Can AGN feedback break the self-similarity of galaxies, groups, and clusters?

0.31 MB·

by  M. Gaspari

Draftversion January22,2014
PreprinttypesetusingLATEXstyleemulateapjv.04/17/13
CAN AGN FEEDBACK BREAK THE SELF-SIMILARITY OF GALAXIES, GROUPS, AND CLUSTERS?
M. Gaspari1,2,5, F. Brighenti2,3, P. Temi4, S. Ettori5,6
1MaxPlanckInstitute forAstrophysics,Karl-Schwarzschild-Strasse1,85741Garching,Germany;[email protected]
2AstronomyDepartment, UniversityofBologna,ViaRanzani 1,40127Bologna,Italy
3UCO/LickObservatory,DepartmentofAstronomyandAstrophysics,UniversityofCalifornia,SantaCruz,CA95064, USA
4AstrophysicsBranch,NASA/AmesResearchCenter,MS245-6,MoffettField,CA94035
5INAF,OsservatorioAstronomicodiBologna,viaRanzani 1,40127Bologna,Italy
4 6INFN,SezionediBologna,vialeBertiPichat6/2,40127Bologna,Italy
1 Draft version January 22, 2014
0
2 ABSTRACT

n It is commonly thought that AGN feedback can break the self-similar scaling relations of galaxies,
a groups, and clusters. Using high-resolution 3D hydrodynamic simulations, we isolate the impact
J of AGN feedback on the L − T relation, testing the two archetypal and common regimes, self-
x x
0 regulated mechanical feedback and a quasar thermal blast. We find that AGN feedback has severe
2 difficulty in breaking the relation in a consistent way. The similarity breaking is directly linked to
the gas evacuation within R , while the central cooling times are inversely proportionalto the core
500
] density. Breaking self-similarity implies thus breaking the cool core, morphingallsystemstonon-cool-
O
core objects, which is in clear contradiction with the observed data populated by several cool-core
C systems. Self-regulated feedback, which quenches cooling flows and preserves cool cores, prevents
. the dramatic evacuationand similarity breakingat any scale; the relationscatter is also limited. The
ph impulsivethermalblastcanbreakthecore-includedLx−TxatT500 <∼1keV,butsubstantiallyempties
and overheats the halo, generating a perennial non-cool-core group, as experienced by cosmological
-
o simulations. Even with partial evacuation, massive systems remain overheated. We show the action
r of purely AGN feedback is to lower the luminosity and heating the gas, perpendicular to the fit.
t
s Keywords: galaxies: active — galaxies: clusters: intracluster medium — galaxies: groups: general —
a galaxies: jets — hydrodynamics — methods: numerical
[

1
1. INTRODUCTION ening in the group regime, ∼4-5 (Mulchaey et al. 2003;
v
Osmond & Ponman 2004; Helsdon & Ponman 2000a,b;
7 In the last decade, feedback due to active galactic nuclei
Sun et al. 2009; Sun 2012; see Fig. 1).
6 (AGN) has allowed to solve crucial astrophysical prob-
In recent years, different authors have studied the
0 lems. The supermassive black hole (SMBH) at the cen-
scaling relations by means of large cosmological sim-
5 ter of galaxies,groups, and clusters can indeed release a
. terrific amount of energy (>1061 erg), providing an effi- ulations with AGN feedback (e.g., Sijacki et al. 2007;
1
Fabjan et al. 2010; McCarthy et al. 2010; Short et al.
cient source to quench cooling flows and star formation
0
2010). In general, they find that the implemented AGN
4 (McNamara & Nulsen 2007). In particular, mechanical
feedback is able to break the self-similarity, lowering lu-
1 AGNfeedbackintheformofjets/outflowsisabletoreg-
minosities by orders of magnitude and, surprisingly, de-
: ulate for several Gyr the thermodynamical state of the
v creasing the global temperature (Puchwein et al. 2008,
systemcore(Gaspari et al.2013aforareview). However,
i fig.2). Oftenoverlooked,thesimulatedsystemsarehow-
X it is far from clear if AGN feedback is able to strongly
ever non-cool-core objects (Planelles et al. 2013, fig. 7,
modify the large-scale gas halo, as the total X-ray lu-
r foracriticaldiscussion). Evenno-feedbackrunsproduce
a minosity and temperature, in other words, breaking the
negativetemperaturegradients,duetoextremeadiabatic
self-similar scaling relations.
