New Astronomy 15 (2010) 96–101

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New Astronomy

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Galaxy shells and the structure of radio galaxies:Clues from Centaurus A (NGC 5128)

Gopal-Krishna a, Paul J. Wiita b,c,*

a National Centre for Radio Astrophysics, TIFR, Post Bag 3, Pune University Campus, Pune 411 007, Indiab Department of Physics and Astronomy, Georgia State University, Atlanta, GA 30302-4106, USAc School of Natural Sciences, Institute for Advanced Study, Princeton, NJ 08540, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 April 2009Received in revised form 26 May 2009Accepted 4 June 2009Available online 13 June 2009Communiated by G. Brunetti

PACS:98.54.Gr98.58.Db98.58.Fd98.58.Nk

Keywords:Galaxies: activeGalaxies: individual (Centaurus A – NGC5128)Galaxies: ISMGalaxies: jets

1384-1076/$ - see front matter � 2009 Elsevier B.V. Adoi:10.1016/j.newast.2009.06.001

* Corresponding author. Address: Department of PhState University, Atlanta, GA 30302-4106, USA. Tel.: +413 5481.

E-mail addresses: krishna@ncra.tifr.res.in (Gopal-K(P.J. Wiita), wiita@astro.princeton.edu (P.J. Wiita).

We show that the northern middle radio lobe of Cen A, an intriguing and much debated manifestation ofradio lobe asymmetry, can be understood in terms of a direct interaction of the northern jet with a gas-eous cloud associated with a stellar shell. This same basic mechanism was proposed earlier for the north-ern inner lobe, but new data allows a more detailed case to be made for the northern middle lobe.Although such an interaction can presently be demonstrated only for Cen A, the nearest radio galaxy,it is likely to be a fairly common occurrence and it provides an alternative to models invoking episodicnuclear activity, possibly accompanied with jet precession, for radio galaxies with multiple lobes andS-shapes. This proposed scenario may also play a key role in the origin of prominent radio galaxy mor-phological classes, such as the Wide-Angle-Tail sources and the Z-symmetric X-shaped radio sources.The strong tendency for radio lobes to be more distorted in double radio sources with jets that are in clo-ser alignment with the optical major axis of the host elliptical galaxy can likewise be understood in termsof jet–shell interactions. In the frequent cases when jet activity is triggered by mergers of a large ellipticalgalaxy with a disk galaxy containing cold gas the impact of the gas associated with stellar shells upon thejets is likely to have significant manifestations.

� 2009 Elsevier B.V. All rights reserved.

1. Introduction

Since a detailed comparison of radio and optical structuresassociated with radio galaxies can be a powerful probe of theunderlying physical mechanisms, much can be learned by studyingtheir nearby examples. Centaurus A, the nearest (and secondbrightest, after Cyngus A, at low frequencies) radio galaxy (RG), ly-ing just 3.7 Mpc away (Ferrarese et al., 2007), is the most spectacu-lar example of a gas-rich disk galaxy consumed in a merger with agiant elliptical galaxy (see the review by Israel (1998) and refer-ences therein). It is also a leading candidate for episodic nuclearactivity accompanied with precession of the jet pair (e.g., Hayneset al., 1983; Morganti et al., 1999; Hardcastle et al., 2009) sinceits two radio lobes, which extend over �10� (�600 kpc), exhibitmultiple radio peaks in an overall S-shaped configuration (e.g.,

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ysics and Astronomy, Georgia1 404 413 6022; fax: +1 404

rishna), wiita@chara.gsu.edu

Cooper et al., 1965; Junkes et al., 1993; Morganti et al., 1999). Itsproximity and huge extent on the sky actually make it difficult tomap on comparable scales at a wide range of radio frequencies,and so only recently have good determinations of spectral indexvariations been obtained at higher frequencies (Hardcastle et al.,2009) by comparing the recent WMAP results (Page et al., 2007; Is-rael et al., 2008) with earlier ground based imaging data. RecentlyCen A has been claimed to be the source of some of the ultra-highenergy cosmic-rays (UHECRs) with energies above 1 EeV � 1018 eVrecorded by the Pierre Auger Observatory (e.g., The Pierre AugerCollaboration, 2007, 2008; Abbasi et al., 2008; Dermer et al., inpress; Fraschetti and Melia, 2008). Clearly, a more complete under-standing of all aspects of this source is particularly important.

