Modeling the Outskirts of Galaxy Clusters with Cosmological Simulations

0.21 MB·English

by  Daisuke Nagai

Mem.S.A.It.Vol.,1
(cid:13)c SAIt 2008 Memoriedella
Modeling the Outskirts of Galaxy Clusters with
Cosmological Simulations
DaisukeNagai
1
1
0 DepartmentofPhysics,YaleUniversity,NewHaven,CT06520,U.S.A.
2 e-mail:[email protected]

n
a
J Abstract. Wepresentcosmologicalsimulationsofgalaxyclusters,withfocusontheclus-
ter outskirts. We show that large-scale cosmic accretion and mergers produce significant
6
internalgasmotionsandinhomogeneousgasdistribution(”clumpiness”)intheintracluster

] medium(ICM)andintroducebiasesinmeasurementsoftheICMprofilesandthecluster
O mass. Wealsoshow that non-thermal pressureprovided bythegasmotions isoneof the
dominantsourcesoftheoreticaluncertaintiesincosmicmicrowavebackgroundsecondary
C
anisotropies. We briefly discuss implications for cluster cosmology and future prospects
.
h for understanding the physics of cluster outskirts using computer simulations and multi-
p wavelengthclustersurveys.
-
o
r
t 1. Introduction as radiative gas cooling, star formation, and
s
a energy injection from active galactic nuclei.
In recent years, galaxy clusters have emerged
[ Dominant physical processes in the outskirts
as one of the most unique and power-
are limited to the gravity-driven collisionless
1 ful laboratories for cosmology and astro-
dynamics of dark matter and hydrodynamics
v physics. Being the largest and most mag-
2 of the intracluster medium (ICM). In the hi-
nificent structures in the Universe, clusters
2 erarchical structure formation model, galaxy
3 of galaxies serve as excellent tracers of the clustersgrowby accretingclumpsanddiffuse
growth of cosmic structures. Current genera-
1 gas from the surrounding large-scale struc-
tion of X-ray cluster surveys have provided
. ture in their outer envelope. Numerical simu-
1
independent confirmation of cosmic acceler-
0 lations predict that the large-scale cosmic ac-
ation and significantly tighten constraints on
1 cretion and mergers give rise to internal gas
the nature of dark energy (Allenetal., 2008;
1 motions and inhomogeneous gas distribution
Vikhlininetal., 2009) and alternative theories
: in the ICM. However, until very recently, ob-
v of gravity (e.g., Schmidtetal., 2009). Several
servationalstudiesofthe ICM havebeenlim-
i
X ongoing and new X-ray (e.g., eROSITA) and itedtoradiiconsiderablysmallerthanthevirial
Sunyaev-Zel’dovich effect (SZE) cluster sur-
r radiusofclusters.
a veys(e.g.,SPT,ACT,Planck)areunderwayto
improvecurrentcosmologicalconstraints. Recently, Suzaku X-ray observations have
Outskirts of galaxy clusters have special extendedX-raymeasurementsoftheICMpro-
importance for cluster cosmology, because fileouttoandbeyondthevirialradiusforsev-
they are believed to be much less suscepti- eral clusters (Bautzetal., 2009; Georgeetal.,
ble to complicated cluster astrophysics, such 2009; Reiprichetal., 2009; Hoshinoetal.,
2 D.Nagai:SimulatingtheClusterOutskirts
2010; Kawaharadaetal., 2010). While these
measurements are still quite uncertain, initial
results suggested that the observed ICM pro-
files may deviate significantly from the pre-
diction of hydrodynamicalcluster simulations
(e.g., Georgeetal., 2009). In addition to test-
ing models of structure formation, these new
measurementswillbeimportantforcontrolling
systematic uncertainties in cluster-based cos-
mologicalmeasurements.
In this work, I will present theoretical
modeling of the outskirts of galaxy clusters
basedoncosmologicalsimulations,withhigh-
lights onimplicationsforthe interpretationof
forthcomingmulti-wavelengthobservationsof
galaxy clusters. The simulations we present
here are described in Nagaietal. (2007a) and
Nagaietal. (2007b), and we refer the readers
tothesepapersformoredetails.
2. GasMotionsinClusterOutskirts
Gasmotionsinducedbycosmicaccretionand
mergersprovidenon-thermalpressuresupport
in galaxy clusters. The top panel of Fig. 2
shows that results of hydrodynamical cluster
simulations, illustrating that the non-thermal
Fig.1. Top panel: Ratio of pressure from random
pressure contributesto 10-20% at r = r of
500 gasmotionstototalpressureasafunctionofradius.
thetotalpressure.Thefractionofnon-thermal
Relaxedclustersarerepresentedbysolidlineswhile
pressure support increases with radius, and it unrelaxed clustersarerepresented bydashed lines.
islargerformoredynamicallyactivesystems. Bottompanel:AveragedmassprofilesM(<r)ofthe
The bottom panel of Fig. 2 shows that the relaxedclusters,normalizedbyM .Thesolidline
500
cluster mass profile based on hydrostatic as- shows the actual mass profilefromsimulation, the
sumption is biased low. We further demon- longdashedlineshowsthemassprofilefromhydro-
strate that it is possible correctthe bias in the staticequilibriumincludingrandomgasandthermal
hydrostatic mass, if one could measure gas pressure,andtheshortdashedlineshowsthemass
