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HHUGE AMOUNTS OF ENERGY

are produced through theaccretion of material ontothe super-massive blackhole situated in the centre

of an Active Galactic Nucleus (AGN).This energy is released into the surround-ing medium in a number of differentways, ranging from collimated radio-plas-ma jets to UV and X-ray emission.The re-gions around the AGN are, therefore,highly complex and host a wealth of phys-ical processes. Gas in different phases(atomic, molecular and ionised) is ob-served in this very hostile environmentand the highly energetic processes relatedto the presence of the AGN are likely tohave a profound influence on the physicaland kinematical properties of this gas.Thus, the study of the gas provides idealdiagnostics to trace the relative impor-tance of the various processes and the ef-fect that the AGN has on the surroundingmedium.

The energy released from the nucleuscan produce gas outflows at very high ve-locities (thousands of km/s) as seen inmany AGN, ranging from Seyfert galaxiesto quasars. Gas outflows are detected asblueshifted absorption or emission linewings in optical, UV and X-ray spectra,(see e.g. Crenshaw 2001 and refs therein).

A number of mechanisms can be con-sidered for the origin of such outflows. Inradio-loud objects, they could be drivenby the interaction of the radio plasmawith the (rich) gaseous medium in the di-rect vicinity of the active nucleus. This ef-fect is, for example, particularly evidentin young radio sources where the newlyborn radio jet is making its way out of thegalaxy. In high-z radio galaxies, such jet-cloud interactions are believed to be amechanism for triggering star formationand indirectly explaining, e.g., the align-ment between the rest frame optical con-tinuum and their associated radio sources(van Breugel 2000).

However, a complication is that fastgas outflows have also been found in ra-dio quiet AGNs. This has led some inves-tigators to suggest that other mechanismsare at work and that some of these out-flows are more likely driven by radiation-or wind pressure from the regions nearthe active super-massive black hole (i.e. aquasar wind).

A very interesting and surprising as-pect of these outflows is that, despite thehigh energies involved, they are not onlyseen in ionised gas but also in neutral gas.A few examples of this phenomenon arenow known. The first case where such afast outflow of neutral gas was detected is

the Seyfert galaxy IC 5063. Using theAustralia Telescope Compact Array, wedetected HI absorption up to 700 km/sblueshifted with respect to the nucleus inthis galaxy, indicating an outflow of neu-tral hydrogen with at least these velocities(see below). More recently, using theWesterbork Synthesis Radio Telescope,we have found even faster outflows of HI(up to 2000 km/s) in the radio galaxies3C293 and 4C12.50. These observationsraise the interesting question of whichmechanism can drive these fast neutraloutflows. Large amounts of energy areneeded to push the gas out at these highvelocities, but despite these high energies,some fraction of the outflowing gas re-mains, or becomes, neutral.

The study of the ionised gas can pro-vide a key complement to the neutral hy-drogen and help in solving this puzzle.For this reason we have made a detailedoptical follow-up study of the Seyfertgalaxy IC 5063 using EMMI on the NTT.In general, Seyfert galaxies are particu-larly interesting objects for investigatingthe effects described above. They havestrong emission lines coming from theionised gas. Their narrow-line regions(NLRs, regions of highly ionised gas im-mediately surrounding the active nucle-us) can be extremely complicated kine-