Here we shall investigate the origin of a distinctly non-symmet-rical part of the radio structure of Cen A, the second Northern radiopeak called the ‘‘Northern Middle Lobe” (NML) and its possiblywide-ranging ramifications for understanding some key structuralaspects of radio galaxies. In Section 2 we summarize observationsof the NML and earlier attempts at understanding its peculiar mor-phology. Section 3 shows that dense gaseous clumps associated witha stellar shell can bend and partially disrupt the jet so as to form the

Gopal-Krishna, P.J. Wiita / New Astronomy 15 (2010) 96–101 97

NML. Newer observations and simulations are fundamentally in ac-cord with the hypothesis that the structure of the NML in Cen A isproduced by the same type of shell/jet interaction that best explainsthe morphology of the northern inner lobe (Gopal-Krishna and Sar-ipalli, 1984). More general ramifications of this mechanism are dis-cussed in Section 4 and we give our conclusions in Section 5.

Fig. 1. Overlay showing various components of the inner and N middle portions ofCentaurus A, adapted from Oosterloo and Morganti (2005) (�ESO). The opticalimage in enhanced contrast gray-scale shows the warped dust lane as well as shells,filaments and young stars. Radio continuum emission regions are indicated by thelabeled white contours while the black contours show the H I cloud.

2. The northern middle lobe

While the inner and the outer radio peaks in the north radiolobe of Cen A have unambiguous counterparts within the southernlobe, the middle peak, or NML, has no southern counterpart (e.g.,Israel, 1998; Morganti et al., 1999). This and other peculiarities ofthe NML have been debated over the past couple of decades (Hard-castle et al., 2009 and references therein). The two inner lobes ex-tend to �8 kpc from the nucleus, while the large-scale outer lobeseach extend out to �300 kpc from the nucleus, with their peaksseen on each side at distances of about 210 kpc. Early VLA mapsshow a jet feeding the N inner lobe (Burns et al., 1983) and the Ein-stein HRI image reveals a closely aligned X-ray jet (Feigelson et al.,1981). The NML stretches out to at least 40 kpc and its flux peaks atabout 23 kpc from the nucleus. Diffuse X-ray emission in excess ofthat from the hot galactic atmosphere comes from the brightest re-gion of the NML (Feigelson et al., 1981) and has recently been re-solved into a series of knots (Kraft et al., 2009).

The exceptional proximity of Cen A has also enabled the discov-ery of multiple optical stellar ‘‘shells” in the regions of both north-ern and southern radio lobes (Malin et al., 1983). These shells aregenerally believed to be phase wrapped remnants of the large diskgalaxy that merged with the massive elliptical host of the doubleradio source (Quinn, 1984) or possibly spatially wrapped accretedmatter forming thin disks (e.g., Dupraz and Combes, 1987). Theonly alternative interpretation of which we are aware posits thatthe shells are the products of a blast wave that enhances star for-mation as it propagates out from the galactic center (Kundt andKrause, 1985; Williams and Christiansen, 1985); it seems to be lesscapable of explaining the variety of observed shell structures andwe will not consider it here.

It was pointed out that the peaks of the northern inner and mid-dle radio lobes coincide with two stellar shells, suggesting that theradio peaks and their S-shaped disposition might have originateddue to an interaction of the northern jet with the shells, presumedto be in a clockwise rotation about the elliptical host (Gopal-Krish-na and Saripalli, 1984, hereafter G-KS; Gopal-Krishna and Chitre,1983, hereafter G-KC). A similar hint was noticed for the southernradio lobe, but the situation there is more complicated because ofthe expected attenuation of the emission from any shells in that re-gion by the massive dust lane seen around the parent elliptical gal-axy. This proposal offered a plausible alternative to the earliermodel for the peculiar morphology of Cen A which invokes anintermittent nuclear activity accompanied by a precession of thecentral engine (e.g., Haynes et al., 1983).

