Shocking news outside of cluster cores

0.39 MB·

by  Lawrence Rudnick

Mem.S.A.It.Vol.75,282
c SAIt 2008 Memoriedella
(cid:13)
Shocking News outside of Cluster Cores
L. Rudnick1,S. Brown2 andD. Farnsworth1
1
1 1 UniversityofMinnesota, DepartmentofAstronomy,116ChurchSt.SE,Minneapolis,
0 MN,55455,USA.e-mail:[email protected], [email protected]
2 2 CSIROAstronomy&SpaceScience,AustralianTelescopeNationalFacility,POBox76,
EppingNSW1710,Australia.e-mail:[email protected]
n
a
J
Abstract. Below 1µG,theminimumpressuresofsynchrotronplasmasstarttoapproach
0 ∼
thoseinthethermalgasinclusteroutskirtsandthemorediffuseWHIMinlarge-scalestruc-
3
turefilaments.Wesummarizesomeofourtechniquestofindthecorrespondinglowsurface

brightness radio sourcesand what hehaveuncovered. Weidentifycluster-likesources in
]
O poor environments, likelyrequiringveryefficientrelativisticparticleacceleration,aswell
as very diffuse radio galaxies with no indications of current activity. More detailed ob-
C
servations of Coma and a new radio-identified cluster highlight some emerging issues in
h. cluster-relatedshocks:Comahasashockontheborderofitshalo,suggestingaconnection
p betweenthetwo;theComarelicappearstobein“infall”shock,associatedwitha500km/s
- infallingcolumnofgalaxies;thenewclusterhasfilamentarypiecesinadditiontoitsmore
o classicalhaloandperipheralrelics,suggestingthatwearebeginningtoseerelicemission
r faceon.Finally,adifferentkindofshockcomesfromourrecentfindingofambiguitiesin
t
s thedeterminationofrotationmeasureswhichwillinfluenceclustermagneticfieldstudies.
a
[

1 1. Searchingforlowsurface Detection of low surface brightness emis-
v brightnessemission sionischallenging;toreachrequiredsensitivi-
5
tieswehavetoaverageoverlargeareas,which
3
rapidlybecome”confused”bymorecompact,
7 Beyondclustercores,intheinfallregionsand
distant radiogalaxies. We have been develop-
5 along the connecting filaments, we expect to
ingtechniquestoremovecompactsourcecon-
.
1 find rich dynamical structures, as seen in nu- fusioninexistinglargesurveys,
0 merical simulations that include the baryonic
1 components. These regions are likely to be Multi-resolution filtering was introduced
1 permeated by stronger (higher Mach number) byRudnick (2002),basedonanalgorithmde-
v: shocks than in cluster cores, althoughthe dif- scribed in Starck,Murtagh&Bijaoui (1998).
i fuse baryons are difficult to detect. Here, ra- It consists of a very fast method to decom-
X
dioemissioncanprovideuniquediagnosticsof poseanimageintoanynumberofcomponent
r theclusterformationprocessandtheenergiza- images each respresenting a different angular
a
tion of magnetic fields and relativistic parti- resolution. We illustrate it here in Figure 1
cles.Sinceoverpressuredradioplasmaswould in its two component mode, where it is used
expand very rapidly, we therefore search for to removethepointsourcecontributionsfrom
lowbrightnessemissionastheindicatoroflow the WENSS survey (Rengelinketal. 1997).
minimumpressureconditions. This allows detection of much lower sur-
Rudnick:ShockingNews 283
Fig.1. Example of multi-resolution filteringto identify low surface brightness emission in existing sur-
veys.Left:originalWENSSsurveyimage.Middle:imageoflowsurfacebrightnessemissionwithcompact
sourcesremoved.Right:OverlayoflowbrightnessradioemissionfromRXJ1053.7+5450 onX-raycon-
toursfromarchivalROSATbroadbandimage,convolvedto240 .(SeeRudnick&Lemmerman (2010)).
