Probing the nature of the ISM in Active Galactic Nuclei through HI absorption

0.42 MB·English

by  Raffaella Morganti

Proceedingsofthe7thEuropeanVLBINetworkSymposium
Bachiller,R.Colomer,F.,Desmurs,J.F.,deVicente,P.(eds.)
October12th-152004,Toledo,Spain

Probing the nature of the ISM in Active Galactic Nuclei through H
absorption
RaffaellaMorganti
NetherlandsFoundationforResearchinAstronomy,Postbus2,NL-7990AA,Dwingeloo,NL;[email protected]
5
0
Abstract. Thephysicalandkinematicalconditionsofthegassurroundinganactivegalacticnucleus(AGN)offerkeydiagnostics
0
forunderstandingtheprocessesoccurringintheinnerfewkpcaroundthenucleus.Neutralhydrogencangiveimportantinsights
2
ontheseregions.Apartfromprobingthepresenceofgasinrelativelysettledconditions(i.e.circumnucleardisks/tori)itcanalso

n tracethepresenceofextremeoutflows.Someexamplesofthesephenomena arebrieflypresented.Forthestudyoftheneutral
a hydrogen around AGN thehigh resolution offeredby theVLBI iscrucial inorder to locatetheregions where theabsorption
J
occursandtostudyindetailthekinematicsofthegas.RecentVLBIresultsarediscussedhere.
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1. Introduction

1
Thepresenceofneutralhydrogenintheregionsurroundingthe
v
4 activegalacticnuclei(AGN)isknownsincemanyyears.This
4 gas can be studied via absorption detected against the strong
5 continuumsource(seee.g.Heckmanetal.1983;vanGorkom
1
etal.1989;Morgantietal.2001,2002;Vermeulenetal.2003)
0
anditisnowknowntobeassociatedwithdifferentstructures.
5
0 Neutral hydrogen can be found in tori very close to the
/ AGN.Althoughithasbeengenerallyassumedthatthetoriare
h
composedofdustymolecularclouds,itisnowclearthat,under
p
- certainconditions,theycanbepartlyformedbyatomichydro-
o
gen (Maloney, Hollenbach& Tielens 1996). The HI can also
r
t be associatedwith largerscale circumnucleardisks(with size
s
a rangingfrom0.1and1kpc).Thesestructuresaresimilartothe
: nuclearoptical disks detectedin a large numberof early-type
v
i galaxies(bothradio-loudandradio-quiet).Thesedisks(mainly
X
detectedbyHST)canbeseeneitherinionizedgasorthrough
r theirstrongdustabsorption(vanderMarel2001,Capettietal.
a
2000andref.therein).
However,theneutralhydrogencanalsobeassociatedwith Fig.1. Histograms of the distribution of optical depth (τ)
more disturbed structures, like bridges or tails left over from
for Seyfert galaxies (Gallimore et al. 1999), radio galaxies
recent mergers. The origin of activity in galaxies is often ex- (Morganti et al. 2001, Morganti et al, in prep) and CSS/GPS
plained as triggered by merger and/or interaction processes.
radio sources (Vermeulen et al. 2003, Morganti et al. 2001).
HI is often seen associated to all these phenomena.The idea Filledregionsindicateupperlimits.
of mergeris supportedby morphologicaland kinematicalev-
idence (e.g. Smith & Heckman 1989, Tadhunter et al. 1989,
Baum et al. 1992). Torques and shocks during the merger canhaveontheinterstellarmedium(ISM).Thisfeedbackcan
can remove angular momentum from the gas in the merging be extremely important for the evolution of the galaxy, up to
galaxies and this providesinjection of substantial amounts of thepointthatitcouldlimitthegrowthofthenuclearblack-hole
gas/dust into the central nuclear regions (see e.g. Mihos & (e.g.Silk&Rees1998,Wyithe&Loeb2003).
