The Activity of the Neighbours of Seyfert Galaxies

1 MB·English

by  E. Koulouridis

Astronomy&Astrophysicsmanuscriptno.aa˙paper (cid:13)c ESO2012
May15,2012
The Activity of the Neighbours of Seyfert Galaxies.
E.Koulouridis1,M.Plionis1,2,V.Chavushyan2,D.Dultzin3,Y.Krongold3,I.Georgantopoulos1,J.Leo´n-Tavares4
1 InstituteofAstronomy&Astrophysics,NationalObservatoryofAthens,PalaiaPenteli15236,Athens,Greece.
2 InstitutoNacionaldeAstrof´ısicaOpticayElectro´nica,AP51y216,72000,Puebla,Me´xico.
3 InstitutodeAstronom´ıa,UnivesidadNacionalAuto´nomadeMe´xico,ApartadoPostal70-264,Me´xico,D.F.04510,Me´xico
4 AaltoUniversityMetsa¨hoviRadioObservatory,Metsa¨hovintie114,FIN-02540Kylma¨la¨,Finland
May15,2012
2
1
ABSTRACT
0
2
Wepresentafollow-upstudyonaseriesofpapersconcerningtheroleofcloseinteractionsasapossibletriggeringmechanismof

y AGNactivity.Wehavealreadystudiedtheclose(≤ 100 h−1kpc)andthelargescale(≤1h−1 Mpc)environment ofalocalsample
a ofSy1,Sy2andBrightIRASgalaxies(BIRG)andtheirrespectivecontrolsamples.Theresultsledustotheconclusionthataclose
M encounter appearscapableof activatingasequence whereanormal galaxybecomesfirstastarburst,thenaSy2andfinallyaSy1.
However,sincebothgalaxiesofaninteractingpairshouldbeaffected,wepresent hereopticalspectroscopy andX-rayimagingof
4 15and16neighbouringgalaxiesaroundtwosamplesof10Sy1and14Sy2galaxiesrespectively.Basedonopticalspectroscopy,we
1 findthatmorethan70%ofallneighbouring galaxiesexhibitstarformingand/or nuclear activity(namelyenhanced starformation,
starburstingand/orAGN),whileanadditionalX-rayanalysisshowedthatthispercentagemightbesignificantlyhigher.Furthermore,
] wediscoveredastatisticallyimportanttrend,ata99.9%level,regardingthetypeandstrengthoftheneighbour’sactivitywithrespect
O to the activity of the central galaxy, i.e. that the neighbours of Sy2 galaxies are systematically more ionized than the neighbours
ofSy1s,afactthatindicatesdifferencesinmetallicity,stellarmassandstar-formationhistorybetweenthesamples.Ourresultsare
C
consistentwiththelinkbetweenclosegalaxyinteractionsandactivityandalsoprovidemorecluesregardingthepossibleevolutionary
.
h sequenceinferredbyourpreviousstudies.
p
Keywords.Galaxies:Active,Galaxies:Seyfert,Galaxies:Starburst,X-ray:Galaxies,Cosmology:Large-ScaleStructureofUniverse
-
o
r
t
s 1. Introduction AGNandtheirenvironmentuptoseveralhundredkpc,cangive
a
usvaluableinformationonthenatureofthegeneralAGNpop-
[ Since the discovery of Active Galactic Nuclei (AGN), signif- ulation as well as on different properties of each AGN sub-
icant effort has been put in the attempt to reveal their nature.
2 type. In addition, the availability nowadays of large, automati-
v Nowadays, we are certain that the accretion of material into a callyconstructed,galaxycatalogues,liketheSDSS,canprovide
4 centrally located massive black hole (MBH) is responsible for the necessary statistical significancefor these type of analyses.
8 the detected excess emission in the AGN’s spectra and such However,greatcautionshouldbetakenwheninterpretingresults
0 black holes do exist in all elliptical galaxies and spiral galaxy
basedonlargedatabases,sincethelargerthesamplesizetheless
4 bulges(KormendyandRichstone1995;Magorrianetal.1998),
controlusuallyonehasonthespectralandotherdetailsofthein-
1. includingourown(e.g.Melia&Falcke2001).However,westill dividualgalaxyentries.Itcouldthenbedifficulttoattempttoad-
1 lackacompleteunderstandingofthevariousaspectsofactivity, dressimportantquestions,suchas:DotheUnificationparadigm
1 for example the triggering mechanism and the feeding of the explainsallcasesoftype1andtype2AGN?Whichisthetrue
1 blackhole,thephysicalpropertiesoftheaccretiondiskandthe
connectionbetweengalaxyinteractions,star formationandnu-
: obscuring torus predicted by the unified scheme (Antonucciet
v clear activity? What is the lifetime of these phenomena? How
al. 1993), the origin of jets in radio loud objects, the connec-
i do LINERs fit in the general picture and can all be considered
X tionwithstarformationandtheroleoftheAGNfeedback.Even AGN?DoevolutionarytrendsaffecttheAGNphenomenology?
