Prateek Sharma (IISc) Collaborators: Deovrat Prasad, Arif Babul

21 Pages·2016·4.41 MB·English

Heating & cooling cycles
in cool cluster cores
Prateek Sharma (IISc)
Collaborators: Deovrat Prasad, Arif Babul
The Astrophysical Journal, 811:108 (21pp), 2015 October 1 doi:10.1088/0004-637X/811/2/108
© 2015. The American Astronomical Society. All rights reserved.
COOL CORE CYCLES: COLD GAS AND AGN JET FEEDBACK IN CLUSTER CORES
The Astrophysical Journal, 811:108 (21pp), 2015 October 1 doi:10.1088/0004-637X/811/2/108
1 1 2
Deovrat Prasad , Prateek Sharma , and Arif Babul
© 2015. The American Astronomical Society. All rights reserved.
1
Joint Astronomy Program and Department of Physics, Indian Institute of Science, Bangalore, 560012, India;
[email protected], [email protected]
2
Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P 1A1, Canada; [email protected]
CORecOeivLed C201O5 RApEril 1C2;YacCcepLteEd 2S01:5CJuOly L28D; pubGlisAhedS20A15NSeDptemAbeGr 2N8 JET FEEDBACK IN CLUSTER CORES
ABSTRACT 1 1 2
Deovrat Prasad , Prateek Sharma , and Arif Babul
1
Using high-resolution 3D and 2DJoi(natxiAsystmromnoemtriyc)PhroygdrraomdyannadmDicepsiamrtmuleantitoonfs Pinhysspichse,riIcnadliagneoInmsetittruyt,ewofe Sstcuiednyceth, eBangalore, 560012, India;
evolution of cool cluster cores heated by feedback-drivendeboivproalta@r pahctyisviecsg.iailsacc.etircnentu.icnl,epi r(aAteGekN@s)pjheytss.icCs.oiinsdce.enrsnaetti.oinn
2
of cold gas, and the consequenDt eepnahrtamnceendt oafccPrheytisoicns, aisndreAqustirroendofmoyr,AUGnNivefreseitdyboafckVticotobraiala,nVciectroardiaia, tBivCe Vco8oPli1nAg 1, Canada; [email protected]
fi fi
with reasonable ef ciencies, and to match theRoecbesievrevded20c1o5olAcporirle1p2r;oapcecrteipetse.dA20fe1e5dJbualcyk2e8f; pcuiebnlicsyhe(dm2e0ch1a5nSiceapltember 28
luminosity » (cid:139)M˙ c2; where M˙ is the mass accretion rate at 1 kpc) as small as 6 × 10−5 is sufficient to reduce
acc acc
the cooling/accretion rate by ∼10 compared to a pure cooling flow in clusters (with M (cid:49) 7 ´ 1014 M ). This
ABSTRA2C00T (cid:58)
fi
value is much smaller compared to the ones considered earlier, and is consistent with the jet ef ciency and the fact
that only a smalUl fsriancgtiohnigohf -graessoatlu1tkiopnc i3sDaccarnedted2Dont(oatxhiesysumpmermetarsisci)vehbyldacrokdhyonleam(SiMcBsHim).uTlhaetiofenesdbianckspherical geometry, we study the
fi
ef ciency in eareliveor lwutoirokns wofascosoolhciglhusttheart cthoerecsluhsteeartecodrebyreafecheeddbaeqcuki-lidbrriivumeninbiapohloatrsatactteivweitghoaulatcmtiucchnuclei (AGNs) jets. Condensation
fi
precipitation, unlike what is observed in cool-core clusters. We nd hysteresis cycles in all our simulations with
of cold gas, and the consequent enhanced accretion, is required for AGN feedback to balance radiative cooling
cold mode feedback: condensation of cold gas when the ratio of the cooling-time to the free-fall time (t t ) is
with reasonable efficiencies, and to match the observed cool core propercotoiles.ff A feedback efficiency (mechanical
10 leads to a sudden enhancement in the accretion rate; a large accretion rate causes strong jets and overheating

luminosity » (cid:139)M˙ c2; where M˙ is the mass accretion rate at 1 kpc) as small as 6 × 10−5 is sufficient to reduce
of the hot intracluster medium suchactchat t t > 1a0c;c further condensation of cold gas is suppressed and the
cool ff
fl