Hernquist1996).Itis,therefore,likelythatintheinitialphase All the above illustrates how important is the gas in the
ofanAGN,thisgas,includingatomichydrogen,stillsurrounds study of AGN. The study of the neutral hydrogenis comple-
–andpossiblyobscurs–thecentralregions.AGN-drivenout- mentarytothestudiesoftheotherphasesofthegas-molecu-
flows have powerful effects on this dense ISM. Surprisingly, larandionized-intheseregions.Here,Ibrieflydiscusssome
the neutral hydrogen has been recently found associated also of the most recent results in this area. While the detection of
withsuchfastoutflows(upto2000 kms−1).Thisfindinggives theHIisusuallydonewitharcsecresolutionobservations,the
furtherinformationonthephysicalconditionsofthegasinthe VLBIfollowupiscrucialinordertobeabletounderstandin
environmentof AGN. Gas outflows generated by the nuclear wichoftheabovementionedstructurethegasislocatedandto
activity are particularly important because of the effects they deriveitsphysicalparameters.
RaffaellaMorganti:ProbingthenatureoftheISMinActiveGalacticNucleithroughHabsorption
2. DetectionofHIinabsorptioninAGN
RadiogalaxiesandradioloudSeyfertshavebeenthesubjectof
manyHI studies. As result, we knownow thatabout10-20%
200
ofradiogalaxiesshowHIabsorptionagainsttheirnucleiwhile 1814−637
100 2.3 GHz
this fraction goes up to more than 60% for Seyfert galaxies Vsys Broad optical
line
(oGfadlelitmecoteredeatbasol.r1p9ti9o9n).isApagrrtoicuuplaorflyobhjiegchtsawrehCeroemthpeacftraSctteioepn MilliARC SEC-1000
Spectrum(CSS)andGigahertzPeaked-Spectrum(GPS,O’Dea
-200
1997)sources.Vermeulenetal.(2003)foundabout50%ofde-
-300
tectionsintheseobjects.Thedepthofanabsorptionline(∆S)
depends on the optical depth (τ), the continuum flux density 300 200 100 MilliAR0C SEC -100 -200
Outflow
(S)andthecoveringfactorc as∆S = c S(1 eτ).Usually,a
f f Velocity
−
coveringfactor c = 1 is assumed.The distribution of optical
f
depth(τ)forthesethreegroupsofAGNisshowninFig.1.
Fig.2. LBA observations of the southern radio galaxy
Thesensitivityofpresentdaysradiotelescopesisthemain
PKS 1814-63 (Tzioumis et al. in prep.). The nucleus is ten-
limitationforthestudyoftheHIabsorption.Thetypicalopti-
tativelyidentifiedwiththenortherncomponent.Ablueshifted
caldepthobservedinthedetectedobjectsisτ 0.01 0.05(i.e.
∼ − shallow component of the absorption is detected. The most
the flux absorbed by the HI is few % of the radio continuum
blueshiftedcomponentappeartobelocatedagainstthesouth-
ofthesourceatthefrequencyoftheredshiftedHI).Thus,fora
erncomponent.Inordertoidentifythisasanoutflow,newac-
typicalobservationwithanoiselevelineverychannelofabout
curatemeasurmentsofthesystemicvelocitywasneeded(from
0.5mJybeam 1 (1-σ),thesevaluesofopticaldepth(detected
− opticaldata,Holtetal. in prep).Thenew value,derivedfrom
at 3-σ level) can be obtainedif the continuumis of the order
thenarrowcomponentintheopticalemissionlines,isindicated
of 50 100mJy.Itisclearthatthisisamajorlimitationfor
∼ − withthedashedline.Thedottedlineindicatethevelocityofthe
thestudyoftheHI(e.g.inweakradiosourcesorsourceswith
broadcomponentdetectedintheopticalemissionlines.
weakradiocores).Objectswithopticaldepthτ 0.10 0.20
∼ −
orlargerdoexistbuttheyarerare.