IC 5063: AIC 5063: AGN GN DRIVENDRIVEN OUOU TFLOTFLOWW OFOF

WWARMARM ANDAND COLDCOLD GG ASAS

THE SOUTHERN SEYFERT GALAXY IC 5063 WAS THE FIRST AGN WHERE A FAST OUTFLOW OF NEUTRALHYDROGEN WAS DETECTED. HERE WE PRESENT NEW OPTICAL SPECTRA THAT WERE TAKEN WITH THEESO-NTT TO COMPARE THE KINEMATICS OF THE IONISED GAS WITH THAT OF THE NEUTRAL HYDROGENCOMPONENT IN ORDER TO STUDY THE MECHANISM THAT COULD DRIVE THIS OUTFLOW. THE DATA REVEALEXTREMELY COMPLEX GAS KINEMATICS, INCLUDING THE PRESENCE OF AN OUTFLOW OF IONISED GAS(WITH SPEEDS OF SEVERAL HUNDRED KM/S) AT THE LOCATION OF THE BRIGHTER RADIO LOBE. THISOUTFLOW IS STRIKINGLY SIMILAR TO THAT OF THE HI. WE CONSIDER THE INTERACTION BETWEEN THERADIO JET AND THE ISM TO BE THE MOST LIKELY MECHANISM FOR THE EXTREME KINEMATICS AND APOSSIBLE SCENARIO IS DESCRIBED.

R. MR. MORGORG ANTIANTI11, , TT. O. OOSOSTERLOOTERLOO11, J, J. H. HOLOLTT2,32,3, , C. C. TTADHUNTERADHUNTER22, J, J.M. .M. VVANAN DERDER HHULSULSTT33

1ASTRON, Netherlands Foundation for Research in Astronomy, NL; 2Dep. of Physics and Astronomy, Sheffield University, UK;

3Kapteyn Astronomical Institute, Groningen, NL

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matically. Some of them are believed torepresent the best examples of regionswhere interactions between the local in-terstellar medium (ISM) and radio plas-ma occur (Wilson 1997, Capetti et al.1999). However, recent results based onHST observations (e.g. NGC 4151 andNGC 1068) have indicated that insteadquasar winds may play a major role. Theytherefore represent ideal objects to studythe importance of the impact of the AGNon its surrounding ISM. IC 5063 is, givenits proximity and its characteristics, a per-fect candidate for such a study. Apartfrom being an interesting case by itself,the results from this galaxy can shed lighton what is going on in AGNs that aremore distant, i.e. objects that cannot bestudied in the same detail, such as radiogalaxies and quasars.

THE INTRIGUINGSEYFERT GALAXY IC 5063

The southern Seyfert galaxy IC 5063 (z =0.0110) is classified as a Seyfert 2.According to the unified schemes forAGNs, this means that the broad-line re-gions (lines coming from ionised gas inclose proximity to the AGN) are ob-scured from our direct view by the nu-clear torus, but that it can be seen indi-rectly in scattered/polarized light. Strongand broad polarized Hα emission has in-deed been observed in IC 5063.

IC 5063 is an early-type galaxy − notvery common among Seyfert galaxies −that shows a complicated system of dustlanes. This is clearly seen from the HSTimage in Figure 1.The extended (about 15kpc in radius) ionised gas has a very pe-culiar X-shaped morphology shown inFigure 2. This gas is ionised by photonsfrom the AGN and the interesting shapeis probably due to the obscuration of the

AGN by the large-scale warped gas discthat is visible as the system of dust lanes.

The most intriguing characteristic ofthis object is seen in the radio (see Mor-ganti et al. 1998 and Oosterloo et al. 2000for details). IC 5063 is among the most ra-dio-loud Seyfert galaxies known. In theradio continuum, it shows triple structure(Figure 1) of about 4 arcsec in size (about1.3 kpc). Two very asymmetric lobes aresituated at each side of the radio core. De-spite being an early-type, the host galaxyis very rich in neutral hydrogen (almost1010 M�). A large, regularly rotating HIdisc (of about 30 kpc in radius) is ob-served (Figure 2). Because of the strongradio continuum source, we were able todetect HI gas in absorption at very lowoptical depth.This absorption is indicatedby the dashed contours in Figure 1, whilethe solid contours indicate the emissionfrom the large HI disc. The absorption ishighly blueshifted with respect to the cen-tre of the galaxy. The systemic velocitycan be accurately derived from the HI

emission, and the absorption is unam-biguously blueshifted. Follow-up VLBIobservations showed that the broadblueshifted absorption occurs against thewestern and brighter radio lobe and notagainst the core (Oosterloo et al. 2000).