accretion rate fatlhlse, cleoaodliinnggt/oacslcorweticoonolirnagteobf yth∼e 1co0recoamndpcaorenddentosataiopnuoref ccoolodlignags, reoswtartiinngcltuhestecyrscle(w. ith M (cid:49) 7 ´ 1014 M ). This
200 (cid:58)
fi
Therefore, therevisalauespirseamd uinchcosrme parloleprerctioesm, psuacrhedastothethjeetopnoewserc,oancscirdeteiorendraetea,rlfioerr,thaensdamise cvoanluseisotfencotrwe ith the jet ef ciency and the fact
fi
entropy or t t . A smaller number of cycles is observed for higher ef ciencies and for lower mass halos because
that only a small fraction of gas at 1 kpc is accreted onto the supermassive black hole (SMBH). The feedback
cool ff
the core is overheated to a longer cooling time. The 3D simulations show the formation of a few-kpc scale,
fi
ef ciency in earlier works was so high that the cluster core reached equilibrium in a hot state without much
rotationally supported, massive (~1011 M ) cold gas torus. Since the torus gas is not accreted onto the SMBH, it is
(cid:58) fi
precipitation, unlike what is observed in cool-core clusters. We nd hysteresis cycles in all our simulations with
4
largely decoupled from the feedback cycle. The radially dominant cold gas (T < 5 × 10 K; v > v ) consists of
∣ ∣ ∣ ∣
r f
cold mode feedback: condensation of cold gas when the ratio of the cooling-time to the free-fall time (t t ) is
fast cold gas uplifted by AGN jets and freely infalling cold gas condensing out of the core. The radially dominant
cool ff
cold gas extends ou1t0tole25adkspctoforathseudfidduecniael nruhna(nhcaelommeansts 7in´th1e01a4cMcreatinodnfereadtbea;cak elaffircgieenaccyc6re×ti1o0n−5r)a,tweicthauses strong jets and overheating

(cid:58)
fl fl
the average masos finthoewhroatte idnotmraincalutinstgerthemoeudtiuomw rsatuecbhy tahafatcttor of t≈2>. W1e0c;omfupratrheeorurcosinmduelnatsioantiorensuoltsf cold gas is suppressed and the
cool ff
with recent observations.
accretion rate falls, leading to slow cooling of the core and condensation of cold gas, restarting the cycle.
Key words: galaxies: clusters: intracluster medium – galaxies: halos – galaxies: jets
Therefore, there is a spread in core properties, such as the jet power, accretion rate, for the same value of core
fi
entropy or t t . A smaller number of cycles is observed for higher ef ciencies and for lower mass halos because
cool ff
the core is overheated to a longer cooling time. The 3D simulations show the formation of a few-kpc scale,
1. INTRODUCTION While there are potential heat sources, such as the kinetic
rotationally supported, massive (~1011 M ) cold gas torus. Since the torus gas is not accreted onto the SMBH, it is
energy of infalling galaxies and sub-halos (e.g., Dekel &
(cid:58)
The majority of baryons in galaxy clusters are in the form of
4
largely decoupled from the feedbackBciyrncbloei.mTh20e0r8a)d, ithaellrymadlocmonidnuacntitocnoflrdomgatshe(Tho<tter5o×uts1k0irtsK; v > v ) consists of
∣ ∣ ∣ ∣
r f
a hot plasma known as the intracluster medium (ICM). In the
fast cold gas uplifted by AGN jets an(ed.gf.,reVeloyigitn&falFlianbgianco2ld00g4a; sVcooitnd20e1n1s)i,nga ogluotbaolflythsetacbloere. The radially dominant
absence of cooling and heating, the ICM is expected to follow
cold gas extends out to 25 kpc for themfiedchuacniiasml r,uwnhi(chhailnocrmeaassess 7rap´idly10w1i4thMan ianncrdeafseinegdbhoatcgkaes fficiency 6 × 10−5), with
fi
self-similar pro les for density, temperature, etc., irrespective (cid:58)
fl density in the cflore, is required to prevent catastrophic cooling.