The histograms of Fig. 1 show that the detected Seyfert
galaxies have, on average, higher optical depth than radio
galaxies. CSS/GPS appear more often detected: the high flux as accurateas possible . Althoughthis soundstrivial, it is not
typicalofthese objectsallow to reachverylowopticaldepth. always an easy information to have available (at least at the
Ontheotherhand,thetypicalcorefluxofaradiogalaxyissel- level of accuracy required for the comparison with HI data).
domstrongenoughtoreachtheselimits.Therefore,thehigher It has been often pointed out (Mirabel 1989, Morganti et al.
detectionrateofCSS/GPSisaffectedbythisbias.However,it 2001) that the systemic velocity derived from emission lines
isalsothecasethatobjectswithhighopticaldeptharemissing can be both uncertain and biased by motions of the emitting
fromtheradiogalaxieswhileareobservedamongCSS/GPS. gas. This is indicated, in the most extreme situation, by the
Inparticularlybrightradiosources,weare,however,inthe detectioninfewobjectsoftwodifferentredshiftsystems,one
positiontolookforHIwithverylowopticaldepth(τ 0.001). derived from the low and the other from the high ionization
Recent observationsmakinguse of the broad band (∼20 MHz) lines. Probably the best example of this has been observedin
now available, e.g. at the upgraded WSRT, have shown that the radiogalaxyPKS 1549-79(Tadhunteretal.2001).Inthis
thekinematicsofthisgascanbeveryextreme.VerybroadHI object,the[OIII]5007Ålinesappearedtobeassociatedtogas
absorptionfeatureshavebeendiscoveredinthisway.Thiswill kinematicallydisturbed,likelyanoutflowduetotheinteraction
bediscussedinSec.5.3. withtheradioplasma.
Thecolumndensitiesoftheneutralhydrogenfollowfrom InthesouthernradiogalaxyPKS1814-63(seeFig.2),only
NHI = 1.83·1018TspinR τdv where Tspin is the spin tempera- thedetailedanalysisoftheopticalspectrumhasshownthatthe
ture in Kelvin and v is the velocity in kms 1. Assuming the emission lines are actually made of two components: a nar-
−
canonicalT = 100K,thecolumndensitiestypicallyfound rowonethatappearstotracethemoreextendedandquiescent
spin
areintherangefromfewtimes1019 atomscm 2 tofewtimes and,therefore,more likely to define the systemic velocity and
−
1021atomscm 2.Itshouldbenoted,however,thatthesevalues a broader component, likely originating from gas interacting
−
of N are likely to be lower limits. In fact, in some physical withtheradioplasma.
HI
situations, such as close to the nuclei of active galaxies or in Similar situations have been found in other radio galax-
outflows,thespintemperatureislikelytobeaslargeasafew ies (e.g. 4C12.50, Holt et al. 2003; 3C 293, Emonts et al. in
1000K(Maloneyetal.1996). prep.). Interestingly, these are often radio sources where out-
flows also associated with the neutral hydrogen are detected
(seeSec.5.3).Thisfurtheremphasizetheimportanceofstudy
3. Accuratesystemicvelocities
both the ionized and the neutral component of the gas in or-
ForaproperinterpretationofthekinematicsoftheHIabsorp- derbuildamorecomplete(andcorrect)pictureofthephysical
tion,itisimportanttohavethesystemicvelocityofthegalaxy conditionsaroundAGN.
RaffaellaMorganti:ProbingthenatureoftheISMinActiveGalacticNucleithroughHabsorption
sorption is detected against the majority of the inner kilopar-
sec of 3C293. This absorption is separated into two dynam-
ically different and spatially resolved systems. This result is
illustrated in Fig. 4. Against the eastern part of the inner ra-
dio jet narrow H absorption is detected and shown to have
higheropticaldepthsinareasco-spatialwithacentraldustlane.
Additionally,thisnarrowlineisshowntofollowavelocitygra-
dientof 50kms 1arcsec 1,consistentwiththevelocitygradi-
− −
∼
entobservedinopticalspectroscopyofionisedgas.Thenarrow
Habsorption,dustandionisedgasappeartobephysicallyas-
sociatedandsituatedseveralkiloparsecsfromthecentreofthe
hostgalaxy.Againstthewesternjetemissionandcorecompo-
nent,broadandcomplexHabsorptionisdetected.Apossible
explanatinforthisisthattheHissituatedinrotationaboutthe
Fig.3.Montageoftheinner250pcofNGC4151fromMundell coreofthisradiogalaxywithsomevelocitydispersionresult-
et al. (2003), showing torus of H2 emission in green (from ingfromin-fallandoutflowofgasfromthecoreregion.Ifthis
Fernandez et al. 1999), ring of HI inferred from absorption explanationis correct, then the mass enclosed by the rotating
measurementsinblueand1.4GHzradiocontinuumemission diskwouldbeatleast1.7 109solarmasseswithinaradiusof
×
fromradiojetinred.Ionizedgas(black)isassumedtofillthe 400pc.
