AGN Winds and the Black-Hole - Galaxy Connection

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by  Kastytis Zubovas

**VolumeTitle**
ASPConferenceSeries,Vol.**VolumeNumber**
**Author**
(cid:13)c**CopyrightYear**AstronomicalSocietyofthePacific
AGNWinds andthe Black-Hole-GalaxyConnection
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1 KastytisZubovas1,AndrewR.King1
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2 1 Dept. ofPhysics&Astronomy,University ofLeicester, Leicester,LE17RH,

n UK;mailto:[email protected]
a
J
Abstract.
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During the last decade, wide–angle powerful outflows from AGN, both on par-
1
sec and kpc scales, have been detected in many galaxies. These outflows are widely

suspectedtoberesponsibleforsweepinggalaxiesclearoftheirgas.Wepresentthean-
]
A alyticalmodeldescribingthepropagationofsuchoutflowsandcalculatetheirobserv-
G able properties. Large–scale AGN–driven outflows should have kinetic luminosities
∼ ηL /2 ∼ 0.05L andmomentumrates∼ 20L /c,where L istheEddington
h. luminEoddsityoftheceEndtdralblackholeandη ∼ 0.1itsErdaddiativeaccreEtidodnefficiency. This
p createsan expandingtwo–phasemediuminwhichmolecularspeciescoexistwith hot
- gas, which can persist after the central AGN has switched off. This picture predicts
o
outflowvelocities∼1000−1500kms−1andmassoutflowratesupto4000M yr−1on
r ⊙
t kpc scales, fixedmainly by the hostgalaxyvelocitydispersion(or equivalentlyblack
s
holemass).Wecompareourpredictionwithrecentobservationaldata,findingexcellent
a
[ agreement,andsuggestfutureobservationaltestsofthispicture.