the average mass in ow rate dominating the out ow rate by a factor of ≈2. We compare our simulation results
of the halo mass (Kaiser 1986, 1991; see also the review by
Observations of several cool-core clusters by Chandra and
Voit 2005). However, sewlf-istihmirlearcietyntisonbosteorbvsaetrivoendsi.n either
XMM-Newton have uncovered active galactic nucleus (AGN)-
groups or clusters (e.g., Balogh et al. 1999; Ponman et al. 1999;
Key words: galaxies: clusters: intracjleut-sdtreirvemn Xed-riauymca–vitgieasl,awxhioesse: mheaclohasni–cagl aploawxeireiss:ejneotusgh to
Babul et al. 2002). Moreover, the core cooling times in about a
balance radiative cooling in the core (e.g., Böhringer
third of clusters are shorter than 1 Gyr, much shorter than their
et al. 2002; Bîrzan et al. 2004; McNamara & Nulsen 2007).
age (∼Hubble time; e.g., Cavagnolo et al. 2009; Pratt
The AGN jets are powered by the accretion of the cooling ICM
1. INTRODUCTION While there are potential heat sources, such as the kinetic
et al. 2009). Thus, we expect cooling to shape the distribution
onto the supermassive black hole (SMBH) at the center of the
energy of infalling galaxies and sub-halos (e.g., Dekel &
of baryons in these cool-core clusters.
The majority of baryons in galaxy clusters daoremiinnantht eclufsotremr gaolfaxy. Thus, more cooling/accretion leads to
The existence of cool cores with short cooling times in a Birnboim 2008), thermal conduction from the hotter outskirts
an enhanced jet power and ICM heating, closing a feedback
a hot plasma known as the intracluster medium (ICM). In the
good fraction of galaxy clusters is a long-standing puzzle.
(e.g., Voigt & Fabian 2004; Voit 2011), a globally stable
loop that prevents runaway cooling in the core.
absence of coolingfland heating, the ICM is expected to follow
According to the classical cooling ow model, cluster cores
AGN feedback has been lomnge-cshusapneicstmed,towphlaicyha irnolcerienasseelsf- rapidly with an increasing hot gas
fi
with such shsoerltf-sciomoliilnagr ptirmoesleswefroer dexepnescitteyd, tteompcoeorlature, etc., irrespective
regulating the ICM (e.g., Binney & Tabor 1995; Ciotti &
density in the core, is required to prevent catastrophic cooling.
catastrophically and to fuel star formation at a rate of
of the halo mass (Kaiser 1986, 1991; see also the review by
Ostriker 2001; Soker et al. 2001; Babul et al. 2002; McCarthy
100–1000 M yr−1 (e.g., Fabian 1994; Lewis et al. 2000). Observations of several cool-core clusters by Chandra and
(cid:58) Voit 2005). However, self-similarity is not oebt saelr.v2e0d08i)n, beuitthaerclear picture has emerged only recently.
However, cooling, dropout, and star formation at these high XMM-Newton have uncovered active galactic nucleus (AGN)-
While AGN feedback should provide feedback heating in
groups or clusters (e.g., Balogh et al. 1999; Ponman et al. 1999;
rates are never seen in cluster cores (e.g., Edge 2001; Peterson jet-driven X-ray cavities, whose mechanical power is enough to
cluster cores (as it is enhanced with ICM cooling), it is not
Babul et al. 2002). Moreover, the core cooling times in about a
et al. 2003; O’Dea et al. 2008). This means that some source(s)
obvious if, for reasonable pabraalmaentceres, AraGdNiahtievaetingccoaonliknegep in the core (e.g., Böhringer
of heating is(athrei)rdabolef ctolursetpelresniasrhetshheocroterer tchoaolnin1g Gloyssre,sm, uch shorter than their
pace with cooling that increases rapidly with an increasing core
et al. 2002; Bîrzan et al. 2004; McNamara & Nulsen 2007).
thereby prevenatigneg ru(n∼awHauybcboloelintgimaned; staer.gfo.,rmCatiaovna. gnolo et al. 2009; Pratt
density. Moreover, the dense core gas is expected to be highly
The AGN jets are powered by the accretion of the cooling ICM
et al. 2009). Thus, we expect cooling to shape the distribution
onto the supermassive black hole (SMBH) at the center of the
1
of baryons in these cool-core clusters.
dominant cluster galaxy. Thus, more cooling/accretion leads to
The existence of cool cores with short cooling times in a
an enhanced jet power and ICM heating, closing a feedback
good fraction of galaxy clusters is a long-standing puzzle.
loop that prevents runaway cooling in the core.
fl
According to the classical cooling ow model, cluster cores
AGN feedback has been long-suspected to play a role in self-
with such short cooling times were expected to cool
regulating the ICM (e.g., Binney & Tabor 1995; Ciotti &
catastrophically and to fuel star formation at a rate of
Ostriker 2001; Soker et al. 2001; Babul et al. 2002; McCarthy
−1
100–1000 M yr (e.g., Fabian 1994; Lewis et al. 2000).