torusinsidetheHIring.Thesegmentsmissingtothenorthand Powerful compact (steep spectrum) radio sources are
south of the H2 torus are due to the limited filter width that uniquelysuitedforinvestigationsintothephysicsofthecentral
excludedthehigh-velocitywingsoftheH2 line;thisprovides engines,inparticulartostudythekinematicsofthegaswithin
evidenceforrotationofthetorusasthenorthernandsouthern 100pcofthecore(seee.g.Vermeulenetal.2003).Pihlstro¨m
segmentscontainthegaswiththehighestradialvelocities(i.e., et al. (2003) have studied the distribution of the HI absorb-
linewings)ifthetorusisrotating. ing gas in a sample of these sources. They find that smaller
sources (< 0.5 kpc) have larger HI column density than the
larger sources > 0.5 kpc) (see Fig.5). This result can be ex-
4. Circumnucleartorianddisks
plainedbothasasphericalandanaxi-symmetricgasdistribu-
Asdescribedabove,HIgascanbeassociatedwithcircumnu- tion, with a radial power law density profile, although these
cleardisksandtori.ToestablishwhethertheHIabsorptionis authorsarguethatthediskdistributionisthemostlikely.
comingfromthese structuresis notalways easy.Often, given EvidenceofHIassociatedwithcicumnucleartorihasbeen
the limited size of the underlying continuum, clear kinemati- reportedfor some ofthese compactsources(see e.g.Conway
cal signatures of a rotation cannot be seen. Thus, one of the 1997, Peck & Taylor 2001).One of the best examplesof this
criteria to distinguishbetweenthese structuresis the width of typeistheCompactSymmetricObject(CSO)1946+708.The
the absorption line. While the HI associated with circumnu- HI absorption in 1946+708consists of a very broad line and
clear disks show relatively broad absorption (typically > 150 a lower velocity narrow line which are visible toward the en-
kms 1), HI associated with larger scale structures is usually tire 100pc of the continuumsource,Peck et al. 1999).with
−
∼
observed as narrow absorption features (see e.g. the case of thicknessof about100 pc and column density of the order of
CentaurusA,vanGorkometal.1986). 1023 cm 2 (with T of several thousand K). The broad line
− spin
In Seyfert galaxies, Gallimore et al. (1999) found that, has low optical depth and peaks in column density near the
withtheexceptionofNGC4151,theabsorbinggastraces100 core of the source. This is consistent with a thick torus sce-
pc-scale rotating disks aligned with the outer galaxy disk. In narioinwhichgasclosertothecentralengineismuchhotter,
NGC4151,HIabsorptionmeasurementsusingMERLINand both in terms of kinetic temperatureand spin temperature,so
VLBAindicateatorus 70pcinradiusand 50pcinheight a longer path-length throughthe torus toward the core would
∼ ∼
(Mundelletal.1995,2003).Acartoonillustratingthegeome- not necessarily result in a higher optical depth. The high ve-
tryofthesystemisshowninFig.3. locitydispersiontowardthecoreof1946+708isindicativeof
Due to the weakness of the radio core in powerful radio fast moving circumnuclear gas, perhaps in a rotating toroidal
galaxies (i.e. Fanaroff-Riley II), evidence for HI associated structure. Further evidence for this region of high kinetic en-
withtorihasbeenfoundonlyinfewcases.Apossiblecandidate ergy and column density is foundin the spectral index distri-
is Cygnus A. In this object, a 50 pc-scale, rotating, flattened bution which indicates a region of free-free absorption along
structurehasbeenfoundfromtheVLBIobservations(Conway thelineofsighttowardthecoreandinnerrecedingjet.TheHI
1999). However, recent optical observations have shown that opticaldepthincreasesgraduallytowardthe recedingjet. The
the situation may be more complicated than this and that the informationderivedfrom the HI can be particularlyusefulto
HIabsorptionincludesalsothesignatureofaninflowingcloud constraincharacteristicsthecentraltoruswhencombinedwith
(seeSec.5.1). hardX-raydata.Thishasbeendoneinthecaseoftwopossible
A more clear case comes from the study of the kinemat- Compton-thickgalaxiesstudiedbyRisalitietal.(2003).
