Cosmological Numerical Simulations of Radio Relics in Galaxy Clusters: Insights for Future Observations

0.38 MB·English

by  Jack O. Burns

Mem.S.A.It.Vol.,1
(cid:13)c SAIt 2008 Memoriedella
Cosmological Numerical Simulations of Radio
Relics in Galaxy Clusters: Insights for Future
Observations
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1 JackO. Burns1,2 andSamuelW. Skillman1,3
0
2
1 CenterforAstrophysics&SpaceAstronomy,UniversityofColoradoatBoulder,Boulder,
n
CO80309USA
a
J 2 NASALunarScienceInstitute,AmesResearchCenter,MoffettField,CA94035USA
3 DOEComputationalScienceGraduateFellow
8
e-mail:[email protected]
1

]
O Abstract.Theaccelerationofelectronsatshockfrontsisthoughttoberesponsibleforradio
C relics, extended radio features in the vicinity of merging galaxy clusters. By combining
highresolutionAdaptiveMeshRefinementHydro/N-body cosmological simulationswith
.
h anaccurateshock-findingalgorithmandamodelforelectronacceleration,wecalculatethe
p expectedsynchrotronemissionresultingfromcosmologicalstructureformation.Fromthese
- simulations,weproduceradio,SZEandX-rayimagesforalargesampleofgalaxyclusters
o
alongwithradioluminosityfunctionsandscalingrelationships.Wefindthatwithupcoming
r
t radioarrays,weexpecttoseeanabundanceofradioemissionassociatedwithmergershocks
s intheintraclustermedium.Byproducing observationallymotivatedstatistics,weprovide
a
predictionsthatcanbecomparedwithobservationstofurtherourunderstandingofelectron
[
shockaccelerationandkinematicstructureofgalaxyclusters.