′′
facebrightnessemissionthanwouldotherwise
be practical. Figure 1 shows a new periph-
eral relic discovered around the X-ray clus-
terRXJ1053.7+5450Rudnick&Lemmerman
(2010).Avarietyofclusterandnon-clusterdif-
fusesourceshavebeenfoundinthisway;one
interestingexampleisdiscussedbelow.
NVSS polarization reprocessing.
Nominally, the NVSS survey (Condonetal.
1998) is not sensitive to structures on scales
greater than 15. However, even structures
′
∼
on the scale of 10s of degrees may be de-
tectable in polarization, if their Q and U
Stokes parameters vary on smaller scales.
Such variations are, in fact, common due to
differentialFaradayrotationandorientationof Fig.2. NVSS (1.4 GHz) total intensity in
greyscale,SWoftheComaClusterintheregionof
themagneticfieldintheplaneofthesky.The
the relic. Only hints of the relic are visible in the
problem in detecting such emission, however,
NVSStotalintensityimages.Contoursshowthere-
isthatonecannotaverageQandUoverlarge
processed NVSS polarization data, with the bright
angles to recover very low surface brightness
southernhalfoftherelicprominentlyseen.
emission. In Rudnick&Brown (2009) we
introduced a technique for recovering very
large scale low surface brightness structures reprocessing.Wehavebeenfollowingupona
through their Q, U variations. We averaged number of the newly detected sources on the
< P>=< √(Q2+U2)>,aprocedurenormally WSRT and the GBT, to disappointing results.
discouraged in polarization work. In this One example is shown in Figure 3. This dra-
way, we were able to recover the polarized matic looking feature is not detected in total
emission from our Galaxy over many 10s of intensity at WSRT (330 MHz), but is instead
degrees, as well as a wide variety of galactic visible as a patchy distribution of rotation
and extragalactic sources. One example is measures to the diffuse galactic background.
shown in Figure 2, the relic to the southwest SuchapatchyRMdistributionwouldproduce
of the Coma cluster, which is marginally small-scalevariationsinQandUintheNVSS
visible in the NVSS total intensity image, but images, and show up as a “pseudo-source”in
seen at high signal:noise in our polarization ourreprocessedpolarizationimages.
284 Rudnick:ShockingNews
Fig.4. Plot of radio luminosity vs. X-ray lumi-
nosity for cluster relics, from Brown&Rudnick
(2009).Thetwocomponentsof0809+39areshown
asN andS ,withupperlimitsfortheirX-ray
diff diff
Fig.3. ExtendedfeatureseeninreprocessedNVSS luminosities.Theyareatleasttwoordersofmagni-
polarizationdataduetoaFaradayscreen,notasep- tudemoreradioluminousthanexpected.
aratesynchrotronsource(Rudnick&Brown 2009).
shown in Figure 4. They are approximately
two orders of magnitude higher in radio lu-
1.1.Cluster-likeemissioninpoor
minosity than would be expected by com-
environments
parison, e.g., with other cluster relic sources.
One of the discoveries from our filtering pro- A filamentary structure in (projected on?)
cedure is a pair of unrelated diffuse sources Abell 1213 Giovanninietal. (2009) also ap-
at0809+30(Delain&Rudnick 2006).Infact, pears too strong for its X-ray luminosity.
these are strong enough to be visible in the In Brown&Rudnick (2009) we frame this
original WENSS images. Detailed mapping dilemmaintermsoftheaccelerationefficiency
of these with both the WSRT and the VLA forrelativisticparticles.
revealed them to be steep-spectrum objects Atpresent,suchobjectsarefairlyrare.We
with little or no associated X-ray emission have followed up on several more of the fil-
(Brown&Rudnick 2009).Thesouthernofthe tering discoveries. Two of these are very low
twosources,S isassociatedwithachainof surfacebrightnesslikelydeadradiogalaxiesin
diff
optical galaxies at a redshift of 0.04. It is not modestgalaxygroups(publicationinprepara-
clearwhetheritoriginatedfromasingleAGN tion). A third is an X-ray cluster source, dis-
oristheresultofa group-wideprocess.Ifthe cussed in the following section. As the next
latter,itishardtosee howtherecouldbesuf- generationofradiotelescopesandsurveysgets
ficientturbulenceorshockstoacceleratefields underway,We will likely see increasingnum-
andparticlesinthisenvironment. bers of “disembodied” diffuse plasmas; with
The identification – but not the physics – lucktheywillhelpusilluminatetheprocesses
of the northern of the two 0809+39 sources by which cluster relativistic plasmasoriginate
(N ) appear clearer. It is apparently as- andarere-energized.