(cid:58) et al. 2008), but a clear picture has emerged only recently.
However, cooling, dropout, and star formation at these high
While AGN feedback should provide feedback heating in
rates are never seen in cluster cores (e.g., Edge 2001; Peterson
cluster cores (as it is enhanced with ICM cooling), it is not
et al. 2003; O’Dea et al. 2008). This means that some source(s)
obvious if, for reasonable parameters, AGN heating can keep
of heating is(are) able to replenish the core cooling losses,
pace with cooling that increases rapidly with an increasing core
thereby preventing runaway cooling and star formation.
density. Moreover, the dense core gas is expected to be highly
1
Cold gas condensation
allows feedback to act sufficiently fast, unlike Bondi
•
t /t ~10 seems robust
•
cool ff
cooling & heating cycles
•
push ε to smallest allowed by observations
•
cold gas inflows & outflows
•
angular momentum: stochastic cold accretion
•
AGN jet-ICM sims.
@⇢
+ ⇢v = S
mass
⇢
@t r ·
@v
⇢ + v v = p ⇢ � + S v ˆr
momentum
⇢ jet
@t · r �r � r
✓ ◆
p d
� 2
ln(p/⇢ ) = n ⇤
� 1 dt �
�
source terms to mimic injection by feedback AGN jets
The Astrophysical Journal, 811:108 (21pp), 2015 October 1 Prasad, Sharma, & Babul
AGN jet-ICM sims.
15
1.8 ´ 10 M , respectively, and adopt c = 4.7 for all in Figure 1). The jet radius r is scaled with the halo mass; i.e.,
(cid:58) 200 jet
@⇢
models.
+ ⇢v = S
mass1 3
⎛ ⇢ ⎞
M
@t r ·
We include the source terms S for mass and S v r for the
ˆ 200
ρ r jet r = 2 kpc .
⎜ ⎟
jet
@v
radial momentum to drive AGN jets (v is the velocity which 14
7 ´ 10 M
⎝ ⎠
jet ⇢ + v v = p ⇢ � + S v ˆr (cid:58)
momentum
⇢ jet
3
the jet matter is put in). These source terms and the cooling @t · r �r � r
✓ ◆
term (in Equation (3)) are applied in an operator-split fashion.
p d
� 2
ln(p/⇢ ) = n ⇤
The jet mass-loading rate is calculated from the current mass
The mass and momentum source terms are approximated
� 1 dt �
accretion rate�(M˙ ) evaluated at the inner radial boundary such
forward in time and centered in space. The cooling term is
acc
source term applied in a small fi
that the increase in the jet kinetic energy is a xed fraction of
applied using a semi-implicit method described in Equations
bipolar cone at the center:
the energy released via accretion; i.e.,
(7) of McCourt et al. (2012).
opening angle of 300, size 2 kpc
2 kpc
Our simulations do not include physical processes like star
2
2
M˙ v = (cid:139)M˙ c . 6
( )
jet acc
formation and supernova feedback. Star formation may deplete jet
some of the cold gas available in the cores (see Li et al. 2015),
v =0.1c, ε=6x10-5, r =1, 2004 kpc −1
We choose the jejett velocity v =in,o3ut ´ 10 km s (0.1 c; c is
jet
but this is unlikely to change our results for a realistic model of robust to variations
the speed of light); such fast velocities are seen in X-ray
star formation. Supernova feedback is energetically subdomi-
fl
observations of small-scale out ows in radio galaxies (Tombesi
nant compared to AGN feedback, and cannot realistically
fi fi
et al. 2010). The jet ef ciency (ò; our ducial value is
fl
suppress cluster cooling ows (e.g., Saro et al. 2006). We only
−5
6 × 10 ) accounts for both the fraction of the infalling mass at
include the most relevant physical processes, namely cooling
the inner boundary (at 1 kpc for the cluster runs) that is accreted
and AGN jet feedback, in our present simulations.
by the SMBH and for the fraction of accretion energy that is
channeled into the jet kinetic energy. Our results are insensitive
2.1. Jet Implementation
to a reasonable variation in jet parameters (v , r , q , s , s ),
jet jet jet r q
fi
Jets are implemented in the active domain by adding mass but depend on the jet ef ciency (ò).