r theexactmechanismthatproducestheobservedIR,X-ray,and etc.
a gamma-rayemission,isstillonlypartiallyunderstood.Theuni-
ficationmodelitself,althoughsuccessfulinmanycases,hasnot
beenabletofullyexplainalltheAGNphenomenology(among 1.1.TriggeringanAGNevolutionarysequence?
others,the roleof interactionsoninducedactivity;Koulouridis
etal.2006a,bandreferencestherein). Despite observational difficulties and limitations, there have
The lack of detailed knowledgeof key aspects of the AGN been many attempts, based on different diagnostics, to inves-
mechanismleavesuswithmanyscatteredpiecesofinformation. tigate the possible triggering mechanisms of nuclear activity.
Theoryisunableinmostcasestoexplainobservations,andob- MostagreethattheaccretionofmaterialintoaMBH(Lynden-
servationsstillcannotclearlyresolvethegalacticnucleitocon- Bell1969)isthemechanismresponsiblefortheemission,butit
firmtheories.Radio-loudandradio-quietAGN,QSOtypeIand is still necessary to understand the feeding mechanism of the
II, Sy1 and Sy2 galaxies, LINERs, transition galaxies between black hole. It is known and widely accepted that interactions
different states (TOs) and starburst galaxies, are some of the between galaxies can drive gas and molecular clouds towards
pieces of the puzzle that we are called to unify. Examination theirnucleusinitiatinganenhancedstarformation(e.g.Lietal.
of the properties of the host galaxies of the different types of 2008;Ellisonetal.2008;Ideueetal.2012).Manyalsobelieve
1
Koulouridisetal.:NeighboursofSeyfertGalaxies.
that the same mechanism could give birth to an active nucleus 1.2.Thiswork
(e.g.Umemura1998;Kawakatuetal.2006;Ellisonetal.2011;
Silvermanetal.2011). Thispaperisthethirdinaseriesof3-dimensionalstudiesofthe
environmentofactivegalaxies(Koulouridisetal.2006a,b),ex-
Indeed, there are several studies that conclude that there is
tendingprevious2Danalyses(Dultzinetal.1999,Krongoldet
anevolutionarysequencefromstarbursttoSeyfertgalaxies(e.g.
al. 2002), in an effort to shed more light to the starburst/AGN
Storchi-Bergmannetal.2001).Basedonthenumberandprox-
connectionandtotheevolutionaryscenarioproposedinourpre-
imity of close (∼<60 − 100 h−1kpc) neighbours around differ- viouspapers.Itisafollow-upspectroscopicstudyaimingatin-
enttypesofactive(Sy1,Sy2andBIRG)galaxies(e.g.Dultzin-
vestigatingthepossibleeffectsofinteractionsontheneighbours
Hacyan et al. 1999; Krongold et al. 2002; Koulouridis et al.
ofourSeyfertgalaxiesandunderstandingtheconditionsneces-
2006a,b)averyinterestingevolutionarysequencehasbeensug-
saryforthedifferenttypesofactivity.
gested,startingwith a closeinteractionthattriggerstheforma-
In§2wewilldiscussourgalaxysamplesandwewillpresent
tion of a nuclear starburst, subsequently evolving to a type 2
ourobservationsanddatareduction.Thespectroscopicanalysis
Seyfert,andfinallytoaSy1.Thissequenceislikelyindependent
andclassificationofthegalaxiesandtheanalysisoftheavailable
ofluminosity,assimilartrendshavebeenproposedforLINERs
X-rayobservationsispresentedinsection§3.Finally,insection
(Krongold et al. 2003) and ULIRGs and Quasars (Fiore et al.
