Young AGN outburst running over older X-ray cavities

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by  Akos Bogdan

Draftversion June22,2014
PreprinttypesetusingLATEXstyleemulateapjv.5/2/11
YOUNG AGN OUTBURST RUNNING OVER OLDER X-RAY CAVITIES
A´kos Bogda´n1,6, Reinout J. van Weeren1,6, Ralph P. Kraft1,
William R. Forman1, Scott Randall1, Simona Giacintucci2,3, Eugene Churazov4,
Christopher P. O’Dea5, Stefi A. Baum5, Jacob Noell-Storr5, and Christine Jones1
1SmithsonianAstrophysicalObservatory,60GardenStreet,Cambridge,MA02138, USA;[email protected]
2DepartmentofAstronomy,UniversityofMaryland,CollegePark,MD20742, USA
3JointSpace-Science Institute, UniversityofMaryland,CollegePark,MD20742-2421, USA
4Max-Planck-Institutfu¨rAstrophysik,Karl-Schwarzschild-str. 1,85741GarchingbeiMu¨nchen, Germanyand
4 5RochesterInstitute ofTechnology, 84LombMemorialDrive,Rochester,NY14623, USA
1
Draft version June 22, 2014
0
2
ABSTRACT

n Although the energetic feedback from active galactic nuclei (AGN) is believed to have a profound
a effect on the evolutionof galaxiesand clusters of galaxies,details of the AGN heating remainelusive.
J
Here,westudyNGC193–anearbylenticulargalaxy–basedonX-ray(Chandra)andradio(VLAand

3 GMRT) observations. These data revealthe complex AGN outburst history of the galaxy: we detect
2 a pair of inner X-ray cavities, an outer X-ray cavity, a shock front, and radio lobes extending beyond

the inner cavities. We suggestthat the inner cavities were produced 78 Myr ago by a weakerAGN
] ∼
outburst, while the outer cavity, the radio lobes, and the shock front are due to a younger (13 26
A
−
Myr) and (4 8) times more powerful outburst. Combining this with the observed morphology of
G −
NGC 193, we conclude that NGC 193 likely represents the first example of a second, more powerful,
. AGN outburst overrunning an older, weaker outburst. These results help to understand how the
h
outburst energy is dissipated uniformly in the core of galaxies, and therefore may play a crucial role
p
in resolving how AGN outbursts suppress the formation of large cooling flows at cluster centers.
-
o
r Subject headings: galaxies: individual (NGC 193) — galaxies: active — radio continuum: galaxies —
t
s X-rays: galaxies — X-rays: ISM
a
[

1 1. INTRODUCTION 2004; Vernaleo & Reynolds 2006). While these stud-
v There is a wide agreementthat the energetic feedback ies addressed various aspects of the interaction, it re-
6 from AGN plays a pivotalrole in the evolution of galax- mains unclear if the models reflect the properties of real
7 ies. Powerful AGN outbursts are capable of quenching AGN feedback. To better understand the interaction
1 between AGN feedback and the surrounding interstel-
the active star formation in massive galaxies, thereby
6 lar/intracluster material, it is crucial to explore nearby
producing the population of “red and dead” ellipticals.
. systems that exhibit signatures of past outbursts.
1 The energy input from AGN can reheat the cooling
In this paper, we study the lenticular galaxy NGC
0 gas, and hence suppress star formation and maintain
193 (aka UGC 408), which hosts a luminous FR-I ra-
4 thepassivenatureofellipticals(e.g.Springel et al.2005;
dio source, 4C+03.01. The galaxy is located in a poor
1 Croton et al. 2006; McNamara & Nulsen 2007). More-
: over,the scalingrelations betweengalaxiesandtheir su- group, where the most optically luminous elliptical is
v NGC 194 (Geller et al. 1983). For the distance of NGC
permassive black holes (BHs) may also be explained by
Xi AGN feedback (e.g. Silk & Rees 1998; Di Matteo et al. 193 we adopt 63 Mpc (1′ = 18.3 kpc). Hubble Space
Telescope imagesofNGC193showprominentdustlanes
ar 20I0n5).galaxies, galaxy groups, and galaxy clusters (Noel-Storr et al. 2003), and low level (< 0.1 M⊙ yr−1)
star formation. The Galactic column density towards
the commonly observed signatures of powerful AGN
NGC 193 is 2.6 1020 cm−2 (Kalberla et al. 2005).