Like Gaspari et al. (2012), the jet energy is injected only in
and momentum source terms as shown in Equations (1) and
the form of kinetic energy; we do not add a thermal energy
(2). The source terms are negligible outside a small biconical
region centered at the origin around q = 0, p, mimicking mass source term corresponding to the jet. We note that Li & Bryan
(2014b) have shown that the core evolution does not depend
and momentum injection by fast bipolar AGN jets.
sensitively on the manner in which the feedback energy is
The density source term is implemented as
partitioned into kinetic or thermal form. Another difference
from previous approaches, which use few grid points to inject
S r, q = (cid:38)M˙ y r, q ,
( ) ( )
r jet
jet mass/energy, is that our jet injection region is well-
resolved.
where M˙ is the single-jet mass loading rate,
jet
2.2. Grid, Initial, and Boundary Conditions
⎡ ⎛ q - q ⎞ ⎛ q + q - p ⎞ ⎤
jet jet
y r, q = 2 + tanh + tanh
( ) ⎢ ⎜ ⎟ ⎜ ⎟ ⎥
Most AGN feedback simulations evolved for cosmological
s s
⎝ ⎠ ⎝ ⎠
⎣ ⎦
q q
timescales (e.g., Gaspari et al. 2012; Li & Bryan 2014a) use
fi
⎡ ⎛ r - r ⎞ ⎤ 1 Cartesian grids with mesh re nement. However, we use
jet
´ 1 + tanh ´
⎢ ⎜ ⎟ ⎥
spherical coordinates with a logarithmically spaced grid in
s 4
⎝ ⎠
⎣ ⎦
r
5 radius, and equal spacing in θ and f. The advantage of a
( )
fi
spherical coordinate system is that it gives ne resolution at
smaller scales without a complex algorithm. Perhaps more
that describes the spatial distribution of the source term which
importantly, a spherical setup allows for 2D axisymmetric
falls smoothly to zero outside the small biconical jet region of
simulations which are much faster and capture a lot (but not all)
radius r and half-opening angle q . We smooth the jet source
jet jet
of essential physics.
terms in space because the Kelvin–Helmholtz instability is
We perform our simulations in spherical coordinates with
known to be suppressed due to numerical diffusion in a fast
0 (cid:45) q (cid:45) p, 0 (cid:45) f (cid:45) 2p, and r (cid:45) r (cid:45) r , with
fl min max
ow if the shear layer is unresolved (e.g., Robertson
1 3
et al. 2010). The normalization factor
⎛ ⎞
M
200
r = 1, 200 kpc .
[ ] ⎜ ⎟
min,max
[ ]
14
7 ´ 10 M
⎝ ⎠
(cid:58)
3
(cid:38) =
3
2pr ( 1 - cos q ) According to self similar scaling, we have scaled all length
jet
jet
scales in our simulations (inner/outer radii r r , r , jet
min max 200
1 3
ensures that the total mass added due to jets per unit time is radius r ) as M .
jet
200
fl
2M˙ . All our simulations use the following jet parameters: We apply out ow boundary conditions (gas is allowed to
jet
leave the computational domain but prevented from entering it)
s = 0.05 kpc, q = p 6, and s = 0.05. The jet source region
r jet q
fi
at the inner radial boundary. We x the density and pressure at
with an opening angle of 30° may sound large but we get
the outer radial boundary to the initial value and prevent gas
similar results with narrower jets. Also, the fast jet extends well
fl
from leaving or entering through the outer boundary. Re ective
beyond the source region and is much narrower (c.f. third panel
boundary conditions are applied in θ (with the sign of v
f
fl
ipped) and periodic boundary conditions are used in f. We
3
We have also carried out narrow-jet simulations with momentum injection
fi
noticed that cold gas has a tendency to arti cially “stick” at the
fi
in the vertical z direction, but do not nd much difference from our runs with
[ˆ]
θ boundaries (mainly in 2D axisymmetric simulations) for our
momentum injection in the radial [r ; see Equation (2)] direction.