ics and distribution within the central kiloparsec of the HI in Inlowluminosityradiogalaxiesthesituationcouldbedif-
the radiogalaxy3C 293(Beswick et al. 2004).StrongH ab- ferent.Thehighdetectionrateofopticalcores,thelackoflarge
RaffaellaMorganti:ProbingthenatureoftheISMinActiveGalacticNucleithroughHabsorption
Fig.4.(Left)Sub-arcsecond continuum structure of the inner few kiloparsecs of 3C293 (from Beswick et al. 2004). The top
contourmapshowsthe1.359GHzradiocontinuumstructureobservedwithMERLINataresolutionof0.23 0.20.Thelower
′′ ′′
×
panel shows the global VLBI, MERLIN and VLA+PT contoured image of the inner jet of 3C293 with angular resolution of
30mas.Thismapiscontouredatmultiplesof √2 times1.3mJybeam 1.(Right)Multi-resolutionposition-velocityplotsofH
−
absorptionagainstthewesternjetcomponentatthecentreof3C293(fromBeswicketal.2004).Ineachofthesediagramsthe
absorptionsignalhasbeenaveragedoverthedeclinationrangeofthecontinuumsource.Thedashedlineshownonallthreeplots
representsavelocitygradientof410kms 1arcsec 1.Thespatialpositionofradiocontinuumcomponentslabelledinthefigure
− −
ontheleftarealsoshownbyarrowspositionedalongthebottomplot.ImagesandplotsaretakenfromBeswicketal.(2004).
Fig.6.HIlinevelocitiescomparedtotheopticalvelocitiesfor
CSSandGPSsourcesfromVermeulenetal.(2003).
Fig.5. Absorbed HI column density versus projected linear object,theVLBIdatasuggestthattheHIabsorptionisduetoa
sizeforCSSandGPSsources.Thereisananti-correlationbe- diskofonly 1.3pcthickprojectedagainstthecounter-jet.In
∼
tween the source size and the amountof absorbinggas (from NGC4261,evidenceforthepresenceofsuchnucleardiskare
Pihlstro¨metal.2003). alsofoundinHSTimages.Theideaofthindiskscanbeinves-
tigatedinmoredetailbycorrelatingthepresence(orabsence)
ofHIabsorptionwiththeopticalcharacteristics.Thishasbeen
absorptionin X-ray (Chiabergeet al. 1999)and possibly also doneforasampleofradiogalaxies(selectedfromCapettietal.
the relatively low detection rate of HI absorption (Morganti 2000)forwhichinformationaboutthepresenceofopticalcores
et al. 2001) suggest that the standard pc-scale geometrically andnucleardustydisks/lanes(fromHSTimages)isavailable.
thick torus is not present in these radio galaxies. The pres- Interestingly, HI absorption was detected in the two galaxies
ence of thin disks has been claimed from HI observations in thathavedustdisks/lanesandnoopticalcores.Inthesecases,
thecasee.g.NGC4261(vanLangeveldeetal.2000).Forthis thecolumndensityoftheabsorptionisquitehigh(>1021cm 2
−
RaffaellaMorganti:ProbingthenatureoftheISMinActiveGalacticNucleithroughHabsorption
for T = 100 K) and the derived optical extinction A (be- temscouldbelocaltotheAGNenvironmentordistributedon
spin B
tween1and2magnitudes)issuchthatitcan,indeed,produce galacticscales.