The association of a gaseous medium within, or extremely closeto, the stellar shells, a prerequisite for the jet–shell interactionhypothesis of G-KC and G-KS, has been borne out by the discoveryof a large amount of neutral atomic hydrogen (Schiminovich et al.,1994) located just outside the shells nearest the radio jets. Furthersupport came when even comparable amounts of clumpy molecu-lar gas were detected overlapping the HI regions (Charmandariset al., 2000) in close proximity to the shells. These new data, to-gether with the recent ATCA image of an HI shell (see below) pro-vide a fresh impetus for revisiting the jet–shell encounter scenarioand for examining its salient ramifications for the physics of radiogalaxies in general. The fact that large amounts of cold gas remainat distances of at least 15 kpc from the galactic core and do not

quickly sink to the center is not obviously expected. However, ifthe ISM of the merging spiral is clumpy, then the dense clouds be-have nearly collisionlessly and should wrap in a fashion very sim-ilar to the stars (Charmandaris et al., 2000).

Recently, VLA observations have revealed the structural detailsof the NML, including a curious jet-like radio feature linking the tipof the northern inner lobe to the south-eastern edge of the NML(see Fig. 2 of Morganti et al. (1999) and Fig. 1, which is anannotated version of Fig. 1 from Oosterloo and Morganti (2005);hereafter OM). The discoverers of this rare feature, the so called‘‘large-scale jet” (Morganti et al., 1999) suggested it was synchro-tron plasma escaping from a gap in the surface of the northern in-ner lobe after being collimated by a de Laval nozzle forming there.In contrast, Saxton et al. (2001) have modeled the NML to haveevolved from a bubble of relativistic plasma that was injected byan intermittently active jet. In their picture the bubble had a den-sity <10�2 of its surroundings, forcing it to rise buoyantly throughthe ISM to its current location over �140 Myr. In this scenario, asubsequent episode of nuclear activity has given rise to the innerradio lobes. A hydrodynamical simulation of just the injected bub-ble could reproduce the gross radio morphology of the NML,namely its bright base and fainter extension to the north (Saxtonet al., 2001). They propose that Morganti et al.’s ‘‘large-scale jet”is actually a stream of synchrotron plasma pulled out of the north-ern inner radio lobe by the same pressure gradient responsible forthe buoyant uplift of the NML and the ‘‘thermal trunk” in its wake.They further argue that this basic picture is consistent with the X-ray detection from the NML region (Feigelson et al., 1981) and thevarious optical emission-line knots and filaments seen near theNML and the large-scale jet (Blanco et al., 1975; OM, and refer-ences therein).

A recent key input to this debate comes from the detailed ATCAimage of the HI ‘‘shell” seen adjacent to the southern edge of the

98 Gopal-Krishna, P.J. Wiita / New Astronomy 15 (2010) 96–101

NML (Fig. 1). While broadly consistent with the original proposalthat the NML is a product of interaction of the northern radio lobe’ssynchrotron plasma with gas associated with the shell (G-KS) thenow available details of the ATCA detected HI shell permit a closerscrutiny of the situation. Recall the suggestion in G-KS that thenorthward deflection of the inner jet, leading to its flaring andthe formation of the northern inner radio lobe, is a result of ajet–shell encounter at a distance of about 6 kpc from the nucleusof Cen A. Subsequent hydrodynamical simulations of jets interact-ing with gas clouds lend support to this interpretation. Non-rela-tivistic simulations (e.g., Balsara and Norman, 1992; Higginset al., 1999; Wang et al., 2000) as well as recent relativistic ones(Choi et al., 2007) all show that jets hitting dense (but initiallynon-moving) clouds at oblique angles either can disperse thecloud, be deflected, or be completely disrupted. The outcome basi-cally depends on the ratios of the effective momentum and energydensities in the jet and cloud (Wang et al., 2000). Other hydrody-namical simulations are consistent with the hypothesis that thejet is disrupted by encountering a shock in the ISM and the North-ern inner lobe forms where a sudden increase in pressure goingoutward is met (Norman et al., 1988); in this case the bend towardthe N (instead of the W) would have to be attributed to the actionof a random turbulent eddy. However, this scenario does not ap-pear to be consistent with the recent deep Chandra study of thehot ISM on Cen A which shows a significant and sharp drop in X-ray surface brightness, and thus, pressure, essentially where the in-ner jet flares into the inner lobe (Kraft et al., 2008).