§4wewillinterpretourresultsanddrawourconclusions.Dueto
2008 and references therein). Furthermore, numerical simula-
thefactthatalloursamplesarelocal,cosmologicalcorrections
tions by Hopkins et al. (2008) outlined such an evolutionary
ofgalaxydistancesarenegligible.Throughoutourpaperweuse
scheme for merging galaxies. The proposed activity evolution
H = 100 h−1 Mpc. Note that we use the term “active” for all
canfullyexplaintheexcessofstarburstsandtype2AGNinin- ◦
galaxiesthatexhibitemissionlinesin theirspectra,theirorigin
teractingsystems,aswellasthelackoftype1AGNincompact
beingeithernuclearactivityand/orstar-formation.
groupsofgalaxies(Mart´ınezetal.2008)andgalaxypairs(e.g.
Gonzalezetal.2008).
In addition,poststarburststellar populationshave beenob-
2. Data
servedaroundAGN(Dultzin-Hacyan&Benitez1994;Maiolino
& Rieke 1995; Nelson & Whittle 1996; Hunt et al. 1997; 2.1.SampleDefinitionandPreviousResults
Maiolino et al. 1997; Cid Fernandes, Storchi-Bergmann &
Schmitt 1998; Boisson et al. 2000, 2004; Cid Fernandes et al. The samples of active galaxies were initially compiled from
2001,2004,2005)andinclose proximityto thecore(∼50pc). the catalogue of Lipovetskyj, Neisvestnyj & Neisvetnaya
Thisfactimpliesthecontinuityofthesetwostatesandadelayof (1987). The original catalogue itself is a compilation of all
0.05-0.25Gyrbetweentheonsetofthestarburstandthelighting Seyfert galaxies known at the time from various surveys
upoftheAGN(Mu¨llerSa´nchezetal.2008;Wild,Heckman,& and in various frequencies (optical, X-ray, radio, infrared).
Charlot2010;Daviesetal.2012),whichmayreachthepeakof It includes all extended objects and several starlike objects
itsactivityafter0.5-0.7Gyr(Kavirajetal.2008,2011).Davieset with absolute magnitudes lower than -24. Multifrequency data
al.(2007),analyzingstarformationinthenucleiofnineSeyfert are compiled and listed, including : coordinates, redshifts,
galaxiesfoundrecent,butnolongeractive,starburstswhichoc- Seyfert type (and sub-type), UBVR-photoelectric magni-
curred10-300Myrago.Furthermore,mostofthesestudies(e.g. tudes, morphological types, fluxes in H and [OIII]5007,
β
Huntetal.1997;Maiolinoetal.1997,Guetal.2001)separate JHKLN fluxes, far-IR (IRAS) fluxes, radio fluxes at 6 and
type I from type II objects implying that recent star-formation 11 cm, monochromatic X-Ray fluxes in 0.3 - 3.5 and 2 -
isonlypresentintypeIIobjects(seealsoColdwelletal.2009). 10 keV. All data can be found online at vizier database
Supportforaninteraction-activityrelationwasrecentlyprovided (http://vizier.cfa.harvard.edu/viz-bin/VizieR?-source=VII/173).
by HI observations of Tang et al. (2008), who found that 94% AbouthalfofthelistedSeyfertgalaxiescanalsobefoundinthe
of Seyfert galaxies in their sample were disturbed in contrast IRAScatalogueandthusareIRselected.
to their control sample (where only 19% were disturbed), but Dultzin-Hacyan et al. (1999) selected from the catalogue
seealsoGeorgakakisetal.(2009)andCisternasetal.(2011)in two volume limited and complete samples, consisting of 72
the AEGIS and cosmossurveysrespectively.We pointoutthat Sy1 and 60 Sy2, to study their circumgalactic environment.In
a great theoretical success of the starburst/AGN connection is Koulouridis et al. (2006a), we used the same samples in order
the quenchingof the induced star formationby the AGN feed- to verify their results, using in addition redshift data from the
back, which can explain the formation of red and dead ellipti- CFA2andSSRSsurveysandourowndeeperspectroscopicob-
cal galaxies (e.g. Springel et al. 2005a; Di Matteo et al. 2005; servations. Our samples were limited in the area of these two
Khalatyanetal.2008).Recentobservationalstudiesandsimula- surveys (48 Sy1 and 56 Sy2) but were tested for consistency
tionshaveshownthatAGNionizedoutflowsmaycarryenough with the original ones. In both studies, well selected control
energytoceasestarformationinthehostgalaxy(e.g.Krongold samples(sameredshift,diameterandmorphologydistributions)
et al. 2007, 2009; Blustin et al. 2008; Hopkins & Elvis 2010; wereusedforthecomparison.