outbursts are cavities in the X-ray gas distribu-
×
NGC 193 has a complex structure with several phe-
tion (e.g. Forman et al. 2005; McNamara et al. 2009;
nomena detected, such as X-ray cavities, surface bright-
Bogd´an et al.2011). JetsemanatingfromtheBHinflate
nessedges,radiolobes,andparsec-scaleradiojets. While
radio lobes, which displace the surrounding hot X-ray
similar features were studied separately in other galax-
emitting gas, possibly drive shocks, and create bubbles
ies, NGC 193 uniquely comprises all of these. Given the
of relativistic plasma. In a handful of systems multiple
complexity of NGC 193, we present our results in two
X-ray cavities are detected originating from consecutive
papers. Here, we provide evidence that a young, more
AGN outbursts (e.g. Randall et al. 2011; Blanton et al.
powerful, AGN outburst overran an older, weaker out-
2011),whichallowsanexplorationofAGNoutbursthis-
burst. In the followingpaper, we will explore the overall
tory.
morphologyof NGC 193,study its large-scaledynamics,
Given the importance of AGN feedback, numerical
and investigate its central regions.
studies have attempted to describe the interaction of
AGN with their surrounding gas (e.g. Ruszkowski et al.
6EinsteinFellow
2 BOGDA´N ET AL.
TABLE 1
The list of analyzedChandra observations.
Galaxy ObsID Center coordinates Tobs (ks) Tfilt (ks) Instrument Obs. date
NGC193 4053 00h39m18.60s+03d19’52” 29.1 14.5 ACIS-S 2003Sep01
NGC193 11389 00h39m18.60s+03d19’53” 93.9 93.9 ACIS-S 2009Aug21
Fig.1.—Topleft: Chandra 0.3−2keVbandimageofa6′×6′ regionofNGC193. Backgroundissubtractedandvignettingcorrection
isapplied. Pointsourcesareexcludedandtheirlocationisfilledwiththelocalbackgroundvalue. TheimageissmoothedwithaGaussian
kernel withawidth of2′′. Top right: 1.4GHzVLAimage ofthe sameregion, showingthe slightlyasymmetric radiolobes. Bottom left:
SameChandra imageasinthetopleftpanel,butoverlaidaretheintensitylevelsofthe610MHzGMRT image. Bottom right: Residual
imageofNGC193inacircularregionwith150′′ radius. Theshockfrontextendstothesouthandpetersattheeast–adetaileddiscussion
willbepresentedinanupcomingpaper.
AGN OUTBURST RUNNING OVER X-RAY CAVITIES 3
2. DATAREDUCTION In a simple picture, the disturbed X-ray gas distribu-
tion,thesurfacebrightnessedges,andtheradiolobesare
2.1. X-ray data
dueto arecentAGNoutburst. Inthis section,we estab-
NGC 193 was observed by Chandra for a total of 123 lish the basic properties of this outburst, specifically the
ks. The data were reduced with standard CIAO1 soft- expansion velocity of the lobes and the occurrence time
ware package tools (CIAO version4.5; CALDB version of the outburst.