Although no simulations of which we are aware have consid-ered the impact of substantial dense moving clouds collidingonto established jets, which is the crux of the G-KS hypothesis,there are additional numerical efforts that should be of relevanceto this situation. These have considered what happens to jetswhen they are buffeted by a turbulent (Loken et al., 1995) orrotating (Heintz et al., 2006) intracluster medium that is pro-duced when two clusters merge. The jets and lobes can be bent,sheared and even disrupted; which fate befalls the jet dependson the jet’s density ratio and Mach number as well as themomentum of the impinging gas. In particular, the Heintz et al.(2006) simulation, based upon a cosmological simulation (Sprin-gel et al. 2001), shows that even a very strong, Cygnus A type jetwith a power of 1046 erg s�1, can be significantly, and asymmet-rically, distorted, by the rotation of the intracluster medium. Thisdistortion implies that radio jets can efficiently heat cluster gasnearly isotropically, thereby explaining why cooling flows arenot seen (Heintz et al., 2006).

Additional observational evidence for the interaction of the jetwith a cloud in Cen A comes from OM’s ATCA observations of theHI shell. They indicate that the various optical emission-line fila-ments and young stars seen in the region of the northern radio lobehave probably arisen from a kinematical interaction between thelarge radio jet and the HI shell present near the NML (theirFig. 4). X-ray maps of the NML made with the XMM-Newton satel-lite were reported (Kraft et al., 2009) after this paper was essen-tially completed. They reveal a series of five X-ray emitting knotswith thermal spectra in close proximity to, but actually anti-coin-cident with, the brightest radio emission within the NML. By farthe most likely explanation for these knots is that they are cloudsof gas, with modest masses around 106 M�, that have been struckby an extension of the large-scale jet (Kraft et al., 2009); as notedbelow, these observations are consistent with the model wepropose.

3. Gaseous shells interrupting the jets

The uniquely detailed set of observations now available forCen A lead us to suggest that the multiple radio peaks in its

lobes are an outcome of intermittent blocking of the jets bygas associated with stellar shells orbiting the elliptical host,rather than the usually postulated episodic nuclear activity. Inthis basic framework, the observed S-shaped distortion of thetwin radio lobes hints that the shells are orbiting clockwiseand so at some epochs they could well move into the radio jetsand the photon beam emerging from the blazar nucleus (G-KS;also, OM, GK-C). The resulting collision of the jet with the shellmeans the latter imparts a side-ways kick to the jet and, simul-taneously, the relativistic plasma in the jet boring through the(much denser) atomic/molecular gas of the shell then gets massloaded via entrainment.

Quite a few persuasive to crystal clear examples of radio galaxyjets smashing into interstellar gas clouds or the gas within com-panion galaxies are now known. ‘‘Minkowski’s object” appears tobe a starburst triggered by a jet from the RG NGC 541 (van Breugelet al., 1985). Aside from the Northern inner lobe of Cen A (GK-S),among the first cases for which a solid argument was made for ajet flaring up upon collision with a shell a few kpc from the nucleuswas the nearby RG 4C 29.30 (van Breugel et al., 1986). A good casefor the collision of a powerful quasar jet in 4C 29.50 with clumps ofgas within the host galaxy was made by Lonsdale and Barthel(1986). Other RGs have radio jets associated with star forming re-gions found from optical observations at much larger distances(e.g., 3C 285, van Breugel and Dey, 1993; Hardcastle, 2008). Inthe moderately distant RG 3C 34 a strong jet/cloud collision is seenas the jet pierces the interstellar medium of another galaxy in thehost cluster (Best et al., 1997). Other cases of jets and gas cloudsaffecting each other, such as 3C 324 and 3C 368 (Best et al.,1998), have also been known for over a decade. A very clear caseof a jet striking a companion galaxy is present in the RG 3C 441(Lacy et al., 1998).

Additional examples of this phenomenon have been reportedmore recently. A multi-wavelength (radio through X-ray) set ofobservations of 3C 321, a powerful RG, clearly shows that a jet ishitting a companion galaxy about 6.2 kpc from the nucleus; thisgalaxy is almost certainly in the process of merging with the hostelliptical and is also an AGN (Evans et al., 2008). The main, nearlystraight, jet is still seen, but a diffuse stream of synchrotron emit-ting plasma is seen to emerge at a large angle to the jet and extendover 100 kpc; this morphology is very similar to that found in somesimulations of relativistic jets striking dense clouds (Choi et al.,2007). In 3C 17, a powerful hybrid morphology radio source, or HY-MORS (Gopal-Krishna and Wiita, 2000), there is an FR II type lobeon one side of the nucleus and an abruptly bent, FR I type jet on theother (Massaro et al., 2009). Here the observed bend may be exag-gerated through projection effects and need not be due to a jet/cloud interaction, but that jet only starts to become visible, pre-sumably because of dissipation induced by entrainment, uponapparently slicing through gas within a spiral galaxy companion(Massaro et al., 2009). Two other HYMORS, 3C 433 and 4C 65.15,show abrupt large bends in the FR I jet sides, and have recentlybeen observed by Chandra (Miller and Brandt, 2009). The combina-tion of radio, X-ray and optical data strongly suggests that thebends in both of these RGs are caused by the jets interacting withmoving gas associated with companion galaxies (Miller andBrandt, 2009).