The obvious question is: what mecha-nism can produce such a fast outflow ofgas, allowing it to remain, or becomeagain, (partly) neutral? As mentionedabove to better understand what is goingon, the radio study needs to be comple-mented by a detailed study of the ionisedgas. In particular, the kinematics and theionisation level of the gas can shed lighton whether a strong jet/cloud interactionis responsible for the outflow, or whetherother processes must be invoked.This wasthe goal of our NTT observations.

THE COMPLEXOPTICAL SPECTRUM

IC 5063 was observed (in service mode)with EMMI on the NTT in the mediumresolution spectral mode. The 0.8 arcsec-

Figure 2: Left: the HI disk (contours) superimposed onto the image of the ionised gas ([OIII] 5007Å).Right: ionised gas (contours) superimposed onto the radio continuum (grey scale). In the centralregions, there is an evident correspondence between the two.

Figure 1: HST imagewith superimposedthe radio continuum(from ATCA at 8GHz). The radioshows three compo-nents, of which themiddle one is thecore. On the right isthe position-velocitydiagram of the HIshowing the broad,blueshifted absorp-tion (dashed). Thelocation in the con-tinuum image wherethe broad HI ab-sorption occurs isindicated.

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wide slit was positioned along the radioaxis. Spectra were taken in the blue andthe red arm simultaneously. Gratings 12and 7 were used, giving a spectral resolu-tion of 0.92 and 0.66 Å/pix in the blue andin the red respectively. Therefore, wecover the wavelength range from [OII]3727Å to [SII].

The spectrum shows a wealth of infor-mation with many emission lines, up tovery high ionisation − in particular highionisation Fe lines (as found by Colina etal. 1991). Most of the lines are spatiallyextended and because of the sub-arcsec-ond seeing of the observations, we canclearly separate the ionised gas at the lo-cation of the radio core from the locationof the lobes, as illustrated in Figure 3.Thisis crucial for the success of the study. Infact, it allows us to investigate how thekinematics of the ionised gas relates withthe radio structure. The extremely com-plex kinematics can clearly be seen in Fig-ure 3. Different components are evident,with emission both from the disturbed gasas well as from the quieter and regularlyrotating gas (i.e. the gas that follows thegalaxy rotation as shown by the largescale HI emission). The region of highlydisturbed kinematics extends almost 6arcsec (almost 2 kpc), mostly in coinci-dence with the region where the radioemission is also present. Indeed, Figure 3already shows an important result of thisstudy, namely that the most extreme kine-matics are detected in the region betweenthe radio core and the bright radio lobe,and that, as for the neutral gas, a largeblueshifted wing is clearly present nearthe western radio lobe. The first ordersimilarity between the kinematics of theblueshifted wing of ionised gas and thatof the HI is illustrated in Figure 4, wherea direct comparison of the two can bemade.

One interesting point to note is thatthe most extreme blueshifted velocitiesare displaced compared to the peak of theradio lobe and they appear to be locatedcloser to the nucleus. The velocities be-come progressively less blueshifted asone approaches the peak of the radiolobe, producing a sort of “comma”-likeshape. This can naively be interpreted asan indication of a decelerated flow: theflow gets slower as it approaches the po-sition of the radio lobe where perhapssome “obstructing” material is present.This is, in fact, consistent with the detec-tion of molecular gas (H2) from NICMOSobservations (Kulkarni et al. 1998) in thewestern region near the radio lobe.

In addition to the blueshifted wing, avery broad component is observed only inthe region between the core and the west-

ern (stronger) radio lobe. This very broadcomponent of ionised gas, however, doesnot appear to have a counterpart in theneutral gas. This may represent a compo-nent of highly shocked gas that is onlyseen as ionised.

Taken all together, these properties clear-ly indicate that the interaction between theradio plasma with the ISM is responsiblefor the extreme kinematics observed bothin the ionised and in the neutral gas.