Novak,Ostriker&Ciotti2011;Cano-D´ıazetal.2012;Zubovas UsingtheCfA2andSSRSredshiftcatalogues,wesearched
& King 2012). In addition, the correlation of morphologiesof forneighbourswithinaprojecteddistanceD≤100h−1kpcand
thehostgalaxieswiththenuclearactivitytypecanalsoleadus aradialvelocityseparationδu≤600km/secandwefoundthat:
toapossibleAGNevolutionarysequence(Mart´ınezetal.2008).
We stress that the proposed evolutionary scenario does not – The Sy1 galaxies and their control sample show a similar
invalidatetheunificationscheme.Itimpliesthattheorientation (consistent within 1σ Poisson uncertainty) fraction of ob-
ofthetoruscandeterminetheAGNphenomenologyonlyatspe- jectshavingatleastonecloseneighbour.
cificphasesoftheevolutionarysequence.Inparticular,thisprob- – ThereisasignificantlyhigherfractionofSy2galaxieshav-
ablyoccurswhentheobscuringmolecularcloudsformthetorus, inganearneighbour,especiallywithinD≤75h−1kpc,with
beforebeingcompletelysweptaway. respecttoboththeircontrolsampleandtheSy1galaxies.
2
Koulouridisetal.:NeighboursofSeyfertGalaxies.
– Thelarge-scaleenvironmentofSy1galaxies(D=1h−1Mpc Basedontheabove,weadoptedthefollowingclassification
and δu ≤ 1000km/sec)is denser than that of Sy2 galaxies, scheme:
althoughconsistentwiththeirrespectivecontrolsamples.
– galaxieswithnoemissionlinesareconsideredtobenormal
Inordertoinvestigatewhetherfainterneighbours,thanthose
foundintherelativelyshallowCFA2andSSRScatalogues,ex- – galaxies with emission lines are active, meaning that they
istaroundourAGNsamples,weobtainedourownspectroscopic exhibitnuclearor/andstarformingactivity.
observationsof all neighbourswith mB∼<18.5 and D ≤ 75 h−1
kpc for a random subsample of 22 Sy1 and 22 Sy2 galaxies. Fluxratiosfortheemissionlinesmentionedabovehavebeen
We found that the percentage of both Sy1 and Sy2 galaxies measured after subtractingthe host galaxycontaminationfrom
thathaveacloseneighbourincreasescorrespondinglybyabout each spectrum. We disentangle the spectral contribution of the
100%whenwe descentfrommB∼<15.5to mB∼<18.5.Thesere- hostgalaxyfromtheobservedspectrabyusingthestellar pop-
sults imply that the originally found difference between Sy1 ulationsynthesiscodeSTARLIGHT3.Spectraprocessingandfits
and Sy2 persists even whenaddingfainter neighboursdown to were carried in the same fashion as described in section 3.1
δm∼<3, which correspond to a magnitude similar to the Large of Leo´n-Tavareset al. (2011).For a detailed description of the
MagellanicCloud. STARLIGHT codeanditsscientificresults,werefertothepapers
For the purposes of the present study we analyzed spectra oftheSEAGalcollaboration(Cid-Fernandesetal.2005,Mateus
ofalltheneighboursaroundtheaforementionedsubsamplesof etal.2006;Asarietal.2007;Cid-Fernandesetal.2007).
the 22Sy1& 22 Sy2respectively.InTable 1 and2we present AlthoughitispossibletodistinguishbetweenaSFGgalaxy
the names, celestial coordinates,O magnitudes1 and red- and an AGN using only the [NII]/H ratio, we cannot distin-
MAPS α
shiftsoftheSy1andSy2galaxieswhichhaveatleastoneclose guish between a low ionization (LINER) and a high ionization
neighbour(withinδu < 600km/sec).Notethatwehavekeptthe (Seyfert)AGNgalaxy.Wehavealsomeasured[OI](λ = 6300)
originalneighboursenumerationofthepreviouspapers(forex- whenpossible,asanextraindicatorofAGNactivity.However,
ample,in table 2, NGC1358hasonlyneighbour2, since 1 had theweaknessofthelineinmostcasesdidnotallowfurtheruse
δu>600km/sec). ofitinaseparateBPTdiagram.