In response to the lateral kick and the mass loading, the decel-erated jet emerging out of the shell would bend and flare. Theresulting radio plume, during the initial stage when it is stillover-pressured relative to the surrounding medium, would ex-pand supersonically. Although no simulations to date have trea-ted this complex three-dimensional situation in sufficient detail,it certainly is plausible to expect that the mass loading wouldbe non-uniform. In that case, the more mass loaded, hence den-ser, portion of the post-collision flow could gradually gravitate

Gopal-Krishna, P.J. Wiita / New Astronomy 15 (2010) 96–101 99

toward the host galaxy, but the lighter and less contaminatedportion of the synchrotron plasma of the plume would be ex-pected to rise buoyantly outward from the galaxy.

In the specific case of Cen A, the NML could arise through thecollision of a moving shell of dense gas passing through (at leastpart of) the jet. The densest portion of the NML, in which themost thermal gas was entrained, gravitates toward the galaxy,and has settled back onto the intervening HI shell, explainingthe striking alignment of the NML base with the HI shell mappedby OM (their, and our, Fig. 1); whereas the lighter radio plasma ofthe NML is rising buoyantly and gradually merging with thenorthern outer radio lobe. If enough thermal matter has been en-trained and the magnetic field is dominated by structures of largeenough size, then more Faraday depolarization would be ex-pected in the denser region. This interpretation is consistent withpolarization measurements of the NML (Morganti et al., 1999),which show that its outer (NE) part has a much higher fractionalpolarization than does the inner (SW) portion. It appears that theshell has recently moved past the jet’s direction, allowing the jetto resume propagation along its original direction, and this is pre-sumably visible as the ridge of radio emission on the SE side ofthe NML, which possesses the lowest fractional polarization inthe NML. Parts of the cool shell material blown away and com-pressed by the jet during their encounter are probably now seenas the young stars and optical emission-line filaments ionized bythe UV nuclear beam, a process already discussed by otherauthors (Morganti et al., 1991; OM; Hardcastle et al., 2009). Theapparently young thermal X-ray emitting knots reported by Kraftet al., 2009) would also be energized by this restarting flow butwould not be massive enough to disrupt it and are consistentwith our scenario. Both the observed close alignment of thesouthern edge/base of the NML with the HI shell, as well as theirobserved segregation, can easily be understood in this scheme,but would just be coincidences in the Saxton et al. (2001) modelmentioned in Section 2.

If the reestablished jet leaving the inner northern lobe is to besignificantly disrupted by the collision with a shell we require sig-nificant transfers of both momentum and energy from the gas tothe jet (e.g., Wang et al., 2000). A criterion for complete disruptionthat must by satisfied is that the rate of mass entering the jet fromthe shell, _Ms > _Mj, the mass flux in the jet (e.g., Hubbard and Black-man, 2006; Gopal-Krishna et al., 2008). If the jet is non-relativistic,

_Mj ¼ pR2j V jqj; ð1Þ

where Rj;Vj and qj are the radius, velocity and density of the jet.More generally, provided the jet power, Lj, is mostly kinetic, thenfor a relativistic flow with bulk Lorentz factor Cj we have_Mj ¼ Lj=Cjc2. At the distance, dj ’ 13 kpc, where the jet interacts

with the shell at the base of the NML, Rj ’ 1:8 kpc. The thermalemission from the core of Cen A strongly indicates that it hosts anAGN with bolometric luminosity, Lbol ’ 1� 1043 erg s�1 (Whysongand Antonucci, 2004) and we can write Lj ¼ �Lbol with� 0:1 < � < 1 as the most likely range. A consistent value ofLj ’ 6� 1042 erg s�1 is independently estimated by Kraft et al.,2009). Combining these expressions we have

