Table Of ContentCold gas & mergers: fundamental difference in HI
properties of different types of radio galaxies?
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Bjorn Emonts∗†
n
a ColumbiaUniversity,DepartmentofAstronomy,MailCode5246,550West120thStreet,New
J
York,N.Y.10027,USA
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E-mail: [email protected]
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Raffaella Morganti,TomOosterloo
]
h
NetherlandsFoundationforResearchinAstronomy(ASTRON),Postbus2,7990AADwingeloo,
p
theNetherlands
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o E-mail: [email protected],[email protected]
r
t
s JacquelinevanGorkom
a
[ ColumbiaUniversity,DepartmentofAstronomy,MailCode5246,550West120thStreet,New
York,N.Y.10027,USA
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v E-mail: [email protected]
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We presentresultsofastudyoflarge-scaleneutralhydrogen(HI)gasinnearbyradiogalaxies.
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4 We findthatthe early-typehostgalaxiesofdifferenttypesofradiosources(compact, FR-I and
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1 FR-II)appeartocontainfundamentallydifferentlarge-scaleHIproperties:enormousregularro-
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tatingdisksandringsarepresentaroundthehostgalaxiesofasignificantfractionoflowpower
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0 compact radio sources, while no large-scale HI is detected in low power, edge-darkened FR-I
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v radio galaxies. Preliminary results of a study of nearby powerful, edge-brightenedFR-II radio
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X galaxiesshow thatthese systems generallycontain significantamountsof large-scaleHI, often
r distributedintail-orbridge-likestructures,indicativeofarecentgalaxymergerorcollision.Our
a
resultssuggestthatdifferenttypesofradiogalaxiesmayhaveadifferentformationhistory,which
couldberelatedtoadifferenceinthetriggeringmechanismoftheradiosource. Ifconfirmedby
largerstudieswith the nextgenerationradiotelescopes, this wouldbe in agreementwith previ-
ousopticalstudiesthatsuggestthatpowerfulFR-II radiosourcesare likelytriggeredbygalaxy
mergersandcollisions, whilethelowerpowerFR-I sourcesarefedin otherways(e.g. through
theaccretionofhotIGM).ThegiantHIdisks/ringsassociatedwithsomecompactsourcescould
-atleastinsomecases-betherelicsofmuchmoreadvancedmergers.
Fromplanetstodarkenergy: themodernradiouniverse
October1-52007
UniversityofManchester,Manchester,UK
∗Speaker.
†FundedbytheNetherlandsOrganisationforScientificResearch(NWO)underRubicongrant680.50.0508
(cid:13)c Copyrightownedbytheauthor(s)underthetermsoftheCreativeCommonsAttribution-NonCommercial-ShareAlikeLicence. http://pos.sissa.it/
Large-scaleHIinradiogalaxies BjornEmonts
1. Introduction
Radiosourcesaregenerallyhostedbyearly-typegalaxies. Asignificantfractionoftheseradio
galaxies, inparticular themorepowerfulones,showopticalpeculiarities (tail,bridges, shells,etc.)
that are indicative of galaxy mergers or interactions (1; 2). This has led to the suggestion that
galaxymergersorcollisions couldbethetriggerfortheradio-AGN activityinthesesystems.
Herewepresent the large-scale neutral hydrogen (HI)properties of different types of nearby
radio galaxies inorder toinvestigate whether these systems have been involved inarecent orpast
gas-rich mergerorinteraction. Simulationshow thatinamajormergerbetween gas-rich galaxies,
large-scale gaseous tidal structures can be expelled from the merging system, which after several
rotations (& Gyr) can partially fall back onto the host galaxy and settle in a low-density, regular
rotating disk or ring (3). HIobservations –in particular when combined witha stellar population
analysisofthehostgalaxy(4)–thereforeprovideanexcellenttooltotraceanddategalaxymergers
onrelativelylongtime-scales, whichcanbecomparedwiththeageoftheradio-loudAGNinthese
galaxies inordertoinvestigate apossible connection withthetriggering oftheradiosource.
2. HIinlowerpower radiogalaxies(compact &FR-I)
Our first study comprises a complete sample of all non-cluster radio galaxies from the B2-
catalog (F &0.2 Jy) up to a redshift of z∼0.04. This sample contains low power compact
408MHz
radiosourcesaswellassub-relativistic, edge-darkened Fanaroff&Riley(5)type-I(FR-I)sources.