heating,whicharenotpresentinhigh-resolutionsimula-
If gravity were the single driver of the evolution
tions(e.g.,Li & Bryan2012). Besidestheunder-resolved
(Kaiser 1986; Kravtsov& Borgani 2012), all systems
black hole/feedback physics and subgrid numerics (see
would scale only with mass, M = (4π/3)∆ρ R3,
∆ c ∆ the analysis in Barai et al. 2014), it remains difficult to
where ∆ is the chosen overdensity. The critical den-
disentangleandisolatetheactionoffeedbackinthecom-
sity of the universe evolves in redshift as ρ (z) ∝
c plex evolutionshaped by mergers,filaments, star forma-
E2(z), where E2(z) ≃ Ω (1 + z)3 + Ω , giving a
m Λ tion, sink particles, and other prescriptions.
characteristic radius R∆ ∝ M∆1/3E−2/3(z). Via hy- The objective of this study is to critically examine
drostatic equilibrium (M∆ ∝ T R∆), we can retrieve if AGN feedback itself can break the self-similarity of
M ∝ T3/2E−1(z). Since the bolometric X-ray lumi- galaxies, groups, and clusters, in a way consistent with
∆
nosityscalesasL ∝n2T1/2R3 (inthe Bremsstrahlung observations. Via controlled high-resolution 3D (mesh)
x x ∆ simulations, we study how the main scaling, L −T is
regime), using gas number density n ∝ M /R3 ∝ x x
∆ ∆ shaped by the two archetypal and commonly adopted
ρ (z) and the above relations, we find the well-known
c feedback models, self-regulated kinetic feedback and a
self-similar scaling L ∝ T2E(z). However, cluster
x x quasarthermalblast. Inaforthcomingwork,weexplore
observations show a slope steeper than 2 (∼3; e.g.,
othermodelsanddifferentrelations(Gasparietal. 2014,
Pratt et al. 2009; Maughan et al. 2012), further sharp-
2 M. Gaspari et al.
in prep.). The normalization of the density profile is set by the
Acrucialconstraintdrivenbyobservationsisthepres- gas fraction at the virial radius, f ≃ 0.15. The
gas,vir
ence of a (strong or weak) cool core in the majority initial gas fraction is intentionally high, near the cosmic
of observed systems (>∼ 65 per cent, Peres et al. 1998; value (Planck Collaboration et al. 2013), since we want
Mittal et al. 2009; Sun et al. 2009; Hudson et al. 2010; to testif AGN feedbackis the originalcauseof gasevac-
Zhao et al. 2013). Such systems show, in the core, uation and hence self-similarity breaking. Using lower
cooling times < 7 Gyr, positive temperature gradients, values (e.g., 0.1) does not change the conclusions.
and low gas entropy (<10s keV cm2). Moreover, cool
cores appear to be long lived and in place since z > 1 2.2. Hydrodynamics, cooling and heating
(McDonald et al. 2013). AGN feedback, or inside-out Using FLASH4 code, we integrate the 3D equations
heating, intrinsically evacuates the central regions, be- of hydrodynamics in conservative form, including to-
fore touching the periphery of the system. Although tal gravity, gas radiative cooling, and feedback heat-
the AGN feedback energetics is in principle capable ing. The latter two source terms are implemented fol-
to breaking the group self-similarity (Cavaliere & Lapi lowing the unified self-regulation model, as described in
2008;Giodini et al.2010),theenergydepositionandhy- Gaspari et al. (2012b). Transport mechanisms, as con-
drodynamics is crucial. We show that breaking self- duction,arenotincludedsincedatasuggestastrongsup-
similarity via AGN feedback implies disrupting the cool pression (e.g., Gaspari & Churazov 2013). The cubical
core, morphing the system into perennial non-cool-core box fully coversthe virial radius, ∼1.2-4.6Mpc (groups
objects;viceversa,self-regulationpreservesthecoreand to clusters). We use concentric grid levels with radius