qj ¼�Lbol

pR2j V jCjc2

’ 4:1� 10�33 �bjCj

g cm�3; ð2Þ

with bj ¼ Vj=c. At first we take the shell to have a uniform density,qs, a velocity perpendicular to the jet, Vs, and a thickness along thedirection of the jet, ds. Then the time the shell takes to cross andfully interact with the jet over its area of � pR2

j is ss ¼ 2Rj=Vs andwe have

_Ms ¼ dspR2j qs=ss ¼ ðds=2RjÞpR2

j qsVs: ð3Þ

The velocity of the shell at the base of the NML can be deter-mined through its H I velocity gradient and corresponds to a circu-lar velocity with Vs ’ 250 km s�1; this is consistent with a mass inCen A out to that distance of about 2� 1011 M� (Schiminovichet al., 1994).

Using Eqs. (1)–(3), the necessary condition for the jet to be dis-rupted due to mass loading by the gas associated with the stellarshell becomes

qs > 4:9� 10�30 2Rj

ds

� ��ð1� b2

j Þ1=2 g cm�3: ð4Þ

The map in Schiminovich et al. (1994)) indicates that ds ’ 3 kpc’ 2Rj so the first quantity in parentheses is ’ 1. The volume of theshell is uncertain, but its extent in the plane of the sky is �6 kpc,and assuming a similar depth along the line of sight we obtain a to-tal volume of � 108 kpc3. Multiplying this by the density and not-ing that both � and ð1� b2

j Þ1=2 are < 1 we find that a gaseous mass

associated with the shell of Ms > 8:2� 103 M� is necessary to dis-rupt the jet. This is much less than the estimated masses in H I of5:7� 107 M� (OM) or H2 of 1:7� 107 M� (Charmandaris et al.,2000) in the relevant shell.

We also need to check that the shell can exert a sufficient forceto bend the jet if the disruption is not complete, as the observedmorphology could also be consistent with this possibility. A de-tailed analysis of jet bending by moving external media for differ-ent types of jets is given by O’Dea (1984) but we follow the slightlysimplified approach of (Walker et al., 1987), where relativistic andadiabatic terms are absorbed into the efficiency factor for conver-sion of bulk energy into energy radiated by the jet. From Eq. (7)of Walker et al., 1987) we have

qsV2s

h¼ 2Lj

VjpR2j Rc

; ð5Þ

where Rc � 5 kpc is the typical radius of curvature of the bending jetin the NML of Cen A and h ’ ds ¼ 3:2 kpc is the scale-height overwhich the bending ram pressure is exerted. Taking the combinedmasses of the atomic and molecular gas in the shell to be7:4� 107 M� and the volume to be 108 kpc3, the mean density isqs ¼ 4:6� 10�26 g cm�3, close to the value used by Saxton et al.,2001) in their simulation. Substituting in the other values fromabove we obtain the bound that Lj < bj6:5� 1043 erg s�1. Since thebolometric power of Cen A is Lbol ’ 1:0� 1043 erg s�1 (Whysongand Antonucci, 2004), this constraint is easily satisfied unlessbj < 0:15�, and since � < 1, this possibility seems highly unlikely.Note that direct measurements of the proper motions in the inner-most kpc of the jet implies bj > 0:5 in that region (Hardcastle et al.,2003).

Therefore, if the shell has an essentially uniform density andcompletely intercepts Cen A’s jet it is to be expected that the jetwill be strongly affected, and very probably completely disrupted.However, the shell’s filling factor is unknown, but the high resolu-tion HI map in OM clearly shows significant structure. So if much ofthe gas is concentrated in dense clumps then the jet disruptionneed only be partial. Each very dense region will exert a strongerimpact on, and provide more gas that can be entrained by, the par-ticular portion of the jet through which it passes than does theintervening lower density gas from the shell moving at essentiallythe same velocity. This non-uniform impact and entrainment couldnaturally produce the substantial spreading of the flow that is seenin the NML but would require too many additional parameters tomodel effectively.

Still, the ratio of >103 between the observed and the likely tocause disruption shell masses implies that such a fate is certainlyto be expected. Furthermore, the shell is presently just offset tothe NW of the jet and forms the inward edge/base of the NML

100 Gopal-Krishna, P.J. Wiita / New Astronomy 15 (2010) 96–101

(Fig. 1). The morphology of the NML (Fig. 4 of Morganti et al., 1999)seems to indicate either that the jet is reestablishing itself along itsoriginal direction to the NE or that it was never totally disrupted.The latter certainly is possible if the shell is not very deep alongthe line-of-sight and part of the jet is somewhat in front of or be-hind it. In that case, the NML can still be produced through the par-tial disruption and/or more a less effective bending of the jet aslong as it has at least a modestly relativistic velocity. However,the apparent age of the X-ray emitting knots in the NML of just afew Myrs (Kraft et al., 2009) argues in favor of the jet having re-cently been reestablished after a (nearly) complete disruption bythe shell.