theobscurationoftheopticalcores.Intheothertwocases,HI HI absorption can be used to trace a particularly rich
absorption has been detected despite the presence of optical medium that is characteristics of some radio galaxies – per-
cores.However,thecolumndensityderivedfromthedetected hapsthoseresultingfrommajormergersorinwhichamerger
absorptionismuchlower( 1020cm−2forTspin = 100K)and happened not so long ago. One example is the radio galaxy
∼
the derived extinction is of the order of only a fraction of a 4C12.50(seeFig.7,Morgantietal.2004a),agalaxythathas
magnitude.Thisis,therefore,consistentwithwhatexpectedif often been suggested to be a prime candidate for the link be-
thecircumnucleardiskarethinintheseradiogalaxies. tween ultraluminousinfraredgalaxiesandyoungradiogalax-
ies. In this object, deep and relatively narrow HI absorption
(observed at the systemic velocity) is associated with an off-
5. Unsettledgas
nuclearcloud( 50to100pcfromtheradiocore)withacol-
∼
5.1.Anyevidenceforinfall? umndensityof 1022 Tspin/(100K)cm−2 andanHImassof
∼
a few times 105 to 106 M . There are more examples of ob-
Evidencefor infallinggaswas reportedby van Gorkomet al. ⊙
jectswheretheHItracestherichmediumsurroundingtheac-
(1989). In a sample of radio galaxies, HI absorption was de- tivenucleus.Examplesofoff-nuclearHIabsorptionarefound
tected either close to the systemic velocity or systematically
in 3C 236 (Conway & Schilizzi 2000) and, more recently, in
redshifted,indicatingthereforeaprevalenceofgasfallinginto
theCSO4C37.11(Manessetal.2004)whereabroad( 500
the nucleus. This result does not appear to be confirmed by kms 1)absorptionlinewasfoundintheregionofthesou∼thern
−
more recent observations. For example, the study of HI ab- hot-spot.
sorptionin compactradio sourcesby Vermeulenet al. (2003)
Thismayhaveimportantimplicationsfortheevolutionof
showsthatthereisevidenceforsignificantgasmotionsandnot
theradiojets.Althoughthisgaswillnotbeabletoconfinethe
onlypositivebutevenmorenegativeHIvelocities(uptomore
radio source, it may be able to momentarily destroy the path
thanv= 1000 kms 1comparedtothesystemicvelocity)are
− − ofthejetasshownalsobynumericalsimulations(Bicknellet
found(seeFig.6).Thisisindicatingthatgasflowingoutofthe
al.2003).Thus,thisinteractioncaninfluencethegrowthofthe
galaxyisalsopresent.Indeed,clearcasesoffastgasoutflows
radiosourceuntiltheradioplasmaclearsitswayout.
havenowbeendetectedasdescribedbelow(seeSec.5.3).
Oneofthemostpromisingcaseofinfallinggaswasfound
in the radio galaxy NGC 315. A very narrow and highly 5.3.FastOutflows
redshifted ( 500 kms 1) HI absorption was reported by
−
Heckmanet∼al.(1983)andDresseletal. (1983).VLBI obser- Gasoutflowsassociatedwithactivegalacticnuclei(AGN)pro-
vations(Peck1999,Morgantietal.inprep.)arenowshowing vide energyfeedback into the ISM that can profoundlyaffect
thatthisabsorptionappearstocoveraregionofabout9pcof the evolution of the central engine as well as that of the host
the source, from the core to the first part of the jet. A likely galaxy. The mass-loss rate from these outflows can be a sub-
explanation for this absorption is that of a cloud at large dis- stantialfractionoftheaccretionrateneededtopowertheAGN.
tance from the nucleus (like tidal debris, Wakker et al. 2002) Fastoutflowshavenowbeendetectedinalargefractionof
detected, in projection, against the nucleus. This seems to be nearbyAGNviaobservationsatvisible,X-rayandUV wave-
morefavorableoverthepossibilityofasmallcloudfallinginto lengthsassociatedtoionizedgas(seee.g.Veilleuxetal.2000,
thenucleusandfeedingtheAGN. Kriss2004,Elvis,Marengo&Karovska2002andref.therein
An interesting case of cloud falling into the nucleus has foranoverview).Itis,therefore,nottoosurprisingtofindsuch
been recently suggested for Cygnus A (Bellamy et al. 2004). outflows also in radio galaxies (see Morgantiet al. 2003a for
Near-IR data show the existence of an off-nucleus molecular asummaryofrecentresults).However,itisextremelyintrigu-
cloud,thatisredshiftedrespecttothesystemicvelocity(mea- ingthediscoveryofanumberofradiosourceswherethepres-
suredaccuratelyfromstellarfeatures).Thissuggeststhepres- ence of fast outflows (up to 2000 kms 1) is associated not
−
ence of a giant molecular cloud falling though the “heart” of onlywithionizedbutalsowithneutralgas.Thisfindinggives
Cygnus A. Interesting, the redshift of this cloud is in agree- new and important insights on the physical conditions of the
ment with that of the HI absorption(or part of it) indicating, gaseousmediumaroundanAGN.Thebestexamplessofarare
therefore,thatthetwophenomenamaybelinked. the radio galaxies 3C 293 (Morgantiet al. 2003b,see Fig.8a)
and4C12.50andtheSeyfertgalaxyIC5063(Oosterlooetal.