In Fig.1a we plot the line ratios log([OIII]/H ) versus
β
log([NII]/H ) (BPT diagram) for those neighbours of Seyfert
α
2.2.SpectroscopicObservations galaxiesforwhichwehaveobtainedthefullspectrum4.Wealso
Wehaveobtainedspectroscopicdataofalltheneighbourgalax- plotthe Kauffmannetal. (2003a)separationlinebetweenSFG
ies in our samples in order to classify their type of activity. andAGNgalaxies,givenby:
Optical spectra were taken with the Boller & Chivens spec-
0.61
trograph mounted on the 2.1m telescope at the Observatorio log([OIII]/H )= +1.3,
Astrono´mico Nacional in San Pedro Ma´rtir (OAN-SPM). β (log([NII]/Hα)−0.05
Observations were carried out during photometric conditions.
andthecorrespondingoneofKewleyetal.(2001):
Allspectrawereobtainedwitha2′.′5slit.Thetypicalwavelength
range was 4000-8000 Å and the spectral resolution R=8Å. 0.61
log([OIII]/H )= +1.19.
Spectrophotometricstandardstarswereobservedeverynight. β (log([NII]/H )−0.47
The data reduction was carried out with the IRAF2 α
package following a standard procedure. Spectra were bias- we also plot in Fig.1b the line ratios log([OIII]/H ) vs
β
subtractedandcorrectedwith domeflat-field frames.Arc-lamp log([SII]/H ).Qualitatively,thesameresultsasthosepresented
α
(CuHeNeAr) exposures were used for wavelength calibration. in Fig.1a are repeated here as well. The dividing line is given
Allspectracanbefoundattheendofthepaper(Fig.3). by Kewley at al. (2006). However, we do not have the respec-
tivelineofKauffmannetal.(2003a),asitisnotavailableinthe
literature, and thus we cannotseparate pure starforminggalax-
2.3.AnalysisandClassificationMethod
iesfromcompositeobjects.Sincethemeasurementofthe[SII]
In this section we present results of our spectroscopic obser- doubletbearsingeneralgreatererrors,wewilldrawourresults
vations of all the neighbours with D ≤ 100 h−1 kpc and basedonthe[NII]forbiddenline.
mOMAPS∼<18.5forthesamplesofSy1andSy2galaxies.Wehave Wecannowclassifyourobjectsinthefollowingcategories:
alsousedSDSSspectrawhenavailable.
– SFG(StarformingGalaxies):alltheobjectswhicharefound
Our aim was to measure six emission lines: H λ4861,
β
belowthelineofKaufmannetal.
H λ6563, [NII] λ6583, [OIII] λ5007, [SII] λ6716 and [SII]
α
– AGNgalaxies:theobjectswhicharefoundabovethelineof
λ6731, in order to classify our galaxies, using the Baldwin,
Kewleyetal.
Phillips&Terlevich(1981,hereafterBPT)diagrams.Notethat
– TOs(transitionobjects):theonesthatarefoundbetweenthe
itwasnotpossibletomeasuretheH and[OIII]emissionlines
β
twolinesandexhibitcharacteristicsofbothanactivenucleus
inESO545-G013N1andthusweclassifieditusingonly[NII]
andastarburst.
andH (Stasin´skaetal.2006).
α
Wechoosetocallallgalaxieswithprominentemissionlines,
1 O (blue) POSS I plate magnitudes of the Minnesota Automated
thatdonotshowAGNactivity,SFGgalaxies.Wedonotattempt
Plate Scanner (MAPS) system. We use O magnitudes because
MAPS
Zwickymagnitudeswerenotavailableforthefainterneighbours, and todividethestarforminggalaxiesintomoresubcategoriessince
weneededahomogeneousmagnitudesystemforallourobjects. such a categorization appears to be highly subjective and de-
2 IRAFisdistributedbyNationalOpticalAstronomyObservatories pendsontheappliedmethodology(e.g.Knapen&James,2009).
operated by the Association of Universities for Research in
Astronomy,Inc.undercooperativeagreementwiththeNationalScience 3 http://starlight.ufsc.br/
Foundation. 4 Wehaveexcludedtwomergingneighbours(UGC7064)
3
Koulouridisetal.:NeighboursofSeyfertGalaxies.
3. Resultsandanalysis.
3.1.Activityoftheneighbours.