Here we suggest that there is a basic similarity between the jet–shell configurations associated with the NML and the northern in-ner radio lobe. In each case, the jet appears to have encountered ashell (presumably rotating clockwise), gotten deflected toward thenortheast and soon flared up into a lobe whose edge facing thegravitational center (the parent elliptical) is fairly flat and runsnearly parallel to the respective HI shell. These jet–shell interac-tions in Cen A are illustrated by Fig. 2a of G-KS and Fig. 1 of Morg-anti et al. (2008) (see also Fig. 1 of Burns et al., 1983). A radio mapbased on all available VLA L-band data (http://www.extragalac-tic.info/~mjh/CENA.LABBCCD-I.FITS) reveals that in the Northerninner lobe some low-level emission is present south of the ‘flatedge’. This low-brightness extension could either mean that thisradio lobe does not cut off sharply on the inner side, or the faintextension seen inward of the shell is actually in front of or behindthe shell in projection rather than passing through it. In any event,the presence of a strong brightness gradient is a highly suggestivemarker of a strong physical interaction between the radio plasmaof the northern inner lobe and the shell just to the south of it.

4. Some wider implications

One basic difference from the repetitive nuclear ejection sce-nario for explaining the multi-peaked radio lobe pairs of the typeseen in Cen A (Section 1) is that the present model does not de-mand that the outer radio peaks/lobes be necessarily older thanthe inner lobes. This is because in this model the formation of a gi-ven radio peak/lobe would have taken place whenever a (rotating)shell moved into a position where it began to interrupt the jet flow.In the case of Cen A, as mentioned previously, there is some evi-dence for a shell interacting with the southern jet as well, whichcould be responsible for the southern inner lobe (G-KS). The ab-sence of a southern middle lobe can be attributed to the lack of ashell at that location to interact with that jet.

Another well known phenomenon where the jet/gaseous shellinteractions may play a significant role is the Wide-Angle-Tail(WAT) radio galaxies whose jets are found to flare up abruptly aftera well collimated propagation for tens of kiloparsecs (e.g., O’Deaand Owen, 1985; O’Donoghue et al., 1990, 1993; Hardcastle andSakelliou, 2004). Higgins et al. (1999) have in fact attributed thejet flaring in WATs to its encounter with a clump of the intraclustergas. However, deep X-ray imaging available for the well knownWAT, 3C 465, shows no discontinuity in the external medium atthe locations of the jets flaring (Hardcastle and Sakelliou, 2004).We suggest the putative jet-disrupting gas clumps could in factbe HI and/or H2 shells, similar to those detected in the nearestradio galaxy Cen A, that would not be expected to be detected asX-ray sources; however, an abrupt reduction in X-ray surfacebrightness can indicate the presence of the colder gas if the obser-vation is sufficiently sensitive (Kraft et al., 2008). Note that, to firstorder, the shells are usually found located symmetrically in oppo-site quadrants of the parent elliptical galaxy (e.g., Quinn, 1984).This symmetry of the shells, in the context of the mechanism pro-

posed here, could explain why the jet in WATs are often found toflare up at similar distances from the host galaxy, a result very hardto understand in some alternative models for jet flaring, e.g., a ran-dom jet-cloud collision scenario suggested by Higgins et al. (1999).While models involving the crossing of an ISM/ICM interface mightalso produce symmetric WAT flaring, we are unaware of any fea-ture indicative of such an interface being found at such locations.

Further, the jet–shell interaction scenario may provide a consis-tent explanation for some other intriguing correlations exhibitedby double radio sources. Recall that successive faint shells detectedaround several nearby ellipticals not only extend up to radial dis-tances exceeding 100 kpc but are distinctly more conspicuouswithin the quadrants along the (optical) major axis of the elliptical(e.g., Malin et al., 1983; Quinn, 1984). It follows that both the epi-sodes of jet blocking and the lateral distortion of the radio lobe orbridge, arising from the interaction of the synchrotron plasma withthe shell material, would be much more pronounced in situationswhere the jets emerge roughly along the major axis of the hostelliptical.