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v
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1. Introduction
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3
. Recently, two sets of observations have allowed us to gain a better understanding of
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theinteraction betweenAGNsandtheirhostgalaxies. Theseareobservations ofhigh–
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2 velocitywide–anglewindsemanatingfromthevicinityoftheSMBH,whichhavebeen
1 detected in a large fraction of AGNs (Tombesietal. 2010a,b); and detection of kpc–
: scale quasi–spherical outflows in active galaxies, with enough power and mass flow
v
i tosweeptheirhostgalaxies clearofgas(Feruglioetal.2010;Rupke&Veilleux2011;
X Sturmetal. 2011; Riffel&Storchi-Bergmann 2011a,b). These outflows have kinetic
r powerequaltoafewpercentoftheEddingtonluminosity ofthecentralblackholeand
a
theirmomentumflowrateisapproximately anorderofmagnitude greaterthan L /c.
Edd
In this paper, we show how the two types of flows can be explained within the
frameworkofAGNwindfeedback. RadiationpressurefromanaccretingSMBHexpels
gasinformofawindfromthenucleus(e.g.Poundsetal.2003a,b),whichthenpushes
theambientgasinthehostgalaxyandproduces anoutflow. Inrecentwork(Kingetal.
2011;Zubovas&King2012)wehaveshownthatlarge–scaleenergy–driven flows(see
Section 3) can indeed drive much of the interstellar gas out of a galaxy bulge on a
dynamical timescale ∼ 108 yr, leaving it red and dead. The remaining mass of the
bulge is then similar to the value set by the observed black–hole – bulge–mass rela-
tion (e.g. Ha¨ring&Rix 2004). The observable features of such outflows – velocities,
kinetic powersand massandmomentum flowrates –areconsistent withobservations.
ThereforeAGNoutflowsappearcapableofsweepinggalaxiesclearofgas.
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2 KastytisZubovas,AndrewR.King
2. ClosetotheSMBH–winds
RadiationpressurefromanAGNaccretingatclosetoitsEddingtonlimitcanexpelgas
fromthevicinity ofthenucleus withamomentumrate
L
M˙ v = Edd, (1)
w w
c
as the wind on average has scattering optical depth ∼ 1 and absorbs all of the radi-
ation momentum. This creates a mildly relativistic diffuse wind (M˙ ∼ M˙ and
w Edd
v ∼ ηc ∼ 0.1c, where η ≃ 0.1 is the accretion radiative efficiency King 2003;
w
King&Pounds 2003). Observations of blueshifted X–ray iron absorption lines cor-
responding to velocities ∼ 0.1c (e.g. Poundsetal. 2003a,b; Tombesietal. 2010a,b)
reveal that the majority of quasars produce such winds. The winds have momentum
andenergyrates
L 1
P˙ ∼ Edd; E˙ = M˙ v2 ∼ 0.05L . (2)
w c w 2 w w Edd
3. Outinthegalaxy–outflows
It is clear that the wind has enough kinetic power to drive the observed large scale
outflow, provided that it can efficiently transfer this power to the ISM. In order for
this to happen, two conditions must be satified. First, most of the sightlines from the
SMBHmustbecoveredwithdiffusemedium. Second,thewindcannotcoolefficiently.
As the wind hits the ISM, it shocks and heats to T ∼ 1011 K. At this temperature,
the most efficient cooling process is inverse Compton scattering of the photons in the
AGNradiationfield(Ciotti&Ostriker1997). Theefficiencyofthisprocessdropswith
increasing shock radius, thus the cooling timescale increases as R2. Since the outflow
velocity does not depend strongly on radius (King 2010; Kingetal. 2011), the flow
timescaleonlyincreasesasR. Therefore,thereisacriticalradius,R ∼ 1kpc,within
cool
whichtheshockcanbecooledefficiently,whereasoutsidemostofitsenergyisretained
andtransferredtotheoutflow. Thetwocasesarecalledmomentum–drivenandenergy–
drivenflows,respectively; theirsalientfeaturesareshownschematically inFigure1.
3.1. Momentum–drivenoutflow
Anefficientlycooledshockedwindgasiscompressedtohighdensityandradiatesaway
almostallofitsoriginalkineticenergy, retainingandcommunicating onlyitspressure,
whichisequaltothepre–shock rampressure P˙ ≃ L /c ∝ M,tothehostISM.
w Edd
For an isothermal ISM density distribution with velocity dispersion σ and gas
fraction f (the ratio of gas density to background potential density) the behaviour of
c
theflowdepends ontheblackholemass M (King2003,2010). For M < M ,where
σ
f κ
M = c σ4 ≃ 4×108M σ4 , (3)
σ πG2 ⊙ 200
with f = 0.16 and σ = σ/(200 kms−1), the wind momentum is too weak to drive
c 200
awaytheswept–upISM,andtheflowstalls. ForM > M thewinddrivestheswept–up
σ
ISM far from the nucleus, quenching its owngas supply and further accretion. There-
fore, M represents an approximate upper limit to the SMBH mass distribution (see
σ
Poweretal. 2011, for more details). The calculated mass is very similar to that ob-
tainedfromobservations ofthe M−σrelation, despite havingnofreeparameter.
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Figure1. SchematicpictureofAGNoutflows. A windwith v ∼ 0.1c)impacts
w
theISMofthehostgalaxy,producingashockoneithersideofthecontactdisconti-
nuity. Within∼1kpcofthenucleus(top),theshockscoolrapidlyandradiateaway
mostoftheirenergy,leadingtooutflowkineticenergy∼ (σ/c)L . Inanenergy–