(a)
Inthissectionwediscussinmoredetailtheresultsofourspec-
1
troscopy and classification. We have excluded from our analy-
sisaSy2galaxywithmultiplenuclei,sincealthoughitqualifies
asAGNwithaclose(merging)neighbour,theindividualnuclei
arenotresolvedbyourspectroscopy.Wehavealsoexcludedthe
mergingneighbourofUGC7064,sincethepropertiesofitstwo
0 nucleiaremoreaffectedbytheirmutualinteractionthanbytheir
neighbouringSeyfert.Wecandrawourfirstresultsforeachsam-
ple separatelyinspectingTable 3. Fromthe analyzed15neigh-
bours of Sy1 only 4 are normal galaxies, while 8 of them are
SFGs, 2 are classified as TOsand oneis classified as an AGN.
SimilarresultsholdfortheneighboursofSy2galaxies.4outof
-1
13neighboursdonotshowactivity,6areSFGsand3areTOs.
Kauffmann et al. (2003a) Therefore,atleast 70%of the neighbours,within 100 h−1 kpc,
------ Kewley et al. (2001)
ofbothtypeofSeyfertgalaxiesareactive.Weshouldnotehere
-2 -1 0 1 that Ho et al. (1997), studying a magnitude limited sample of
galaxies(B ≤12.5),cameupwithasimilarhighpercentageof
T
activity(86%).However,theresultsofoursampleoffaintneigh-
bourscannotbedirectlycomparedwiththoseofHoetal.dueto
thebrightermagnitudelimitofthelatter.
(b)
We can extract one of the most interesting results of our
1
analysisbyexaminingFig.1,i.e.theneighboursofSy2galaxies
have systematically larger values of [OIII]/H than the neigh-
β
boursofSy1galaxies.Especiallyforthosegalaxiesthatexhibit
onlystar-formation,theratio[OIII]/Haismainlyanindicatorof
theirionizationlevel,whichinitsturnisanindicatorofmetallic-
ity.Furthermore,metallicityiscloselyrelatedtothestellarmass
0
(M −Z relation). The average age of the stellar population
⋆ neb
alsochangesalongthe[OIII]/H axis,thehigherionizedgalax-
β
ies having higher ratio of current to past star-formation rates
(Asarietal.2007,CidFernandesetal.2009).Summarizingall
oftheaboveweconcludethattheneighboursofSeyfert2galax-
-1 ies show higher ionization, lower metallicity, less stellar mass
and more importantly younger stellar populations than those
------ Kewley et al. (2006)
of Seyfert 1 galaxies. Using a two-dimensional Kolmogorov-
Smirnovtwo-sampletestwehaveverifiedthatthenullhypothe-
-2 -1 0 1
sis,thatthesamplesaredrawnfromthesameparentpopulation,
isrejectedata99.9%level.
Fig.1. BPT classification diagrams of the neighbours of Sy1
Finally we should note how close to a composite state are
and Sy2 galaxies.The neighboursof Sy2 and Sy1 galaxiesare
the neighboursofactivegalaxies,in agreementwith Kewleyat
indicatedbytrianglesandcrosses,respectively.
al. (2006a)whoshowed thatthe starformingmembersof close
pairs, lie closer to the classification line than the starforming
field galaxies. We suggest that galaxies between the curves of
We also classify our objects using only the [NII]/H ratio,
α Kauffmannetal.(2003)andKewleyetal.(2001)migratefroma
asproposedbyStasin´skaetal.(2006).Inmoredetail,we clas-
purestarformingphasetoapureAGNphase.Thissuggestionis
sify as AGN those objects with log([NII]/H ) > −0.2, as SFG
α ofgreatimportancetoapossibleevolutionaryscenarioandwill
those with log([NII]/H ) < −0.4, and as TO the rest. We find
α bediscussedfurtherin§4.
that it is accurate in all cases, except one (see Table 3), with
the onlycaveat thatit cannotdistinguish between LINERsand Summarizingourmainresultsofthissection:
Seyferts. Hence, we use this classification for ESO 545-G013
N1 forwhichwe couldnotmeasurethe H and[OIII](λ5007)
β
lines. – Morethan70%oftheneighboursofthetwo AGNsamples
Further classification of the Seyfert galaxies in type 1 and exhibitactivity,asindicatedbyopticalspectroscopy.
type2wasobtainedbydirectvisualexaminationofthespectra – Around30%oftheneighborsofSy1andSy2galaxiesshow
and the broadeningof the emission lines. No broad lines were thepresenceofAGNactivity.
discovered in the spectrum of the two neighboursclassified as – The neighbours of Sy2s are systematically more ionized,
AGNandthereforetheyshouldbeconsideredastype2.InTable thantheneighboursofSy1s, indicatingthattheir currentto
3welistforallneighbourstheirlineratiosandthetwodifferent paststarformationrateishigher.
classifications.Ascanbeobserved,inallcasesexceptNGC3786 – No broad lines (type 1 activity) were detected in the two
N1,theclassificationsareabsolutelyconsistent. neighbourswhichexhibitpureAGNspectra.