the large-scale structure. of ∼60 cells, centered on the BCG, where the maximum
resolution reaches ≈ 290 pc. The system is integrated
2. PHYSICS&NUMERICS for at least 5 Gyr. Boundary conditions are set in diode
2.1. Initial conditions mode.
Inordertofullyisolatetheroleoffeedbackinalteringthe 2.2.1. Self-regulated mechanical feedback
scalingrelations,westartwithavirializedgroup/cluster
In Gaspari et al. (2011a,b, 2012a,b, 2013a,b) was found
havingaformedcoolcore,whichcharacterizesthemajor-
ity of observed systems (§1). Groups and clusters share that the most consistent model able to solve the cool-
ing flow problem is mechanical feedback, self-regulated
many common properties, allowing to build an initial
bycoldaccretion. Inturbulentregionswherethecooling
‘universal’ system defined only by its mass. Following
Vikhlinin et al. (2006)1, the observed average tempera- time dropsbelow∼10×the free-falltime, thermalinsta-
bilities become quickly nonlinear,leadingto the conden-
ture profile can be modeled as
sationofcoldgas outof the hot phase. Such coldclouds
0.45+(rˆ/0.045)1.9 1 andfilamentscollideinaninelasticandchaoticwaywhile
T(r)=T0 1+(rˆ/0.045)1.9 (1+(rˆ/0.6)2)0.45, (1) rainingonto the blackhole, boostingthe accretionrate.
Bipolar massive sub-relativistic outflows are then trig-
where rˆ= r/R ; the normalization is T ≃ 1.4 T ≃ geredwithkineticpowerproportionaltothecentralcool-
500 0 500
3 keV(M500/1014M⊙)0.6 (cf., Sun et al. 2009). Eq. 1 ing rate, Pjet = ǫM˙coolc2 (Gaspari et al. 2012b for the
models the positive gradient of the cool core and the numerical details), with optimal mechanical efficiencies
gentle decrease at large r; the peak temperature (r ∼ ǫ ∼ 5 × 10−4 − 5 × 10−3. The self-regulated outflow
0.15R ) is ∼2× the central value, which is reached generates the cocoon shock, two buoyant bubbles, and
500
again at r ∼ R . Albeit some groups have slightly gas/metal uplift. The kinetic feedback rises the central
500
higher T peak and steeper decrease (Sun et al. 2009), gas entropy, quenching cooling and stifling the accretion
such minor differences have no impact on the results. rate; the self-regulated loop starts then over again.
The system is initially in hydrostatic equilibrium The gentle self-regulation with either kinetic or ther-
within the gravitationalpotential φ, dominated by dark mal injection (the latter commonly used in cosmologi-
matter,modeledviatheusualNFWprofileintheconcor- calsimulations2)producesanalogousimpactonthescal-
dance ΛCDM universe. The halo concentrationis linked ing relations, although thermal feedback induces again
to the virial mass as c ≃ 8.5(Mvir/1014 M⊙)−0.1 (e.g., excessive core overheating (Brighenti & Mathews 2003;
Bullock et al. 2001). In addition, each group/cluster is Gaspari et al. 2011b).
dominatedbyacentralmassiveellipticalgalaxy(‘BCG’),
modeled with a de Vaucouleurs stellar density profile. 2.2.2. Quasar thermal blast
The BCG K-band luminosity increases with the halo In the opposite spectrum of feedback models resides the
mass as LK ≃ 4.7 ×1011(Mvir/1014 M⊙)0.39L⊙ (e.g., sudden and powerful release of thermal energy. This
Lin & Mohr 2004). The stellar mass is then retrieved can be justified by a quasar event, emitting large ra-
adoptingM∗/LK ∼1(e.g.,Mannucci et al.2005). Since diative power absorbed by highly dense clouds, or in
BCGs are large ellipticals, we keep the effective radius alternative, by an Eddington wind fully thermalizing
Reff ≃9kpc. AsdescribedinGaspari et al.(2012b),the in the inner core. Numerically, the thermal energy is
BCG injects a low amount of energy and mass due to injected in the inner ∼4 kpc, with Eddington power
SNIa and stellar winds; however, the energetics is domi- PEdd ≃1.5×1047(Mbh/109M⊙)ergs−1 lasting∼6Myr.
nated by the AGN feedback. ThetotalreleasedenergyisE ≡ηM c2 ≃3×1061
AGN bh
1 Note a typo in the published version, missing the 0.45 expo- 2Insomecosmologicalworks,the‘quasarmode’issimplyquasi-
nent;A.Vikhlinin,privatecommunication(seeastro-phversion). continuous thermalfeedback.