ˆ
4
Dependence on halo
mass & efficiency
larger ε
1.8x1015 M
sun
suppresses
7x1014 M
sun
accretion
more massive
halos require
larger ε
depends on where
Mdot calculated
Density movie
150 kpc
BCG+NFW in PLUTO
256x128x32 in (logr,θ,φ)
r =0.5 kpc, r =0.5 Mpc
min max
evolution for ~2.8 Gyr
made by Deovrat Prasad
r-θ slices
The Astrophysical Journal, 811:108 (21pp), 2015 October 1 Prasad, Sharma, & Babul
sound/weak
shock waves
pressure
50 kpc
buoyant
bubbles
bubble mixing
with ICM
density
Figure 1. Pressure (upper panel), electron number density (middle panel), and temperature (lower panel) contour plots (R–z plane at f = 0) in the core at different
times for the 3D fiducial run. The density is cutoff at the maximum and the minimum contour level shown. The low-density bubbles/cavities are not symmetric and
there are signatures of mixing in the core. The left panel corresponds to a time just before a cooling time in the core. The second panel from the left shows cold gas
dredged up by the outgoing jets. The rightmost panel shows infalling extended cold clouds. The pressure maps show the weak outer shock, but the bubbles/cavities so
prominent in the density/temperature plot are indiscernible in the pressure map, implying that the bubbles are in pressure equilibrium and buoyant. Also notice the
outward-propagating sound waves in the two middle pressure panels in which the jet is active. The infalling/rotationally supported cold gas has a much lower
temperature and pressure than the hot phase. The arrows in the temperature plots denote the projected gas velocity unit vectors.
5
r-θ slices
The Astrophysical Journal, 811:108 (21pp), 2015 October 1 Prasad, Sharma, & Babul
sound/weak
shock waves
pressure
50 kpc
buoyant
bubbles
bubble mixing
with ICM
density
Figure 1. Pressure (upper panel), electron number density (middle panel), and temperature (lower panel) contour plots (R–z plane at f = 0) in the core at different
times for the 3D fiducial run. The density is cutoff at the maximum and the minimum contour level shown. The low-density bubbles/cavities are not symmetric and
there are signatures of mixing in the core. The left panel corresponds to a time just before a cooling time in the core. The second panel from the left shows cold gas
dredged up by the outgoing jets. The rightmost panel shows infalling extended cold clouds. The pressure maps show the weak outer shock, but the bubbles/cavities so
prominent in the density/temperature plot are indiscernible in the pressure map, implying that the bubbles are in pressure equilibrium and buoyant. Also notice the
outward-propagating sound waves in the two middle pressure panels in which the jet is active. The infalling/rotationally supported cold gas has a much lower
temperature and pressure than the hot phase. The arrows in the temperature plots denote the projected gas velocity unit vectors.
5
Cold rotating torus
6 Deovrat Prasad & Prateek Sharma
few kpc scale
molecular torus
Fig. 3.— The 2-D (z = 0) contour plots of density in the very inner region at different times for the fiducial 3-D run, with the projection
of the velocity unit-vector represented by arrows. The top-left panel shows the beginning of the infall of cold gas with random angular
momentum. The top-middle panel shows an anti-clockwise transient torus. All times after this show a clockwise torus which waxes and
wanes because of cooling and AGN heating cycles. Even at late times the cold torus is not stable and gets disrupted by jets.
it reforms over a few cooling times. Figure 3 shows the that these explosive ‘events’ are rare and the jet mate-
evolution of the torus at various stages of the simulation. rial is quickly mixed with the ICM, and the core settles
The top-left panel of Figure 3 shows the cluster center at back to a quiescent state (see the top panel of Fig. 9
0.5 Gyr. Small cold gas clouds are accumulating in the which shows a large peak in jet energy at 2.4 Gyr). In
core after the first active AGN phase. At 1.3 Gyr, cold reality, the cold gas in the torus is mainly consumed by
gas accreting through the inner boundary has an anti- star-formation and only a part of it reaches the SMBH,
clockwise rotational sense. At 1.98 Gyr, cold gas (and and that too at the slow viscous timescale. In addition,
the hot gas out of which it condenses) is rotating clock- the rapidly reorienting AGN jets can disrupt the massive
wise. Jet activity leading up to this phase has reversed cold torus (e.g., see Babul et al. 2012).