4.5.6). To characterize the X-ray surface brightness edge at
The data analysis was performed following 2′ central distance, we build surface brightness and
Bogd´an & Gilfanov (2008). First, we filtered the ∼temperatureprofilesinthe 0.3 2keVbandusing circu-
flare contaminated time intervals, which resulted in a lar wedges along the eastern a−nd western sides of NGC
total net exposure time of 108 ks. Bright point sources 193 with an opening angle of 60◦ (Figure 2). The pro-
were identified and masked out. We subtracted the files show a sharp surface brightness edge and a tem-
instrumental and sky background components by using perature jump at the east of NGC 193, which suggest
the ACIS “blank-sky” files. To correct for temporal that this is a shock front produced by the supersonic
variations in the normalization of the background expansionof the radio lobes – for the 610 MHz radio in-
components, the “blank-sky” files were renormalized by tensitylevelsoverlaidontheX-rayimageseethebottom
the ratio of the count rates in the 10 12 keV band. left panel of Figure 1. To estimate the expansion veloc-
−
We built exposure maps – assuming a thermal model ity,weusetheRankine-Hugoniotjumpconditionsacross
with kT = 0.9 keV, 0.3 Solar metallicities, and Galactic the shock front and the gas densities/temperatures up-
column density – to account for the vignetting effects. stream and downstream (Markevitch & Vikhlinin 2007;
Russell et al. 2010). From the density drop of n /n =
2.2. Radio data 1.60 0.15,weobtainaMachnumber ofM =1.421+01.12,
NGC 193 was observed with the Very Large Array ± −0.10
while from the temperature drop of T /T =1.40 0.09
(VLA) at 1.4 GHz and 4.9 GHz between March 2007 we deduce M = 1.13+0.12. The lower2Ma1ch numb±er ob-
andMay2008(Laing et al.2011). Theinitialdatareduc- −0.13
tained from the temperature jump is presumably due to
tionstepswerecarriedoutwiththeNRAOAstronomical
projection effects. To account for this effect, deprojec-
Image Processing System package. Gain solutions were
tion could be employed, which would result in a some-
obtained for the calibrator sources and transferred to
whathigher Machnumber. However,we do notperform
NGC 193. Theflux-scalewassetbythe Perley & Taylor
a deprojection analysis, since the hot gas distribution
(1991) extension to the Baars et al. (1977) scale. Each
exhibits significant deviations from spherical symmetry,
dataset from a specific VLA configuration was then im-
which would result in large systematic uncertainties in
agedandseveralroundsofphaseselfcalibrationwerecar-
the deprojected gas parameters.
ried out to refine the calibration. A final step of phase
To estimate the age of the AGN outburst, we assume
and amplitude self calibration was performed. We im-
ported the data in CASA2 and combined the data from that the radio lobes are on the plane of the sky (θ =
90◦), they expandwith aconstantvelocityofM =1.0
allconfigurations. Wethenimagedthetwodatasetswith 1.5 (480 720 km s−1 in a 0.9 keV plasma), and the−y
the multi-scale clean algorithm. To make spectral index
extend to−112′′ ( 34.2 kpc). Thus, the age of the radio
maps,wecreatedimageswithuniformweighing,tocom-
∼
lobes is 47 70 Myr. This estimate may be modified by
pensateforthedifferentsamplingintheuv-lane. Wealso
−
two factors. First, the expansion may have been faster
employedGaussianuv-taperstomatchtheresolutionsof
in the past (Churazov et al. 2000), which could decrease
both datasets.
The Giant Metrewave Radio Telescope (GMRT) ob- the expansion time by a factor of 3/5 to 28 42 Myr.
−
Second, if the axis of the radio lobes does not lie in the
served NGC 193 at 235 MHz and 610 MHz in
plane of the sky (i.e. θ = 90◦), as suggested by the
August 2008 and August 2007, respectively. The
6
GMRT data presented in this work come directly from asymmetric radio lobes, then their real extent is larger
than the projected one. Therefore, the expansion time
Giacintucci et al. (2011).
that takes into account these effects is (28 42)/sinθ
3. RESULTS Myr. −
3.1. A simple picture of NGC 193 Another age estimate on the radio lobes can be ob-
tained from computing their radiative age. To this end,
In Figure 1 (top left panel) we show the 0.3 2 keV
− we extracted the flux densities at 1.4 GHz and 4.9 GHz
bandChandra imageofNGC193,whichrevealsthepres-
across the lobes and computed the corresponding spec-
ence of diffuse X-ray emission originating from hot ion-
tral index (α). As shown in the spectral index map
izedgas. Thegasdistributionisdisturbed,andshowsthe
(Fig. 3), α remains fairly flat along the east-west axis
presence of cavities and sharp surface brightness edges.