Breaking galaxies, groups, and clusters with AGN feedback 3
Figure 1. X-ray bolometric luminosity versus X-ray temperature, including (top) or excising (middle) the core, r < 0.15R500. The
bottom panels show the central coolingtime(∼15-20 kpc), withthecool-core thresholdtcool∼7Gyr. Left: self-regulatedkineticmodels
(§2.2.1;ǫ=5×10−3). Right: quasarthermalblast(§2.2.2);theblackpointsshowthesimulated5Gyrevolutionevery500Myr(theinitial
point has magenta contour). The observational data are from Maughanetal. (2012, Chandra; red), Prattetal. (2009, XMM; green),
Sunetal. (2009,Chandra;blue),Mulchaeyetal.(2003)andOsmond&Ponman(2004;magenta),Helsdon&Ponman(2000a,b,ROSAT;
cyan); wealways useh=0.7(e.g., forLx ∝h−2). Theself-regulatedAGN feedback prevents overheating, at thesametimeavoidingthe
self-similaritybreaking. Conversely,thepowerfulthermalblastcanbreaktheLx−Txrelationatthegroupscale,butmorphingthesystem
intoaperennialnon-cool-coreobject.
4 M. Gaspari et al.
erg, i.e., the characteristic energy of a SMBH with typi- points towards lower luminosities and higher tempera-
calMbh ∼109M⊙ andradiativeefficiencyη ≃1.5×10−2 tures. Similarly, radiative cooling moves the system to-
(Novak 2013). The isotropic blast triggers once the nec- wards higher L and lower T (cf., Ettori & Brighenti
x 500
essary mass of cold gas has been accreted; due to the 2008). In other words, the L −T secularly moves due
x x
powerful heating, no second event ever occurs. Conclu- to heating/cooling perpendicular to the fit, particularly
sionsareunalteredwithdifferentE andcompactde- as the core is included. Works based on cosmological
AGN
position windows. simulations (§1) show instead a decrease of T , adding
500
Boosting ǫ above the optimal values (e.g., ∼ 0.1) subgrid AGN feedback. This could be linked to the re-
transformsthe previousgentle self-regulatedfeedback in duction of the extreme adiabatic heating present in the
the impulsive blast. Conversely, significantly lowering under-resolved pure cooling flow. A more serious prob-
E morphs the feedback in the self-regulated regime. lem is that, although the similarity breaking occurs, no
AGN
The presented models constitute thus the two opposing object can be observationally described as a cool core
archetypes of inside-out feedback. with the positive T gradient depicted in §2.1.
Theself-regulatedmodelsshowthatavoidingthecom-
3. RESULTS pleteself-similaritybreakingimpliespreservingthe cool-
Figure 1 presents the key results of the high-resolution core structure – at the same time reducing the cool-
hydrodynamic simulations, testing kinetic or thermal ing rate below 10 per cent of that of the pure cooling
AGN feedback in the range of systems with T ≃0.5- flow. In the bottom panel (left), the central cooling
500
6 keV (Mvir ∼ 1013-1015 M⊙). In the top and mid- timestaysinanyhalobelow∼7Gyr,thecommonupper
dle panels, we show the X-ray luminosity versus X-ray limitusedtodefinecool-coresystems(e.g.,Hudson et al.
temperature3 within R , including or excisingthe core 2010). These systems also preserve the positive tem-
500
(r < 0.15R ), respectively. The bottom panel depicts perature gradient and low central entropy. As found in
500
the gas central cooling time (in the shell ∼15-20 kpc, Gaspari et al. (2011b, 2012b), ǫ ≃ 5×10−3 is the best
contained within <∼ 0.06R500). The Lx − Tx relation value for clusters; indeed, the two groups switch to a
is shaped by the global amount of cooling and heating, weaker cool core after the initial heating. Overall, op-
while t ∝T/nΛ assesses the core thermal state (Λ is timal self-regulation induces the system to oscillate be-
cool
the cooling function; see Gaspari et al. 2012b). tween a state of weak and strong cool core, preventing
The self-similar relation is expected to be L ∝ T2, both the cooling and heating runaway. The duty cycle
x x
even shallower in the group regime due to line emission is very efficient with cold accretion, while much weaker
(L ∝ T3/2Λ ∝ T ). Figure 1 (top) reveals that the ob- with hot Bondi regulation (Gaspari et al. 2011b). Since
x x x most of the observed systems host a cool core (§1), self-
servationaldatarelativetotheclusterregime(red,green)
regulated mechanical heating represents the long-term
are already deviating, with a slope α∼3. Below 2 keV,
maintenance mode of AGN feedback, while avoiding the
i.e., for small and massive groups, the relation steepens
break of the scaling relations.