Wedetectlarge-scaleHIinemissioninsix(∼25%)ofoursamplesources. Twooftheseareshown
inFig.1(imagesoftheotherHIdetections canbefoundinEmontsetal. 2007(6)). Detailsofthe
HIstructures aregiveninTable1. Thetwomoststrikingresultsfromthislowpowersampleare:
• Large-scaleHIstructuresaremainlyfoundaroundthehostgalaxiesofseveralcompactradio
sources, while for none of the extended (>20 kpc) FR-I sources HI is detected outside the
optical bodyofthehostgalaxy(seealsoFig.3);
• These detected HI structures are mostly large-scale regular rotating disks and rings with
diameters upto190kpcandHImassesupto2×1010M⊙!
In order to investigate the nature of these large-scale HI disks/rings in more detail, we studied
one of the radio galaxies, B2 0647+27, in great detail. As is published in Emonts et al. 2006
(7), a detailed spectroscopic stellar population analysis of this system shows that a 0.3 Gyr post-
starburst stellarpopulation ispresentthroughout thissystem,whichcouldhavegivenB20648+27
theappearance ofan(Ultra-) Luminous Infra-Red Galaxyinthefirstepoch after thestarburst was
triggered. Newdeep optical imaging ofthis system (see Fig. 1-left) showsthat afaint stellar tail
is present in the HI ring. The morphology of the HI ring, the faint stellar tail and the presence
of apost-starburst stellar population inB20648+27 suggest that this galaxy isanadvancedmajor
merger. The merger event in this system must have occurred roughly 1.5 Gyr ago, after which
HI and stellar tails – which were expelled during the merger –had time tofall back onto the host
galaxy and settle in the rotating ring-like structure that we observe. B2 0648+27 is an excellent
example of a galaxy in which the different stages of merger, starburst and AGN activity can be
traced(seeEmontsetal. 2006(7)formoredetails).
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Large-scaleHIinradiogalaxies BjornEmonts
Figure1:TwoHI-richradiogalaxiesfromourB2sample.Left:B20648+27(7)–theHIring(contours)in
thisadvancedmergersystemfollowsafaintstellartailseenindeepopticalB+Vbandimaging(gray-scale).
ContoursHI:from0.22to2.1×1020cm−2.Right:B20258+35–thehostgalaxyisclassifiedasearly-type,
but deep optical B-band imaging(gray-scale)revealsfaint spiral structurein the disk. ContoursHI: from
0.34to3.0instepsof0.33×1020cm−2. SeeTable1formoredetails.
We note, however, that the formation mechanism of the large-scale HI disks in the other B2
radiogalaxiesremainsuncertain. Figure1(right)shows,forexample,thattheHIdiskB20258+35
hasafaintandtightlywoundspiralstructure indeepoptical imaging. Thiscouldbetheresultofa
much older merger event, but perhaps itindicates that B20258+35 israther amore typical spiral-
typegalaxy. Furtherstudiesarenecessary todeterminetonatureofthisandtheotherHIdisks.
3. HIinhigher powerradio galaxies(FR-II)
In order to investigate the large-scale HI properties of higher power, edge-brightened FR-
II radio galaxies (with relativistic radio jet ending in a hot-spot), we observed a sample of the
nearest (z .0.06) powerful radio galaxies. Because the volume density of FR-II sources in the
Table1: PropertiesoftheHI-detectedradiogalaxies
Host Ref. Sourcetype M D Morph. Remarks
HI HI
Galaxy (+diam./kpc) (M⊙) (kpc) HI
B20258+35 (6) C (1.4) 2×1010 160 disk faintopticaldisk
B20648+27 (7) C (1.3) 9×109 190 ring advancedmerger
B20722+30 (6) FR-I (14) 2×108 15 disk spiralhostgalaxy
B21217+29 (9) C (>1) 7×108 37 disk classicalelliptical(NGC4278)
B21322+36 (6) FR-I (19) 7×107 20 blob +resolvedHIabsorption
NGC3894 (6) C (1.6) 2×109 105 ring faintopticaldust-lane
NGC612 (8) FR-II (473) 2×109 140 disk+bridge S0withstar-formingdisk
3C305∗ FR-II (11) 6×108 106 tail +prominentcentralabsorption
3C382∗ FR-II (284) 2×109 92 bridge/tail HImaybepartofenvironment?