diff
sociated with a poor cluster of galaxies at
a redshift of 0.2, with a wide-angle-tail
2. Theclustershockmenagerie
(Owen&Rudnick 1976) in the center. In lo-
cation,size,morphology,luminosity,spectrum Asthenumberofobservedhaloandrelic-type
and polarization it appears to be a quite ordi- sourcesincreases,andthedistinctionsbetween
naryrelic,exceptforthelackofdetectableX- them blur, we have the opportunity to rede-
rayemissionfromthecluster. fine our notion of shocks and turbulence in
The relationship between radio and X- these very dynamic regions. We need to look
ray luminosities for these two sources are againto theincreasinglysophisticatednumer-
Rudnick:ShockingNews 285
ical simulations that include thermal and rel-
ativistic plasmas. Battagliaetal. (2009), e.g.,
showusanetworkofshocksinsidethecluster,
generated by previous accretion, mergers and
turbulence. These shocks are typically weak
(Miniatietal. 2000)becauseofthehightem-
peratures of the thermal plasma. The various
typesofshocksweexpecttoeventuallyseeas-
sociated with cluster and large scale structure
formationinclude:
– Internalclustershocks–mayplayarolein
keepinghalosenergized;
– Outgoing merger shocks, e.g.,
generating peripheral relics,
(Roettiger,Burns&Stone 1999);
– Accretion shocks – this term applies in Fig.5. WSRT 330 MHz observations of Coma
the literature to the approximately spher- cluster; 1D cut showing shock at edge of halo
ical shocks due to continuing infall into (Brown&Rudnick 2010).
the cluster. They typically appear beyond
the virialradius(Tozzi,Scharf&Norman
2000)andhavenotyetbeenseeninthera- analysisoftheXMMdataofSchueckeretal.
dio; (2004),ascitedinBrown&Rudnick (2010).
– Infall shocks – a term we introduce in Thisisasurprise.Itcould,ofcourse,sim-
Brown&Rudnick (2010) to describe the ply be a coincidence that an ordinary outgo-
shock at the termination of a column of ingmergershockisfoundattheouteredgeof
galaxiesinfallingintoComa; a halo. However,the halo at 330 MHz blends
– Filament shocks – accompanying infall smoothly into the shock structure, which is
intoandalongfilamentsduringlarge-scale why it has been hard to detect. More likely,
structureformation(Ryuetal. 2003). andmoreinteresting,isthatthehaloandshock
are physically connected, Markevitch (2010)
As we seek to disentangle the roles of these
suggests that there may be a number of other
shocks, e.g., for relativistic particle accelera-
examplesof shocksat halo edges. At present,
tion,itisimportantthatweidentifythemprop-
halos are not thought to be caused by a sin-
erlyandassignthemthecorrectdynamicalfea-
glecluster-wideshockstructure,andsothede-
tures. The simple picture of outgoing merger
nouement of this issue will undoubtedly give
shocksisbecominginadequatefortherichva-
usinsightsintohaloformation.Wenotethata
rietyofstructuresnowseen.
detached,filamentaryfeatureisalsoseenatthe
ComahaloshocklocationbyPizzo (2010)in
WSRTobservationsat150MHz.