Apart from the complex kinematicsdetected in the western region, theionised gas in the eastern side also showscomplex kinematics, in particular linesplitting. Interestingly, this seems to ex-tend beyond the radio emission (see be-low). It is clear that the gas kinematics onthe two sides of the radio core are com-pletely different, possibly reflecting somemajor differences in the properties of theISM and/or that there are two differentmechanisms acting.

Although it is obvious from Figure 3that the kinematics are very complex, aquantitative analysis is required to fullyidentify the various kinematical compo-nents and to study their characteristics.As a starting point, we have used thestrong [OIII] 4959, 5007Å doublet. Agood description of each line typically re-quires between two and four Gaussiancomponents. The same components wereused to fit other emission lines (e.g. Hβ,[SII], Hα+[NII]) where only the relative

amplitudes of the various componentswere allowed to change. These restrictedfits gave good results in almost all cases.

Using the components found from theGaussian fits, one can study the ionisationof the gas by, for example, looking at theratio (for each component) of the [OIII]5007Å and the Hβ emission lines.Although the extreme kinematics nearthe western lobe appear to be due to ajet/cloud interaction, that does not neces-sarily mean that all the gas is ionisedthrough this mechanism. The ratio[OIII]/Hβ is found to be very high (> 10)for the narrow components close to thecentre (up to a few hundred parsec fromthe nucleus). This component, beingnarrow, is likely not to be influenced at allby the interaction with the radio plasmaand therefore the high ionisation is likelyto be due to the UV radiation from thenucleus. In the remaining regions/compo-nents, the [OIII]/Hβ ratio is between5 and 10, without any clear differencebetween the regions with extreme kine-matics or the broad components.This sug-gests that the shocks, produced in theinteraction between the radio plasma andthe ISM, are not the dominant ionisationmechanism (even in the NW region).

This confirms what was derived fromthe energy budget argument (Morganti etal. 1998) and also what has been foundfrom similar studies of radio galaxies, e.g.Cygnus A.

Morganti R. et al., IC 5063

Figure 3: (Top) The spectral region from [OI] 6300Å to [SII], including the Hα+[NII] lines.With the contrast used, the different intensity of the lines allows to see different features:from the regular galaxy rotation at larger distance from the centre to the complex kinemat-ics clearly seen especially from the [OI] lines. (Bottom) A zoom-in of the [OIII] lines. The ra-dio image on similar scale is also shown. This allows a direct comparison between the struc-tures observed in the radio and the kinematics of the ionised gas.

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Finally, the density can be measured us-ing the ratio of the [SII] 6717/6731Å lines.The narrow component traces a decreasein the density going from almost 1000particles cm−3 close to the nucleus to thelow-density limit (<100 particles cm−3) inthe outer regions. Part of the reason forthe decline of the [OIII]5007Å /Hβ can bethe result of this density decrease.

A POSSIBLE SCENARIOThe kinematics of the ionised gas and thesimilarities compared to that of the neu-tral hydrogen, support the idea that weare observing a gas outflow in the regionof the stronger radio lobe due to interac-tion between the radio jet and the ISM.This situation is conceivable because inIC 5063, the radio jet appears to be ex-panding in the galactic disc, thereforemoving through a rich medium, as evi-denced e.g. by the large-scale HI disc seenin emission, as well as by the presence ofmolecular gas from NICMOS observa-tions (Kulkarni et al. 1998) detected in thewestern region near the radio lobe.

However, the question remains: why, insuch a hostile environment, is some frac-tion of the gas neutral?