and steepens along the lobes. Assuming that the last
The 1.4 GHz VLA image (Figure 1 top right panel) re-
electron acceleration occurred at the outer edge of the
veals somewhat asymmetric bipolar radio lobes, extend-
lobes, we fitted the observed steepening using a Jaffe-
binigpotloar∼ra2d′ioinlotbheeseaarset-nwoetsotndlyiredcettieocnt.edInatter1e.s4tiGnHglzy,,bthuet Perola model assuming that νbreak ∝ d−2, where d is
thedistancefromtheinjectionsource. Usingthebest-fit
alsointhe4.9GHzVLAandthe235MHzand610MHz
ν = 2.0 1.2 GHz and the “revised” equipartition
GMRT images. These evidencesindicate that the BHof break ±
magnetic field of 16 µG obtained from Beck & Krause
NGC 193 was active in the recent past.
(2005), we estimate that the age of the lobes is 13 26
−
1 http://cxc.harvard.edu/ciao/ Myr. This estimate should be considered as an upper
2 http://casa.nrao.edu/ limit, since we assumed that the magnetic field is con-
4 BOGDA´N ET AL.
1.5
East East
2m] West 1.4 West
2c/c10-17 1.3
e
s
s/arc keV] 1.2
s [erg/ ature [ 1.1
brightnes10-18 Temper 0. 91
e 0.8
c
a
urf 0.7
S
10-19 0.6
50 100 150 200 250 0 50 100 150 200 250
Radius [arcsec] Radius [arcsec]
Fig. 2.— Surface brightness (left panel) and temperature (right) profiles of NGC 193 extracted from circular wedges with an opening
angleof60◦towardthewestandeast. Thethick(red)verticallineindicatesthepositionofthesurfacebrightnessjump,whichiscoincident
withtheextent oftheradiolobetowardstheeast. Thethin(blue)verticallineshowstheextent oftheradiolobetowardsthewest.
NGC193,(iii)brightrims surroundingtheinnercavities,
and (iv) sharp surface brightness edges. While the outer
cavity and the surface brightness edge is in good agree-
mentwiththedistributionoftheeasternradiolobe(Fig-
ure1bottomleft),theinnercavitypairisorientedinthe
north-south direction as opposed to the east-west align-
mentoftheradiolobes. Additionally,the radioemission
is not enhanced at the inner cavity, and the radio lobes
extendsignificantlybeyondthem. Thissuggeststhatthe
inner and outer cavities may originate from two distinct
AGN outbursts.
To estimate the occurrence time of the outburst that
inflated the inner cavities, we derive their buoyant rise
time following McNamara & Nulsen (2007). Assuming
that the cavities have a radius of 37.5′′ and using the
gas temperature of 0.9 keV we find that t = 78 Myr.
b
Weemphasizethatduetopossibleprojectioneffectsthis
valueshouldbeconsideredasalowerlimit. The absence
Fig.3.—Spectralindexmapofa6′×6′regionaroundNGC193 of radio emission directly associated with the inner cav-
inferredfromthe ratio of the 1.4GHz and 4.9 GHz VLA images.
Overplotted are the 4.9 GHz VLA contours, where the contour ities also suggests an old outburst. Using the 235 MHz
levelsarespacedat[1,2,4,8,...]×3σrms. GMRT image and assuming a magnetic field of 10 µG
(Parma et al. 2007), we estimate that the fading time of
theradioemissionis 50Myr,whichplacesalowerlimit
stant and uniform across the lobe and expansion energy ∼
on the age of the outburst. Note that the precise value
losses are negligible. At the face value, the two different
of the magnetic field is not known, yielding a somewhat
ageestimates arebroadlyconsistent,albeitthe radiative
uncertain estimate.
age may be somewhat shorter.