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v Keywords.Cosmology:theory–Galaxies:clusters:intraclustermedium–radiationmech-
1 anisms:nonthermal
6
3
3
1. Introduction electrons gyrating in ICM B-fields producing
.
1 steep-spectrum,so-called“radiorelics”.
0 Colliding galaxy clusters are insightful astro-
1 physicalplasmalaboratories.Shocksproduced Radio relics are rarely found in radio sur-
1 during mergers heat the intracluster medium veys of galaxy clusters, possibly because of
v: (ICM), assisting the gas to achieve roughly theirsteepradiospectrumanddiffuse,lowsur-
i hydrostatic equilibrium with the cluster grav- face brightness emission at cm wavelengths.
X
itational potential well. Shocks also play a However, with new and improved radio ar-
r key role for the nonthermalcomponentof the rays such as LOFAR, GMRT, and the EVLA,
a
ICM. Shockscompress and amplify magnetic new high sensitivity observations may reveal
fields. Shocks accelerate cosmic rays (CR) an abundance of cluster radio relics. To assist
via a diffusive Fermi process. Thus, merger insearchingfortheserelics,wehaverunhigh
shocks are illuminated via the resulting syn- spatial dynamic range cosmological adaptive-
chrotronradiationarisingfromrelativisticCR mesh-refinement(AMR)simulationsusingthe
2 Burns&Skillman:SimulationsofRadioRelics
Fig.1.Projectionsofdensity(top),temperature(middle),and1.4GHzradioemission(bottom)forarep-
resentativesampleofclustersfromEnzoAMRsimulationsatz = 0.Eachimageis4h−1 Mpconaside.
Peakresolutionis3.9h−1kpc.
Enzocodethatproduce≈2000clustersinvari- esting points are worth noting in comparing
ousstagesofmergerevolution(Skillmanetal. these images. First, there is a distinct differ-
2010). Using new shock-identification tools enceinmorphologiesbetweenthedensityand
and analytical models for diffusive shock ac- radio images. The density (≈X-ray emission)
celeration (Hoeft&Bru¨ggen 2007), we have iscenter-filledwhereastheradioisoftenedge-
constructed radio maps of a large sample of brightened. The curved radio arcs are illumi-
simulatedclusterstostudytheproductionand nated bow shocks produced as two clusters
observational properties of relics in different pass between their cores. Such shocks are ef-
merger states. Details of these Enzo simula- fectively invisible on the density/X-ray maps
tionsarefoundinSkillmanetal. (2010). and only partially visible on the temperature
images. Thus, the radio relics light up im-
portant evolutionary features in clusters (i.e.,
2. SimulatedRadioRelics shocks)thatarenotapparentatX-rayenergies.
Images of density, temperature, and radio Second, in cases where the merger is
emission for a small representative sample largely along the plane of the sky, the radio
of clusters in a single projection along one relics fall on the edges of sharp temperature
axis are shown in Figure 1. Several inter- gradients.Thisisparticularlyapparentforthe
Burns&Skillman:SimulationsofRadioRelics 3
Fig.3. Scaling relation between 1.4 GHz radio
powerand0.2−12keVX-rayluminosityforsimu-
latedclusters(solidline).Alsoshown(dashedline)
isalinewiththeslopeofthebest-fitobservedscal-
ingrelationforradiorelicsfromFeretti (2002).
Fig.2. Profiles of X-ray flux, dimensionless
ComptonySZEparameter(∝gaspressure),andra- 3. ScalingRelations
diofluxdensityforamergingclusterwithacenter-
filledradiorelicintheupperleft-handimageofthe In Fig. 3, we show the scaling relation be-
lowerpanelinFigure1. tween radio power at 1.4 GHz and soft X-ray
luminosity for the most X-ray luminous sim-
ulated clusters in our sample. Although there
cluster in the lower right of Figure 1. This isaclearcorrelationbetweentheradioandX-
strongX-raytemperature/radiospatialcorrela- ray luminosities within r ≈ 0.8r , there
200 virial
tionagreeswithrecentobservations. isconsiderablescatterintherelationship(fac-
Third, depending upon the projection, torof≈104inP1.4GHz).Thisscatterisrealand
there is a wide variety of radio morpholo- represents differentmerger states for different
gies. Although there is a preferencefor edge- clusters. High radio power clusters have suf-
brightened radio emission coinciding with feredrecentmergerswhereaslowpowerclus-
shocks, there are a few clusters that demon- ters have not experienced a merger in over a
strate more diffuse, center-filled radio emis- Gyr. To date, only the most radio luminous
sion (upperleft-hand clusters in bottom panel clusters have been observed and this possibly
of Figure 1). This corresponds to cluster pro- represents an observational bias that may be
jectionswhere the mergeris largelyalongthe remediedwithmoresensitive,highbandwidth
line-of-sight.Suchemissionmayqualitatively observationsinthenearfuture.
resemble“radiohalos”,buttheydonotdemon- Thereisalsogoodagreementintheslopes
strate a strong correlation between X-ray and of the scaling relation between observed and
radioemissionprofilesseenforradiohalos.In simulated clusters in Fig. 3. Although the ob-
Figure2,weshowprofilesofX-ray,Sunyaev- servedsamplefromFeretti (2002)issmall(9
Zeldovich Effect (SZE), and radio emission clusters), the generalagreementbetween sim-
foraclusterwithapparentlycenter-filledradio ulationsandobservationsisencouraging.
relic emission. For all the radio relics, the ra-
dio profile shapes and slopes are inconsistent
4. PredictedNumberofRadioRelics
with those for the densities (and X-ray emis-
sion).So,oneshouldbeabletodistinguishbe- From our numerical simulations, we con-
tweenrealradiohalosandprojectedrelicsvia structed a radio luminosity function for radio
theirX-ray/radioprofiles,alongwithexpected relicclusters.Thatis,wecalculatedthecumu-
differencesinspectralindex. lative number of clusters with P greater
1.4GHz
4 Burns&Skillman:SimulationsofRadioRelics
improve, SZE will complement radio images
inmappingmergershocks.
6. Conclusions
Usingarobustshock-findingalgorithmandan
analytical model for diffusive shock accelera-
tion applied to a large-volume AMR cosmo-
logicalsimulation,wehaveproducedsynthetic
X-ray and radio images with characteristics
similartothoseobservedinclusterswithradio
relics.Thedual-arcradiomorphologiesandX-
ray/radio scaling relations are good matches
toobservations.We predictthatanincreasein
10-100 in the number of radio relic clusters
detected with new and improved radio arrays
(e.g.,EVLA,LOFAR,andfuturelunarfarside
Fig.4. Synthetic SZE/radio images. Contours are
lowfrequencytelescopes)shouldbepossible.
SZEfluxandgrey-scaleisradioemission.
Acknowledgements. Thisworkhasbeenfundedby
thanagivenlevel,wheretheradioemissionis grants from the U.S. NSF (AST-0807215) and the
theresultofclustermergershocks.Sinceradio NASA Lunar Science Institute (NNA09DB30A)
to J.O.B. S.W.S. has been supported by a DOE
power scales with cluster mass in our simula-
tions(P ∝ M3.2) (Skillmanetal. 2010), Computational Science Graduate Fellowship (DE-
1.4GHz FG02-97ER25308).Computationsdescribedinthis
our computational volume produces clusters
work were performed using the Enzo code de-
with M < 1015M and P < 1024
200 ⊙ 1.4GHz veloped by the Laboratory for Computational
W/Hz.Toextendtheselimitstohighermasses AstrophysicsattheUniversityofCaliforniainSan
and radio powers, we extrapolated our clus- Diego(http://lca.ucsd.edu).Analysisofthesimula-
tersampleusingtheWarrenetal. (2006)mass tionswasperformedusingyt(Turketal. 2011).
functionat0<z<1.Withinthiscosmological
volume of ≈26 (Gpc/h)3, our simulations and
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