This enhanced probability of jet–shell interaction could simul-taneously explain two known statistical predilections of radio gal-axies. First, off-axis distortions of radio lobes/bridges in doubleradio sources are found to be much more common in sourceswhose radio jets happen to emerge close to the major axis of theelliptical host (Capetti et al., 2002; Saripalli and Subrahmanyan,2009). Second, double radio sources oriented closer to the opticalmajor axis have significantly smaller physical dimensions, as indi-cated by early work (Palimaka et al., 1979; Guthrie, 1980) and nowconfirmed for a larger data set by Saripalli and Subrahmanyan(2009). This too can be readily understood as an outcome of the en-hanced jet–shell interactions expected in the major-axis quad-rants. On the other hand, it is possible that this secondcorrelation might also arise merely from those jets encounteringless galactic gas if they propagate along minor axes, so the jet–shellinteraction is not a unique explanation for this particularcorrelation.

Note also that since the lobe distortions (‘wings’) arising fromthe jet–shell interactions are basically inversion symmetric in nat-ure (G-KC), this specific mechanism may also account for some ofthe interesting class of X-shaped RGs (XRGs) (e.g., Leahy andParma, 1992; Capetti et al., 2002). The numbers of known XRGshas recently burgeoned thanks to a search of FIRST Becker et al.,1995 radio maps by Cheung (2007). In cases where the jets remainstalled by shells for very long times the lateral extensions of thebridges (‘wings’) may even become similar in size to the mainlobes, as found for many XRGs. Further, the Z-symmetry of thewings detected in several XRGs could naturally arise in this picture,where jets encounter rotating shells on both sides of the galaxy(Gopal-Krishna et al., 2003). As discussed above, the shells are be-lieved to be engendered during the galactic merger process. Thatsame merger could reorient the central engine during the subse-quent supermassive black hole merger and ‘‘spin-flip” that wouldyield copious gravitational radiation and can also account for thenew direction of jets that can produce the basic XRG morphology(e.g., Zier and Biermann, 2001).

5. Conclusions

The jet–shell interaction scenario (Gopal-Krishna and Chitre,1983) originally invoked to explain the multi-peaked, inversionsymmetric radio lobes of Centaurus A (Gopal-Krishna and Saripalli,1984), received a major boost from the discovery of large amountof dense atomic and molecular gas associated with the shells(Schiminovich et al., 1994; Charmandaris et al., 2000). Here wehave argued that this model can also explain the intriguing

Gopal-Krishna, P.J. Wiita / New Astronomy 15 (2010) 96–101 101

northern middle radio lobe in Cen A. The combined morphologiesof the NML and the stellar and gaseous shells very strongly suggestthat we are witnessing the final phase of a large shell crossingthrough the jet. Gas from the associated dense clouds has at leastpartially, and probably only temporarily, affected the jet whilethe jet’s passage has induced star formation in some of the denserfilaments. Deeper X-ray studies of the NML/shell region could pos-sibly provide further support to this model for the NML, as the re-cent Kraft et al. (2008) Chandra observations of the inner jet andshell did for the GK-S proposal.

Because many RGs seem to be triggered by mergers, particularlymergers of cold gas-rich spirals with large ellipticals, their jets mayfrequently find themselves traversing recently produced stellarshells, at least some of which will have substantial associated gas-eous components. These interactions can be important and we sug-gest that such encounters may have a significant role to play in theorigin of prominent radio morphological classes, such as theZ-symmetric X-shaped radio galaxies and Wide-Angle-Tail radiogalaxies. Jet–shell interactions also can naturally explain the dis-covery of stronger distortions of the radio bridges occurring insources whose radio axes are more closely aligned with the majoraxes of the corresponding parent elliptical galaxies. However,those more distant galaxies cannot be studied to a similar levelof detail as currently possible for Cen A, so testing this hypothesiswill require the next generation of radio and X-ray telescopes.

Acknowledgements

We thank Tom Oosterloo for permission to use their figure andthe anonymous referee for suggestions that improved the presen-tation of our arguments. PJW is grateful for hospitality at NCRA,where much of this paper was written.

P.J.W. acknowledges support from a sub-contract to GSU fromNSF Grant AST-0507529 to the University of Washington.

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