the azimuthal velocity of the cold gas. At all times af- Li & Bryan (2014b) show that after 3 Gyr the cold
ter this the dynamic cold gas torus rotates in a clockwise gas settles down in form of a stable torus, with no further
sense, essentially because the mass (and angular momen- condensation of extended cold gas. This is inconsistent
tum) in the rotating torus is much larger than the newly with observations. The bottom panels in Figure 3 from
condensing cold gas. our fiducial run shows that the torus is unsteady even at
The middle panels of Figure 3 show the dynamic na- late times. Moreover, unlike them, we see extended cold
ture of the rotationally supported torus. The torus gets gas condensing out till the end. We compare our results
disrupted due to jet activity as seen in the middle panel in detail with Li & Bryan (2014b) in section 4.1.
of Figure3, but forms again quickly. The snapshots at 2.4 To test the role of cooling in maintaining the cold ro-
and 2.4 Gyr show that the inner region is covered by the tating torus, we restarted the 3-D fiducial run after a
very hot/dilute jet material. This unphysical behavior massive cold torus had formed (3 Gyr), and re-ran it
is mainly because of our feedback prescription; we scale without radiative cooling or feedback heating. While
the jet power with the instantaneous mass inflow rate the cold torus is long-lived even without cooling (Kelvin-
through the inner boundary (see Eq. 6). Even small os- Helmholtz instability does not grow for at least the next
cillations of the cold torus can sometimes lead to a large Gyr), it is heated (by numerical dissipation) to 105 K,
instantaneous mass inflow through the inner boundary and is no longer maintained at the temperature of the
and hence an explosive jet feedback in which the jet ma- stable phase (104 K). Thus, radiative cooling function is
terial encompasses the inner core. The reassuring fact is what dictates the temperature of the cold phase. The
6 Hamer et al.
Cold torus in Hydra A
6 Hamer et al.
[Hamer et al. 2014]
~5 kpc cold torus
more examples from ALMA, Hershel
Figure 2. This figure shows the IFU maps of the Hα emission as taken from fits to the Hα/[NII] triplet observed in the VIMOS cubes.
Panel (A) shows a continuum image made by collapsing the cube, the contours show the Hα emission clearly centred on the BCG. Panel
(B) is a Hα Flux map which shows a disc of bright emission running across the BCG. In panel (C) we show the relative velocity of
the Hα line to the galaxy redshift, a strong velocity gradient of ∼ 600 km s−1 can clearly be seen. Contoured on this plot are lines of
constant velocity created by fitting a disc model to the velocity map. The final panel (D) shows the measured Full Width Half Maximum
(FWHM) of the line which can be seen to broaden at the centre of the velocity gradient.
is likely to be due to the lower signal–to-noise as the lines are BCG. The luminosity of [FeII] emission has a high depen-
present within the total spectrum of this region (extracted dence on the gas density (Bautista et al. 1994) so we would
1×1 arcsec2 centred on the offset Paα peak) though the line expect it to be brightest in the central regions where the
is weak compared to Paα. gas density is higher. Despite being compact the line does
appear to be extended to the east on scales slightly greater
[FeII] emission was the only line detected in the H-band
than the seeing. Within this small extent there appears to be
observations. The maps presented in Figure 5 show that the
a velocity change of ∼ 200 km s−1 across the emission. This
[FeII] emission is compact and located at the centre of the
Figure 2. This figure shows the IFU maps of the Hα emission as taken from fits to the Hα/[NII] triplet observed in the VIMOS cubes.
Panel (A) shows a continuum image made by collapsing the cube, the contours show the Hα emission clearly centred on the BCG. Panel
(B) is a Hα Flux map which shows a disc of bright emission running across the BCG. In panel (C) we show the relative velocity of
the Hα line to the galaxy redshift, a strong velocity gradient of ∼ 600 km s−1 can clearly be seen. Contoured on this plot are lines of
constant velocity created by fitting a disc model to the velocity map. The final panel (D) shows the measured Full Width Half Maximum
(FWHM) of the line which can be seen to broaden at the centre of the velocity gradient.
is likely to be due to the lower signal–to-noise as the lines are BCG. The luminosity of [FeII] emission has a high depen-
present within the total spectrum of this region (extracted dence on the gas density (Bautista et al. 1994) so we would
1×1 arcsec2 centred on the offset Paα peak) though the line expect it to be brightest in the central regions where the
is weak compared to Paα. gas density is higher. Despite being compact the line does
appear to be extended to the east on scales slightly greater
[FeII] emission was the only line detected in the H-band
than the seeing. Within this small extent there appears to be
observations. The maps presented in Figure 5 show that the
a velocity change of ∼ 200 km s−1 across the emission. This
[FeII] emission is compact and located at the centre of the