4
Koulouridisetal.:NeighboursofSeyfertGalaxies.
farbelowthedashedline.OneneighbourwhichliesintheAGN
regime(NGC7682-N1)canbeclearlyseen.Thishasbeenclas-
sifiedasaTOgalaxyintheopticalspectroscopicanalysisandis
one of the three neighbours(for which XMM observationsare
available, see Table 4) having an active nucleus based on op-
tical spectroscopy.Additionalinformationon the nature of our
sourcescanbe extractedfromthe hardnessratios. Twosources
NGC526-N2andNGC1358-N2havehardnessratiossuggesting
absorption N ≈ 1022 cm−2, consistent with the presence of a
H
moderatelyobscuredactivenucleus.Boththesegalaxiespresent
noopticalemissionlinesandthusareclassifiedasnormal,based
ontheiropticalspectra.Inotherwords,thelackofopticalemis-
sion lines from the nucleus of these objects could be a result
of obscuration.In addition,we should mentionhere thatin the
groupof objectswith an X-ray observation,all galaxiesclassi-
fiedas“normal”throughopticalspectroscopypresentanX-ray
detection,possiblyalsoindicatingthepresenceofnuclearactiv-
ity. In contrast, all SFG galaxies except one in the X-ray sub-
sampledonotshowanX-raydetection.
WeshouldnoteherethatunobscuredlowaccretionrateSy2
objectsand/orlow luminosityAGN, where the NLR cannotbe
Fig.2.TheX-ray(0.2-12keV)tooptical(B-band)fluxdiagram detectedbymeansofopticalspectroscopy,orevenX-raybina-
forboththeAGNtargets(solidcircles)andtheneighbors(open ries, may account for the X-ray detection of unobscured ”nor-
circles). The triangles (upperlimits) denote the neighborswith mal”galaxies.Thisanalysisthereforeimpliesthatthetotalfrac-
noX-raydetection.Theupper,lowersolidlineandthedashline tionof neighboursofAGN thatshowactivity,basedonoptical
correspondto f /f =+1,−1,−2respectively. spectroscopy,isatleast70%andpossiblyquitehigher.Thismat-
X B
terwillbefullyaddressedinfuturework.
3.2.TheXMM-Newtonobservations
4. Discussion&Conclusions
We have also exploredwhether the neighboursshow X-rayac-
tivity, using the XMM public archive. We find that 13 target Basedonopticalspectroscopy,wehavefoundthatthelargema-
fields have been observed by XMM. However, some of them jority(> 70%)ofcloseneighboursofAGN(beingSy1orSy2)
areverybrightandhavebeenobservedinpartialwindowmode, showactivity,mostlyenhancedstar-formationbutAGNaswell.
renderingtheobservationsincenteroftheField-of-Viewunus- Furthermore,the close neighboursof Sy1 galaxies, being SFG
able(NGC5548,NGC863,1H1142-178,NGC7469).Thelistof or AGN,are less ionizedand thusseem to be a different,more
theremainingobservations(13neighboursand9centralSeyfert evolved,populationthanthoseofSy2s.
galaxies)isshowninTable4,inwhichwepresentX-rayfluxes In addition, our X-ray analysis of a subsample of neigh-
for the detections as well as upper limits for the non-detected bours with public XMM observations showed that the neigh-
sources.Thefluxeshavebeentakenfromthe2XMMcatalogue bours which are classified as ”normal” based on optical spec-
(Watson et al. 2009). The fluxes refer to the total 0.2-12 keV troscopy, might have an active core, since all of them are X-
bandforthePNdetectororthecombinedMOSdetectorsinthe raydetected,whiletwooutoffiveappeartohaveamoderately
case wherePN fluxesarenotavailableand areestimatedusing obscured active nucleus. On the contrary, the pure star form-
aphotonindexofΓ = 1.7andanaverageGalacticcolumnden- ing neighbours do not show any X-ray emission, down to the
sityofN = 3×1020 cm−2.Luminositieswereestimatedusing fluxlimitoftheavailableobservations.Fromboth,opticalspec-
H
the same spectral parameters. In the same table we quote the troscopyandX-rayobservations,itbecomesclearthatthefrac-
2XMMhardnessratios,derivedfromthe1-2keVand2-4.5keV tionof neighbours,within 100h−1 Mpc,of AGNorBIRG that
bands (hardness ratio-3 according to the 2XMM catalogue no- areactiveis>70%andpossiblymuchhigher.