2.1.RevisitingtheComacluster
On the GBT, we found that the relic to
New observationalprogramson the GBT and the southwest of the Coma cluster (first no-
WSRT of the well-studied Coma cluster have ticedbyJaffe&Rudnick (1979)butidentified
uncoveredsomeexcitingnewphenomena.The byGiovannini,Feretti&Stanghellini (1991)),
first is the discovery that the western edge of wasapproximatelytwiceaslongaspreviously
the halo is formed by a shock structure as seen.Thenewextentoftherelic 2Mpc,isnot
∼
shown in Figure 5. The shock nature of this outsidethe norm.However,we foundthatthe
feature is confirmed by the coincident sharp relicisadjacenttoa“wall”ofgalaxies,theend
jump in X-rays (Markevitch 2010) and the ofa500km/sinfallingcolumnpreviouslyseen
temperature jump by a factor of 3 in a re- as individual clumps and groups of infalling
∼
286 Rudnick:ShockingNews
Fig.6. Opticalgalaxiesandradiorelicaroundthe
Comacluster(Brown&Rudnick 2010).
Fig.7. VLA 1.4 GHz image of cluster of galax-
ies at 0217+70 (Brown,Duesterhoeft&Rudnick
galaxies(Adamietal. 2005).Amazingly,this 2011)
structurecanbeseeninthedistributionofneb-
ulae by Wolf (1902). The interesting thing sociated with merger shocks (Skillmanetal.
aboutthisjuxtapositionoftheopticalandradio 2010). However, the brightest examples are
structures is the likely dynamical state. Since likelytobethoseseenprojectedalongtheline
thecolumnofgalaxieshasnotyetmergedwith of sight, biasing our samples. In the near fu-
Coma,thereliccannotbea“classical” outgo- ture, we shoulbe be able to get a much better
ing shock. However,in this early stage of the pictureoftheir3Dstructure.
encounter, the column can generate shocks at
its leading and trailing edges; we see the lat-
3. Someshockingresultsonrotation
ter. We have suggested designating such fea-
tures as “infall shocks”, rather than accretion measures
shocks(asalreadyusedintheliterature)orout-
Weclosewithabriefnoteonsomerecentwork
goingmergershocks, becausetheir energetics
on ambiguities in the determination of rota-
anddynamicalevolutionwillbequitedistinct.
tionmeasures(Farnsworth,Brown&Rudnick
2011). The problem is illustrated in Figure 8.
The left-most image shows a model of obser-
vationswith quite good coveragein λ2 space,
2.2.Aradio-identifiedclusterofgalaxies
of a source with a rotation measure of 1 rad.
m2
One of the diffuse features found through The other panels show a variety of two com-
our filtering work turned out to be a ponentmodels,allof themdoingan excellent
previously unidentified cluster of galaxies jobofmatchingtheobservationsatlongwave-
(Brown,Duesterhoeft&Rudnick 2011)atthe lengths, and two of the four matching the ob-
lowgalacticlatitudeof9o,asseeninFigure7. servationsatbothshortandlongwavelengths.
Ithasbothahaloanddoubleperipheralrelics, To date, mostrotation measuredata are much
seen as filamentary structures on a circle of sparserthanshownin thesemodels;thereisa
1Mpcradiusaroundtheclustercore.Ofspe- correspondingenormousgrowthinthenumber
∼
cialinterestaretheotherfilamentarystructures of alternative fits to the data. Rotation mea-
seen with various orientations and at various sure synthesis (Brentjens&deBruyn 2005)
positionsaroundthehaloandrelic.Onepossi- cansometimesshowtheexistenceoftwocom-
ble explanation for these is that they are pro- ponents, but fails in the very important case
jectionsofthethree-dimensionalstructuresas- where two components are within the broad
Rudnick:ShockingNews 287
Fig.8.Alternativemodelstodescribethesame(model)rotationmeasureobservations.
FWHM of the rotationmeasuretransferfunc- partially supported at the University of Minnesota
tion.Thereisnotasimpleprescription,suchas byU.S.NSFgrantAST0908688.Thecitedpapers
the ratio ∆RM/FWHM to tell whether an am- detail support for the VLA and WSRT observato-
biguitywillbepresent,sinceitalsodependson ries,onwhichmuchofthisworkisbased.
therelativeamplitudesandevenintrinsicposi-
tionanglesofthetwocomponents.
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