Edd
drivenoutflow(bottom),the shockedregionsexpandadiabatically,communicating
mostofthekineticenergyofthewindtotheoutflow,whichisthenabletosweepthe
galaxyclearofgas.
3.2. Energy–driven outflow
Alarge–scale (& 1kpc)outflowbecomesenergydriven. Itisessentially adiabatic, and
has the wind energy rate, i.e. E˙ ≃ E˙ ∼ 0.05L (from Equation 2). The hot bub-
out w Edd
ble’sthermalexpansion makesthedrivingintothehostISMmorevigorous thaninthe
momentum–driven case. Observed galaxy–wide molecular outflows must be energy–
driven, as demonstrated directly by their kinetic energy content (cf. Equation 2). The
adiabatic expansion oftheshocked windpushes theswept–up interstellar mediumina
‘snowplow’. In Kingetal. (2011) wederive the analytic solution for the expansion of
the shocked wind in a galaxy bulge with an isothermal mass distribution. With AGN
luminosity lL ,allsuchsolutions tendtoanattractor
Edd
1/3
2ηlf
R˙ = v ≃ cσ2c ≃ 925σ2/3(lf /f )1/3 kms−1 (4)
e " 3f # 200 c g
g
4 KastytisZubovas,AndrewR.King
untiltheAGNswitchesoffwhentheshockisatsomeradiusR = R . Subsequently, the
0
expansion speeddecayswith x = R/R ≥ 1as
0
10 1 2 10
R˙2 = 3 v2+ σ2 − − σ2. (5)
(cid:18) e 3 (cid:19)(cid:18)x2 3x3(cid:19) 3
In Eq. (4), the current gas fraction f may be lower than f (cf. Eq. 3). The outflow
g c
persists for an order of magnitude longer than the duration of the quasar outburst that
is driving it, and reaches radii of 104 − 105 pc. It is evident that energy–driven out-
flowsarecapableofsweepinggasoutofgalaxies,quenchingfurtherstarformationand
establishing theSMBH–bulgemassrelationship (Poweretal.2011).
3.3. Observableoutflowparameters
The solutions (4, 5) describe the motion of the contact discontinuity see Figure 1).
Outflows are usually observed in molecular gas, which is embedded in the outflowing
shell(seeZubovas&King2012,formoredetails), whichmoveswithvelocity
γ+1 lf 1/3
v = R˙ ≃ 1230σ2/3 c kms−1 (6)
out 2 200 f !
g
fromadiabatic shockconditions, usingγ = 5/3,andthemassoutflowrateis
dM(R ) (γ+1)f σ2 η(γ+1) f R˙c
M˙ = out = g R˙ = g M˙ , (7)
out dt G 4 f σ2 Edd
c
assuming M = M . If the AGNluminosity is still close to Eddington and f = f , the
σ g c
massloadingfactor(f ≡ M˙ /M˙ )andmassoutflowratesare
L out Edd
2ηc 4/3 f 2/3 l1/3 l1/3
f = g ≃ 460σ−4/3 ; M˙ ≃ 3700σ8/3l1/3 M yr−1. (8)
L 3σ! f ! m˙ 200 m˙ out 200 ⊙
c
Ifthecentralquasarisnolongeractive, M˙ islowerbyR˙/v ,withR˙ givenbyEq. (5).
out e
One can show from Equations (6) and (8) that M˙ v2 /2 ≃ 0.05L , i.e. most
out out Edd
of the wind kinetic energy istransferred to the outflow, asexpected for energy driving
(more precisely, while the quasar is active, the outflow contains 2/3rds of the total
energy). WecanalsoderiveanexpressionforthemomentumflowrateP˙ intheoutflow:
L L
P˙ = Edd f1/2 ∼ 20σ−2/3l1/6 Edd. (9)
out c L 200 c
4. Discussion
Weseethatinprinciple,large–scalewide–angleoutflowsdrivenbyamildlyrelativistic
wind launched by the AGN radiation pressure can sweep galaxies clear of gas. The
observableproperties ofsuchoutflowsaretypicalvelocitiesv ∼ 1000−1500kms−1
out
and mass flow rates up to M˙ ∼ 4000 M yr−1 (Equations (6) and(8)). The outflows
out ⊙
should have mechanical luminosities E˙ ∼ (η/2)L ∼ 0.05L , but (scalar) mo-
out Edd Edd
mentumrates P˙ ∼ 20L /c,consistent withobservations (seeTable1).
out Edd
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Table1. Outflowparameters:observationversuspredictionforasampleofAGN
Object M˙ v E˙out M˙outvoutc f M˙ v f
out out 0.05Lbol Lbol L pred. pred. L,pred.
Mrk231(a) 420 1100 0.66 18 490 880 810 840
Mrk231(b) 700 750 0.51 20 820 880 810 840
Mrk231(c) 1200 1200 1.0 25 1400 1150 1060 1110
IRAS08572+3915(c) 970 1260 2.1 50 1200 950 875 910
IRAS13120–5453(c) 130 860 0.88 31 1080 220 610 1870
Firsttwocolumns: observedmassflowrate(inM yr−1)andvelocity(inkms−1)oflarge–scale
⊙
outflows in molecular (Mrk231, IRAS 08572+3915 and IRAS 13120–5453) and warm ionised
gas(Mrk1157). Middlethreecolumns: quantitiesderivedfromobservations. Lastthreecolumns:
massflowrate,velocityandmassloadingfactorderivedfromourequations(6)and(8).Allderived
quantitiesshowgoodagreementwiththoseobservedandwitheachother.
References:a-Rupke&Veilleux(2011);b-Feruglioetal.(2010);c-Sturmetal.(2011).
Such outflows leave several observable signatures. Cold gas clumps entrained
within the shell produce the observed molecular emission. The inner wind shock ac-
celerates cosmic ray particles, which can emit synchrotron radiation in the radio band
and produce gamma rays when interacting with the ISM. These signatures resemble
those of the gamma–ray emitting bubbles in our Galaxy recently discovered by Fermi
(Suetal.2010),whichcanbeexplainedasrelicsofashortquasaroutburstabout6Myr
ago(Zubovasetal.2011,alsothecontribution byZubovastothisvolume).
Acknowledgments. We thank the conference organizers for their hospitality. Re-
search in theoretical astrophysics atLeicester is supported byan STFCRolling Grant.
KZissupported byanSTFCresearchstudentship.
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