These results hint that the morphological features of
NGC193originatepresumablyfromtwoconsecutiveout-
3.2. Two consecutive AGN outbursts
bursts. In this picture, an old outburst ( 78 Myr ago)
Although previously we considered a simple scenario, inflatedtheinnerX-raycavitiesthatareb∼uoyantlygrow-
inwhichtheobservedX-rayandradiofeaturesaredueto ing at the present epoch. This outburst was followed
a single AGN outburst, this picture cannot give a com- by ayoungeroutburst,that producedthe supersonically
plete descriptionofNGC 193fortworeasons. First, this expanding outer cavity, the shock front, and the radio
scenariocannotaccountfor all majormorphologicalfea- lobes. A consequence of this interpretation is that the
tures. Second,thispictureresultsintimescaleestimates inner cavities were inflated before the outer cavities, and
that are in tension with each other. hence the presently observed younger radio jets overran
Toexploreandbetteremphasizethemorphologyofthe the older X-ray cavity.
X-ray gas, we construct a residual X-ray image. There- The scenario, in which the younger outburst overran
fore,wesubtractanazimuthallyaveragedsurfacebright- an older outburst, can resolve the mismatch between
ness image from the 0.3 2 keV band Chandra image. the estimated time scales of the younger AGN outburst
−
The thus obtained image (Figure 1 bottom right) shows (Section 3.1). Specifically, the expansion time scale of
the following features: (i) a pair of inner cavities in the (28 42)/sinθMyrobtainedfromtheRankine-Hugoniot
centralregions,(ii) anouter cavity in the easternside of −
AGN OUTBURST RUNNING OVER X-RAY CAVITIES 5
jump conditions is likely overestimated. Indeed, a high how do AGN outbursts suppress the formation of large
momentum jet can cross the low density region nearly cooling flows at cluster centers. It is clear that regu-
with the speed of light, which implies that the propaga- lar AGN outbursts can provide sufficient energy to off-
tion time within the X-ray cavities is virtually negligi- set radiative losses and prevent the formation of cooling
ble. Assuming that the cavities extend to 50′′, the radio flows. How the energy of the outburst is dissipated uni-
lobes expand at the estimated M = 1.0 1.5 velocity formly in the core and not at larger radii is uncertain
in the 50 112′′ region. Taking into acco−unt the vari- (O’Neill & Jones 2010). Buoyantly rising bubbles will
−
ouscorrections(Section3.1),theageofAGNoutburstis dissipatetheirenergynon-isentropically,butmuchofthis
(16 23)/sinθ Myr,whichisconsistentwiththe 13 26 will occur outside the region of largest radiative losses
− −
Myr age estimate obtained from the radio data. (Vernaleo & Reynolds 2006). Likewise, regular low-level
By measuringthe cavity power(P ) we estimate the AGN outbursts will generate sounds waves (Begelman
cav
energy injected to the X-ray emitting gas by the two 2001; Churazov et al. 2002), but efficient heating of the
AGN outbursts. We derive P from the minimal en- gas in the core requires that the viscosity of the gas is
cav
ergy required to inflate the cavity and the age of the a significant fraction of the Spitzer value. The entropy
cavity (McNamara & Nulsen 2007; Bogd´an et al. 2011). increasedue to weakshocks is the mostlikely method of
For relativistic plasma, the minimal energy is obtained increasing the entropy of the gas in the core and offset-
from the cavity enthalpy, H =4pV, where p is the aver- tingtheradiativelosses(McNamara & Nulsen2007),but
age pressure and V is the volume of the cavity. how this energy input is uniformly distributed through-
Wedescribetheinnercavitieswithtwocircularregions out the core remains unclear.
with 37.5′′ radius (Figure 1 bottom right) and assume If a strong AGN outburst is driven through an older,
spherical symmetry. Using the best-fit spectrum, we de- presently buoyant, outburst, the shock from the current
rivetheaverageelectrondensityofne =4.2 10−3cm−3, outburstwillberapidlydriventhroughtheoldradiobub-
×
and the average pressure as p = 1.9nekT = 1.0 blessincethe soundspeedis high(muchhigherthanthe
10−11 erg cm−3. Thus, the total AGN work done b×y shockspeedinthethermalgas,ashighasc/√3iftheold
the cavities is 1.5 1058 erg. Assuming that the age radio bubble is relativistic plasma). Thus, the old radio
×
of the inner cavity is 78 Myr, the total cavity power is bubble will effectively “isotropize” the shock from the
Pcav =6.1 1042 erg s−1. more recent outburst, more uniformly distributing the
×
The outer cavity is described with a circular region heating throughout the core. Additionally, if the older
with a circular region with 40′′ radius (Figure 1 bot- radiobubbleispartiallyshredded,thepassingshockwill
tom right). Since the outer cavity is not detected on turnthesmallerclumpsofradioplasmaintovortexrings,
the western side of NGC 193, we base our calculations andthe vorticityinthe gascansignificantlyincreasethe
on one cavity and assume spherical symmetry. From efficiency of heating the ICM (Heinz & Churazov 2005).