3C192∗ FR-II (331) 3×108 110 bridge HIbridgedetectedinabsorption
3C390.3∗ FR-II (349) 2×109 172 tail absorption;maybeenvironment?
*preliminaryresults–needtobeverified AssumedH =71kms−1 Mpc−1
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Large-scaleHIinradiogalaxies BjornEmonts
Figure2:Preliminarytotalintensitymapsoflarge-scaleHIdetectedaroundthreenearbyFR-IIradiogalax-
ies. ThearrowspointtotheHIstructuresthatweassigntotheradiogalaxies(seeTable1fordetails). The
different contours are indicated in the figure (for clarification, the extended radio source in NGC 612 is
omittedfromtheplot).
nearbyUniverseisextremelylow,HIcanbemappedonlyforaverylimitednumberofFR-IIradio
galaxies withthesensitivity ofcurrent-day radiotelescopes (andeventhentheseFR-IIsourcesare
notaspowerfulastheirhigh-zcounterparts).
Preliminary results show that 5 out of 7 FR-II radio galaxies that we studied so far appear to
contain large-scale HI (three examples are shown in Fig. 2, while Table 1 summarizes the pre-
liminary results of the remaining detections). This HI detection rate is significantly higher than
the detection rate for the lower power FR-I radio galaxies. NGC 612 (Fig. 2 -left) contains a
large-scale rotating disk (8), very similar in morphology to the disks and rings detected around
somecompactsourcesfromourB2-sample. TheHIfeaturesdetectedaroundtheotherFR-IIradio
galaxies resemble more irregular tail- or bridge-like structures. For 3C 192 (Fig. 2 -middle) a
large-scale HI bridge is detected in absorption against the strong radio continuum (although the
signal is weak and only detected at the 4s level in the individual channel maps). We note that
for 3C 382 (Fig. 2 -right) and 3C 390.3 (Table 1), it is uncertain whether the HI is directly re-
latedtotheradiogalaxyorpartofitsHI-richenvironment. OurpreliminaryHIdetectionsneedto
beverifiedand additional observations are planned in order to accurately map the large-scale HI
structures aroundtheseFR-IIradiogalaxies.
4. Discussion
Ourresults oflarge-scale HIaround nearby radio galaxies imply that there could beafunda-
mentaldifferenceinlarge-scaleHIcontentbetweenthevarioustypesofradiogalaxies, which is
visualized in more detail in Fig. 3. While the host galaxies of several low power compact radio
sourcescontainalarge-scale regularrotatingHIdiskorring(possiblyastheresultofanadvanced
merger), the extended FR-I radio galaxies in the same sample lack any large-scale HI structures
outside the optical body of the host galaxy up toa conservative detection limit of afew ×108M⊙.
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Large-scaleHIinradiogalaxies BjornEmonts
Figure 3: Large-scale HI mass plotted against the total linear extent of the radio source. Black circles
andarrowsrepresentthe sourcesfromourB2 sample(compact+ FR-I),whileredsquaresandarrowsare
preliminaryresultsfromourFR-IIsample. Incaseofnon-detectionaconservativeupperlimit(3s across
400kms−1)wasestimated.
Ontheotherhand,preliminaryresultsforasmallsampleofthemorepowerfulFR-IIradiosources
show that most of their host galaxies do contain significant amounts of HI, often distributed in
tail- or bridge-like structures. If confirmed by future studies with greater statistical significance
and lowerdetection limits, this could meanthat thereisafundamental difference inthe formation
history of different types of radio galaxies and, related, the triggering of their radio source. This
would be in agreement with optical studies that suggest that powerful FR-IIsources are generally
fueledbymajormergers,whileFR-Isourcearelikelypoweredinanotherway,forexamplethrough
theaccretion ofgasfromthehotinter-galactic medium(1;2).
Futureinstruments,witheventuallytheSquareKilometreArray,willbeessentialtoverifyour
results byobserving HIemissioninlargestatistical samplesofradiogalaxies.
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