A possible scenario, illustrated in Fig-ure 5, is that the radio plasma jet is inter-acting strongly with a molecular cloud inthe ISM. Part of the gas is kinematicallydisturbed by the shock that is producedby the interaction. Once the shock haspassed, some fraction of this gas may havethe chance to recombine and becomeneutral, showing up as neutral gas at highvelocities. To understand whether such ascenario is feasible, it is worth consideringthe model proposed for the evolution ofclouds in radio galaxy cocoons when theyare overtaken by a strong shockwave

(Mellema, Kurk & Röttgering 2002). Thismodel predicts that, as the shock runsover a cloud, a compression phase starts(as the cloud gets embedded in an over-pressured cocoon) and the shock wavesstart travelling into the cloud and thecloud fragments. By taking cooling intoaccount, they derived that the excess ofenergy is radiated away on a time scalemuch shorter (a few hundred years) thanthe typical lifetime of a radio source(106−7 yr). This process results in the for-mation of dense, cool and fragmentedstructures at high velocities. These cloudscan contain neutral gas that can be seen inabsorption, if located in front of the radiosource. Whatever the details of theprocesses involved, the observations ofIC 5063 show that neutral and ionised gasoutflows co-exist and that the interactionbetween the radio plasma and the ISM islikely to produce both. This puts strongconstraints on the physical processes inthe centre of this galaxy.

What is the situation on the other side(i.e. the SE side) of the nucleus? Apartfrom the emission associated with theregularly rotating large-scale gas disc, asecond component is seen, blueshiftedcompared to the quiescent gas and ex-tending beyond the radio emission. It ishard to explain this component only by ajet-cloud interaction and the line split isperhaps more reminiscent of an expand-ing cocoon. It is worth noting that in oth-er Seyfert galaxies radiation pressurefrom the nuclear emission has been pro-posed as the dominant accelerationmechanism. In addition to this, the clearasymmetry between the two sides of theradio source suggests that the conditionsof the medium around the AGN are actu-ally very inhomogeneous. This is likely to

strongly affect the evolution of the radioplasma and its effect on the environment.

The detailed study of the nearbySeyfert IC5063 has a number of implica-tions for other objects that cannot bestudied with the same detail as IC 5063. Inparticular, we are now also finding broadblueshifted HI absorption in more distantpowerful radio galaxies (the best exam-ples so far are 3C293 and 4C12.50, Mor-ganti et al. 2003). Moreover, in the highredshift universe, radio galaxies appear tolive in very gas rich environments. Stronginteraction between the radio plasma andthis medium is likely to be even morecommon.What we learn from the study ofnearby objects can therefore be of greathelp in interpreting what happens in theirfaraway cousins.

ACKNOWLEDGMENTSJH acknowledges a PPARC PhD stu-dentship and a NOVA-Marie Curie Fel-lowship. She also wishes to thank theKapteyn Institute, Groningen, for its hos-pitality where most of this work was done.

REFERENCESCapetti A., et al. 1999, ApJ 516, 187Colina L., Sparks W.B., Macchetto F. 1991, ApJ

370, 102Crenshaw D.M. 2001, Science, 292, 1500Kulkarni et al. 1998, ApJ, 492, L121Morganti R., Oosterloo T., Tsvetanov Z. 1998,

AJ 115, 915Morganti R., Oosterloo T. Emonts B., van der

Hulst J., Tadhunter C., 2003 ApJ Letters, inpress

Oosterloo T., Morganti R., Tzioumis A.,Reynolds J., King E., McCullogh P., Tsve-tanov Z. 2000, AJ 119, 2085

Van Breugel W. 2000, Proc. SPIE Vol 4005, p.83-94, in Discoveries and ResearchProspects from 8 – to 10-Meter-Class tele-scopes, J. Bergeron eds. (astro-ph/0006238)

Wilson A. 1997, in ASP Conf. Ser. 113, EmissionLines in Active Galaxies, ed. B. Peterson,F.Z. Cheng & A. Wilson, p. 264

Figure 4: Comparison between the width of theHI absorption (white profile) and that of the ionisedgas (from the [OIII]5007Å). The first order similar-ity between the amplitude of the blueshifted com-ponent is clearly seen.

Figure 5:A schematicdiagramillustratingthe proposedscenario (asdescribed inthe text) anda possiblegeometricarrangementof the variousemittingcomponentsin IC 5063.