further,reachingα∼ 4-5,with amuchmoresignificant
Wetestnowthe otherextremeofAGNfeedbackmod-
scatter. Excising the core in both quantities reduces the
els, i.e., the impulsive thermal blast (Fig. 1, right). The
scatter and the steepness (α ∼ 2.5), avoiding an abrupt
suddenisotropicenergyrelease(∼3×1061 erg,compara-
decline. The relation becomes nearly self-similar consid-
bletothatofatypicalSMBH)candramaticallyevacuate
ering only cool-coreclusters (Maughan et al.2012). Un-
the atmosphere of the compactgroup,decreasing the X-
fortunately, the excised relation for small groups is not
ray luminosity by 2.5 orders of magnitude; T initially
covered by observational data. 500
increases by ∼2× (the rightmost circle). The strong
In the left column, the simulations (black; the ini-
breakingoccursbecause the grouptotalbinding energy4
tial state has magenta contour) show that the impact
of self-regulated jet feedback (ǫ = 5 × 10−3) on the is ≃ 1061 erg. After the initial blast, the BCG is slowly
replenished by the stellar mass loss, mildly increasing
L − T relation is limited. The remarkable aspect is
x x the luminosity (<R ) to ∼ 1041 erg s−1, while restor-
that no break – a deviation of orders of magnitude – 500
ing T near the initial value (this is not due to the ac-
occurs, even at the scales of small halos. In the mas- 500
tionofAGNfeedback). Thesystemhascompletelymor-
sivegroup(pentagons),the maximumdeviationinlumi-
phed. The central t (bottom) is always >60 Gyr, in
nosity/temperature is ∼30/10 per cent (0.1/0.05 dex), cool
a perennial non-cool-core state. Excising the core (mid-
strongly diminishing to ∼10/1 per cent in massive clus-
dle), aggravates the breaking, since the compact group
ters. In the compact group (circles), the luminosity de-
is substantially devoid of gas outside the BCG, with an
creases by ∼2.8×, while temperature increases by ∼80
unrealistic drop down to ∼1040 erg s−1 (cf., Sun 2012).
per cent. T showsthe weakerscatter;the mainaction
500 Overall, the inside-out heating able to fully break self-
of feedback, especially kinetic, is to evacuate gas and,
similarity in the core-included L −T , violently alters
secondarily, to heat the global atmosphere. x x
the excised L −T relation, which is instead observed
Excising the core within 0.15R (middle panel), sig- x x
nificantly reduces the scatter to5<∼001/3 of the previous to have tighter scatter.
In the massive group (pentagons), the total binding
values. By removing the core separately for each vari-
energy is ≃ 1062 erg (> E ). The thermal blast
able, we see that internal heating can only move the AGN
can thus only partially evacuate the gas from the core
3 Wecomputedboththeemission-weightedT500 withChandra (∼ 0.1R500), halted by the extended atmosphere before
sensitivity(Tx>∼0.3keV;Gasparietal.2012b)andspectroscopic- reaching R500. The result is a decrease in luminosity by
liketemperature(Vikhlinin2006);sinceourflowisnotmultiphase,
tthheeydaiffreerveenrcyesiwmitihlart.heAsspinheorbicsaerlvvaatliuoensi,swmeinuoser.theprojectedT500; 4 Equaltothegravitational energyEb=R0RvirρgasφdV.