2000).Itisalsoworthnoticingthatoutflowsofionizedgasare
5.2.GascocoonaroundAGN
alsoassociatedwiththeseneutraloutflows(seeMorgantietal.
As mention in the introduction, HI absorption can also trace 2003a).
gasdistributedinamorecomplexwayaroundtheAGN.Inthe An otherinterestingobjectwhere an HI outflowhasbeen
low luminosity active galactic nucleus, NGC 1052,the VLBI detected is the Compton-thick, broad-line and GPS radio
studyoftheHIhasrevealedatomicgasinfrontoftheapproac- galaxyOQ208.TheHIspectrumofthissource(thatisonly10
ingjetaswellastherecedingjet(Vermeulenetal.2003b).The pcinsize)isshowninFig.8b.Guainazzietal.(2004)suggest
gasappearedtobeassociatedwiththreevelocitysystems.One that in this source we coulld be seeing the jets piercing their
systemcanbeascloseas1-2pcfromthecore.Theothersys- way through a Compton-thickmedium pervading the nuclear
RaffaellaMorganti:ProbingthenatureoftheISMinActiveGalacticNucleithroughHabsorption
+0.05
0)
00 0 CORE
2
J
ec, m)
arcs bea
et ( Jy/
offs −0.05 S(m
c VLBI
e
D
WSRT
−0.10
Vhel(km/s)
+0.06 +0.03 0 −0.03 −0.06
RA offset (arcsec, J2000)
Fig.7.(Left)VLBIcontinuumimage(greyscaleandthincontours)of4C12.50(fromMorgantietal.2004a)superimposedonto
thetotalintensityofthe(narrow)HIabsorptionobservedatthesystemicvelocity(thickcontours).Thepositionoftheradiocore
isalsoindicated.Thecontourlevelsforthecontinuumimageare:5mJybeam 1to800mJybeam 1instepsoffactor1.5.(Right)
− −
HIabsorptionprofileobservedwiththeWSRT(black)andVLBI(red).Abroad,shallowHIabsorptionisdetectedintheWSR
Tobservations.Duetothenarrowerband,thisbroadabsorptionisnotdetectedintheVLBIobservations.
environment. The outflow detected in HI (see Fig.8b) would studies is the sensitivity of present day radio telescopes, the
be an other indicationof this process. Guainazziet al. (2004) possibility now becoming more and more available of broad
also suggestthatif the jets have to interactwith such a dense band observationsallows to explore the presence of kinemat-
medium, one could largely underestimating the radio activity ically disturbed HI. This may represent a relatively common
dynamicalage determinatedfor this kind of sourcesfrom the phenomena,perhapsrelatedwiththeevolutionarystageofthe
observedhot-spotrecessionvelocity.Asimilarsituationcould radio sources. The importance of understanding the physical
beoccuringthetheGPS4C12.50describedabove. conditionsof the gasin the environmentof the AGN (bothin
A number of possible hypotheses can be made about the the circumnuclear tori as well as in the AGN-driven outflow)
originofthegasoutflow(e.g.,starburstwinds,radiationpres- isillustratedbytheinterestandthewealthofobservationsper-
sure from the AGN, adiabatically expanded broad emission formed,e.g.,intheopticalandX-raybands.However,thepos-
lineclouds).Insomecases,thepossibilitythattheyaredriven sibilityofimagingthisgasatveryhighresolution–byobtain-
by the interaction of the radio jet with the ISM seems to be ingHIabsorptionwithmilli-arcsecresolution–isuniquetothe
favored. To investigate whether this is indeed correct, high- radio band and the VLBI technique. This combined with the
resolution(VLBI) studiesare in progressto find the exactlo- broadband(i.e.toinstanteneouslycoverupto 4000 kms 1)
−
∼
cation of the ouflowing gas. So far these outflows have been isprovidinganextremelypowerfultooltoinvestigatethecon-
foundinobjectsthatareeitherin theearly-stageoftheirevo- ditions(includingtheextremeonethatnowweknowcanexist)
lution(like 4C 12.50)or,perhaps,in a phaseof re-startedac- oftheatomicgasaroundAGN.