the best-fit spectrum we obtain the average density of ThecombinationofvariablepowerAGNjetswiththedy-
ne = 3.5 10−3 cm−3 and the average pressure of namic ICM (Heinz et al. 2006) are likely the key to bal-
p = 1.9nek×T = 1.1 10−11 erg cm−3, and hence com- ancingtheradiativelossesinclustercoresandpreventing
pute the cavity ene×rgy of 1.0 1058 erg. Since this the formationof large cooling flows. A more careful and
×
value refers to one cavity, the total cavity energy is systematicexaminationofarchivalChandra observations
2.0 1058 erg. Assuming the cavity age of 13 26 of groups and clusters would provide stronger limits on
× −
Myr, the total cavity power of the young outburst is the frequency with which AGN outbursts overlap.
Pcav = (2.4 4.9) 1043 erg s−1. Thus, the more re- The observationaldata points out that the inner cavi-
− ×
cent outburst, that overran the inner X-ray cavities, is ties that originate fromthe older outburst have a north-
(4 8) times more powerful than the older outburst. south orientation,while the younger radio lobes and the
−
outercavityarepositionedalongtheeast-westdirection.
4. DISCUSSION It is feasible that the jet axis changed by 90◦ between
Our observation of two AGN outbursts in NGC 193 the two AGN outbursts, which may have∼been triggered
is the first example of a second, more powerful, AGN byarecentmergerthatchangedtheblackholespindirec-
outburst overrunning an older, weaker outburst. Clear tion,andhence the orientationofthe jet (Nixon & King
examples of multiple outbursts in group environments, 2013; O’Dea et al. 2013). However, the observed mor-
such as NGC 5813 (e.g. Randall et al. 2011) have been phology can also be obtained without the reorientation
reported, but all these cases the outbursts are clearly ofjets. AccordingtothescenariooutlinedinSection3.2,
spatially separated, and the oldest outburst is also the the inner cavities are due to a relatively weak outburst.
mostpowerful. This is almostcertainly anobservational Following a weak and short outburst with an arbitrary
bias since it is easiest to distinguish the outbursts when jet orientation, the inner cavities could have expanded
they are well separated, and the oldest outburst is only and moved to follow the gravitational potential or even
detectable if it is the strongest since the shock gener- just the motion of gas in the core. Since the galaxy is
ated by the oldest outburst will also have weakened the approximately elongated in the northeast-southwest di-
most. A weak pressure wave (i.e. transsonic/subsonic) rections,the buoyantlymovinglobeswouldmostlymove
from an old outburst is likely undetectable in any but in the same direction. The younger and more powerful
the brightest, best-observed systems. jet was presumably not affected by buoyant forces, and
Given the small number statistics and difficulty in henceitcouldpropagateintheeast-westdirection,which
identifying outbursts that have overrun each other, it would make it appear that the new and old jet axis are
is not clear how common they are. Multiple, interacting different. Thus,while a changein the jet axis is feasible,
outbursts could be crucial in resolving one of the long it is not required to explain the observational data.
standing issues in galaxy group/galaxycluster physics –
6 BOGDA´N ET AL.
Acknowledgements. We thank Paul Nulsen for helpful dis- AssociatedUniversities,Inc. A´BandRvWacknowledgesup-
cussions. This research made use of Chandra data provided port provided by NASA through Einstein Postdoctoral Fel-
by the CXC. The VLA is a facility of NRAO, which is a fa- lowship awarded bytheCXC,which is operated bytheSAO
cility of the NSF operated under cooperative agreement by for NASAundercontract NAS8-03060.
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