profilefromhydrostaticequilibriumincludingther-
motions in clusters and hence correct for the
malpressureonly.Hashedregionshowsthe1-σer-
bias. Upcoming Astro-H X-ray satellite mis-
rorofthemean.FromLauetal.(2009).
sion (scheduled to be launched in 2014) will
providea first directmeasurementof gas mo-
tionsinclustersviadopplerbroadeningofiron a major source of systematic bias in X-ray
lines(Inogamov&Sunyaev,2003). measurementsofICMprofilesintheenvelope
of galaxy clusters (r & r ) (Nagai&Lau,
200
2011). Using hydrodynamical simulations of
3. GasClumpinginClusterOutskirts
cluster formation,we show that gas clumping
Recently, Suzaku X-ray observations revealed introducestheoverestimateoftheobservedgas
thattheobservedentropyprofileoftheICMis density and causes flattening of the entropy
significantly offset from the prediction of hy- profile at large radius. This is illustrated in
drodynamical simulations of galaxy clusters. Fig.2.Thetoppanelshowsthattheclumping
Here,we pointoutthatgasclumpingislikely factor of the X-ray emitting gas (T & 106 K)
D.Nagai:SimulatingtheClusterOutskirts 3
8 εf, εDM Ac
2K] 6
µ
[ 4
Dℓ
2
08 α0 β
2K] 6
µ
[ 4
Dℓ
2
0 103 104 103 104
ℓ ℓ
Fig.3.AstrophysicaluncertaintiesintheSZpower
spectrum.Differentlinesindicatetheoreticaluncer-
taintiesassociatedwith(1)heatingofgasbyenergy
injectionfromstarsandAGN(indicatedwithsolid
lines)andmergerdynamics(indicatedwithdashed
lines in top-left panel), (2) dark matter structures
(top-right),and(3)non-thermalpressurebygasmo-
tions (bottom-left) and its time evolution (bottom-
right).Ineachcase,thethickredlinerepresentsour
fiducialmodel.FromShawetal.(2010).
C(r)1/3. Results of our analyses indicate that
Fig.2. Top panel: Median clumping factor pro-
filesofgaswithdifferentminimumtemperaturefor gas clumping causes the flattening of the ob-
servedentropyprofilesatr & r .Whilecur-
the simulations with gas cooling and star forma- 200
tion. Bottom panel: Impact of gas clumping on X- rent Suzaku measurements are still uncertain,
raymeasurements of theICMentropy profile.The our results indicate that gas clumping is im-
dashedlineindicatesthetrueentropyprofileofthe portantforreducingthetensionbetweenrecent
simulatedclusters,whilethesolidlineindicatesthe Suzakuobservationsandtheoreticalprediction
observed entropy profile,obtained by assuming no oftheΛCDMmodel.
clumping(C=1).BlackpointsareSuzakuobserva-
tionsofPKS0745-191(circles),A1689(triangles),
A1413(stars),andA1795(blackdot-dashedlines). 4. ImpactontheSZpowerspectrum
FromNagai&Lau(2011).
Recent measurements of the SZ power spec-
trum by SPT and ACT telescopes revealed
that the SZ power is significantly below the
signal predicted by the current cosmic struc-
is C ≡ hρ2gasi/hρgasi2 ∼ 1.3 at r = r200, ture formation model (Luekeretal., 2010;
and it increases with radius, reaching C ∼ 5 Shirokoffetal.,2010;Dunkleyetal.,2010).
at r = 2r200. In the bottom panel, the solid Inourrecentwork,wearguedthatthecur-
line indicates the true entropy profile, which rent SZ power spectrum template is overesti-
is consistent with the self-similar prediction, mated by 50-100%, due to lack of important
K ≡ T/n2/3 ∝ r1.1 (Voitetal., 2005). From astrophysical process in theoretical modeling
e
the definition of entropy, the overestimate of oftheSZpowerspectrum(Shawetal.,2010).
gas density due to clumping causes an under- Fig. 3 illustrates theoretical uncertainties in
estimate of the observed entropy profile by the thermal SZ power spectrum. Our model
4 D.Nagai:SimulatingtheClusterOutskirts
accounts for star formation and energy feed- clusterswithdeepimagingdataextendingout
back(fromsupernovaeandactivegalacticnu- to large radii. Theoretical modeling based on
clei)aswellasradiallydependentnon-thermal detailed numerical simulations is also under-
pressure support due to random gas motions, way. A plethora of activities (in both theory
the latter calibrated by recent hydrodynami- andobservation)willadvanceourunderstand-
calsimulations.Varyingthelevelsoffeedback ing of cluster physics and provide foundation
and non-thermal pressure support can signif- fortheuseofgalaxyclustersaslaboratoriesfor
icantly change both the amplitude and shape cosmologyandastrophysics.
of thethermalSZpowerspectrum.Increasing
the feedbacksuppressespowerat small angu- Acknowledgements. I would like to thank my col-
larscales,shiftingthepeakofthepowerspec- laborators Andrey Kravtsov, Erwin Lau, Laurie
Shawforrewardingcollaborationswhichproduced
trum to lower l. On the other hand, increas-
resultsdescribedhere.Thisresearchwascarriedout
ing the non-thermal pressure support signifi-
attheYaleUniversityandwassupportedinpartby
cantly reduces power at large angular scales.
NSFundergrantAST-1009811.
In general, including non-thermal pressure at
the level measured in simulations has a large
effectonthepowerspectrum,reducingtheam- References
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