tivity(asmightbethecasefor3C293).Anothercharacteristic
The next step is, however, the dramatic improvementand
of these galaxies is the presence of a “young” stellar popula-
possibilitiesthatthenewgenerationofradiotelescopewillof-
tion(fromtheiropticalspectra,seee.g.Tadhunteretal.2002).
fer. A summary of the possibilities that the Square Kilometer
Such a component(with ages between 0.5 and 2 Gyr) can be
Arraywill openfor the studyof th eenvironmentof the AGN
considered an indication that the galaxy is indeed in a stage
of its evolution,when largeamountsof gas/dust- likely from are summarized in Morganti et al. (2004b). Apart from the
the merger that triggered the activity - are still present in the moredetailed studyof the HI in single objects,the SKA will
providethepossibilityofperforminglargesurveysandunder-
innerregionandtheradiojetisstronglyinteractingwithit.A
stand the occurence of the phenomena described above and
moresystematicsearchforfastgasoutflowsinradiogalaxiesis
theirrelationwith the propertiesofthe hostgalaxy.TheSKA
nowinprogressandhasalreadyrevealedmorecasesofbroad,
will allow to investigate an unexplored region of parameter
blueshiftedHIabsorptions.
space.Atpresent,serendipitousHIsurveyscanalreadybecar-
riedoutineverydeepfield,forexamplebyusingspectral-line
6. Conclusions
mode, in which continuumobservations(see e.g. Morgantiet
The HI is an important tool to study the physical conditions al.2004cforthecaseoftheSpitzerSpaceTelescopeFirst-Look
ofthegasaroundAGN.Althoughthemainlimitationofthese DeepsurveydonewiththeWSRT).
RaffaellaMorganti:ProbingthenatureoftheISMinActiveGalacticNucleithroughHabsorption
3C 293
H I absorption
10
−1
1400 km s
0
y)
mJ -10
S ( V
sys
-20
-30
12000 12500 13000 13500 14000
V (km s−1)
hel
Fig.8.(Left)TheHIabsorptionprofiledetectedin3C293fromtheWSRTobservations.Thespectrumisplottedinflux(mJy)
against optical heliocentric velocity in kms 1 (from Morganti et al. 2003).(Right)Broad HI absorption detected (using the
−
WSRT)againstthecompactsourceOQ208.ThesystemicvelocityderivedbyMarzianietal.(1997)isalsoindicated.TheVLBI
radiocontinuum(thatisonly 10pcinsize)istakenfromStanghellinietal.(1997).
∼
With the highsensitivity expectedfrom the SKA, we will MihosJ.C.&HernquistL.1996ApJ,464,641
beabletosearchforHIabsorptionatτ 0.01level(thetyp- MorgantiR.,GreenhillL.J.,PeckA.B.,JonesD.L.,HenkelC.2004b,
∼
icalabsorptionfoundincasesofcircumnucleartori)onevery in”SciencewiththeSquareKilometerArray”eds.C.Carilliand
source stronger than only a few mJy of any observed field. It S.Rawlings,NewAstronomyReviews(Elsevier:Amsterdam)in
press
willbelikesearchingeverysourceoftheNVSScataloguefor
Morganti R.,Garrett M., Chapman S., Baan W., Helou G., Soifer T.
HIabsorption.Thelargeinstantaneousbandwidthwillensure
A&A424,371
thatalargerangeinredshiftiscoveredtodetectthisabsorption.
MorgantiR.etal.2004aA&A424,119
Morganti et al. 2003a, in Recycling Intergalactic and Interstellar
Matter, IAU Symposium 217, eds. P.-A. Duc et al., p. 332
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