Breaking galaxies, groups, and clusters with AGN feedback 5
maximum 0.9 dex and T oscillating within 0.1 dex. one order of magnitude at large radii implies decreas-
500
The extended evacuation is confirmed by the excised re- ing the core density n by at least 10× more, inducing
0
lation. Both simulated L −T are consistent with the t ≫ t . The problem is further aggravated by the
x x cool H
observed data. However, the similarity deviation occurs increaseoftemperature(t ∝T1/2). Thedirectaction
cool
againattheexpenseofthecoolcore. Infact,thecentral of AGN feedback is to lower the luminosity and heating
cooling time stays above tens Gyr, signaling a strong thegas,notmovingthesystemparalleltotheL −T fit.
x x
non-cool-core group. Analyzing the poor and massive Overall,AGNfeedbackappearsnaturallysuitedtoregu-
cluster simulations (squares, triangles), we see no sim- latethethermodynamicalstateofcosmicsystems,inthe
ilarity breaking, due to the larger Eb. The maximum core, but not over large radii (r >∼0.2R500). We remark
deviation in the core-included relation is highly limited, that any feedback mechanism, that is able to break the
∼0.17/0.08 dex in Lx and ∼0.03/0.015 in T500, for the self-similarity, needs to properly solve the cooling flow
poorandmassivecluster,respectively. Excisingthe core problem.
stifles the scatter by at least 1/3. The powerful AGN In a forthcoming work, we discuss other heating mod-
heating has again the side effect of destroying the core els, parameters, and scaling relations. We found, nev-
(tcool ≫ tH). Only the massive cluster can partially re- ertheless, that AGN feedback models fall in the two
cover after several Gyr. Cool cores are common in the archetypalcategoriespresentedhere: self-regulatedheat-
universe,hencethistypeofbreakingshouldberare. No- ing,preventingthebreaking,orstrongimpulsiveheating,
ticethatcombiningthe twofeedbackmechanismsaggra- which breaks the scaling relations but destroys the core.
vates the core overheating. For instance, using either thermal or kinetic feedback
The global Lx−Tx property seems overallmore likely with self-regulation has the same minor impact on the
linked to a primordial imprint that the group/cluster scaling relations (§2.2.1). Injecting energy in the center
experienced, rather than an internal breaking after for- or at a distance of a few 10 kpc (<∼ 0.05R500) has also
mation. However, we note that when the external ‘pre- thesameeffect,consideringthatthefeedbackmustaffect
heating’athighredshift–whoseagencyisstillunclear– extremely large regions (R , several 100s kpc); a too
500
is high enough to bring Lx consistent with observations, distant injection allows instead central runaway cooling.
thegasentropybecomesusuallytoohigh,inhibitingcool Further, boosting ǫ transforms the self-regulated feed-
corestoformabinitio(e.g.,Brighenti & Mathews2001). back into the quasar-like blast; vice versa, diminishing
the impulsive E slightly lowers t , but prevents
4. CONCLUSIONS AGN cool
the similarity breaking. In other words, even with a dif-
We showed that AGN feedback has severe difficulty in ferent parametrization of the archetypal AGN feedback
breaking the self-similarity of galaxies,groups,and clus- models,breakingself-similarityimpliesbreakingthecool
ters, in a consistentway. Via high-resolution3D simula- core.
tions,weisolatedtheimpactofthetwocommonregimes
ofAGNfeedbackontheprincipalscalingrelationLx−Tx. ACKNOWLEDGMENTS
• Self-regulated kinetic feedback prevents the simi- The FLASH code was in part developed by the DOE
larity breaking, inducing a limited scatter (<∼0.1 NNSA-ASC OASCR Flash center at the University
dex). Self-regulation allows to properly quench of Chicago. MG is grateful for the financial sup-
cooling flows preserving the cool-core structure; port provided by the Max Planck Fellowship. SE
avoiding overheating translates thus in a modest and FB acknowledge financial contribution from ASI-
central gas evacuation, maintaining low core INAF I/009/10/0, PRIN INAF 2012, PRIN MIUR
cooling times (t < 7 Gyr) and avoiding the 2010LY5N2T. High-performance computing resources
cool
L − T breaking at any halo scale. Since the wereprovidedbytheNASA/AmesHECProgram(SMD-
x x
majority of observed systems display a cool core 13-3935, SMD-13-4373, SMD-13-4377; Pleiades). We
(§1), this mode should represent the long-term thank M. Sun for providing the groups data, E. Chu-
maintenance phase of AGN feedback. razovandtheanonymousrefereeforinterestinginsights.
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