhundredsofmergingclustersand
inthefilamentarycosmic-webofthenearand
whichresultedintheradiodata.TheGMRTisana-
distantUniverse.
tional facility operated by the National Centre for
RadioAstrophysicsoftheTIFR,India.
4. Conclusions
WehavepresenteddeepX-rayandradioobser- References
vations which show that ZwCl 2341.1+0000
Bagchi,J.etal.2002,NewAstron.,7,249
constitutesacomplex,large-scaleensembleof
Bonafede, A. et al. 2010, Proc. ISKAF2010
clusters, which are merging at the intersec-
ScienceMeeting,TheNetherlands
tion of filaments identifiable in optical and
Bykov, A. M., Dolag, K. & Durret, F. 2008,
X-ray images. Energeticsof multiple mergers
SSRv.,134,119
andaccretionflowshasresultedinwide-spread
Dolag,K. 2006,AstronomischeNachrichten,
shocks, acceleration of cosmic ray particles
327,575
and amplification of weak magnetic fields.
Ferrari,C.etal. 2008,SSRv.,134,93
Thisisevidentinapairofgiant,peripheralra-
Giovannini, G., Bonafede, A., Ferreti, L.,
diorelicsandafaint,diffuseradiohaloextend-
Govoni,F.,&Murgia,M. 2010,A&A,511,
ingmorethan∼1Mpcbetweenthetworelics,
L5
followingthelarge-scalefilamentarystructure.
Raychaudhury,S.etal. 2011,Inpreparation
Acknowledgements. We thank the operations team vanWeeren,R.J.etal. 2009,A&A,506,1083
of the NCRA–TIFR GMRT observatory for help