tation). The upperlimits, derivedusing theFLIXsoftware, are Sincethephysicalpropertiesoftheneighboursshouldbere-
estimated following the method of Carrera et al. (2007). This flected on the state of the central AGN galaxy, we argue that
provides upper-limits to the X-ray flux at a given point in the these results are in the same direction as those of our previous
skycoveredbyXMMpointings.Theradiususedforderivingthe papers (Koulouridis et al 2006a,b), supporting an evolutionary
upper limit was 20 or 30 arcsec depending on the presence of sequence of galaxy activity, induced by interactions, the main
contaminatingnearbysources. path of which starts from inactivity and then follows the se-
InFig.2wepresenttheX-raytoopticalfluxdiagram f − f quenceofstarburst,Sy2andfinallySy1phase.
X B
(e.g. Stocke et al. 1991). This diagram provides an idea on We now attempt to interpret our present results within the
whether a galaxy may host an active nucleus. This is because aboveevolutionaryscheme.Somethingthatisnowadaysundis-
AGN have enhanced X-ray emission at a given optical magni- puted is the role of interactions in the creation of a starburst.
tuderelativetonormalgalaxies.Thespaceusuallypopulatedby Molecular clouds are being forced towards the galactic cen-
AGNisshownbetweenthecontinuouslines.ThecentralSeyfert ter, become compressed and light up the galaxy as a starburst.
galaxiesareshownasfilledpoints,butsinceX-rayfluxhasnot Despite the fact that the exact mechanism is still unknown, in
beencorrectedforX-rayabsorption,anumberofabsorbedAGN the local universe an accretion rate of ∼ 0.001− 0.1M⊙/yr is
galaxiesliebetweentheloweranddashedline,whiletheheavily needed in order to fuel the black hole. Theoretically this can
absorbedSy2NGC7743(Akylas&Georgantopoulos2009),lie onlybeachievedbymeansofa nonaxisymmetricperturbation
5
Koulouridisetal.:NeighboursofSeyfertGalaxies.
whichinducesmassinflow.Suchperturbationsareprovidedei- can be accessed at http://aps.umn.edu/). The STARLIGHT
therby barsorby interactions.Whicheverthe mechanismmay project is supported by the Brazilian agencies CNPq, CAPES
be,theresultisthefeedingoftheblackholeandtheactivation and FAPESP and by the FranceBrazil CAPES/Cofecub pro-
oftheAGN phase,maybe∼ 0.5Gyr afterthe initialinteraction gramme.FundingfortheSDSSandSDSS-IIhasbeenprovided
tookplace.Aninteractioncertainlypredictssuchadelay,since bytheAlfredP.SloanFoundation,theParticipatingInstitutions,
after the material has piled up around the inner Linblad reso- the National Science Foundation, the U.S. Department of
nance,producingtheStarburst,itcanbechanneledtowardsthe Energy,theNationalAeronauticsandSpaceAdministration,the
nucleus by loosing significant amounts of angular momentum, Japanese Monbukagakusho, the Max Planck Society, and the
aprocesswhichisnotinstantaneous.Therefore,adelayshould HigherEducationFundingCouncilforEngland.TheSDSSWeb
exist between the pure starformingand AGN phase, where ac- Siteishttp://www.sdss.org/.
tive nucleus and circumnuclear starburst coexist. In this initial
phase, the nucleus is heavily obscured by the still starforming
molecularcloudsanditcanbeobservedasatransitionstageof
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Koulouridisetal.:NeighboursofSeyfertGalaxies.
Table1.ThesampleofSy1galaxiesandtheirneighbours1
NAME RA DEC O z
MAPS
J2000.0 J2000.0 integrated
NGC863 021433.5 −004600 14.58 0.0270
neighbour1 021429.3 −004605 18.25 0.0270
MRK1400 022013.7 +081220 17.07 0.0293
neighbour1 021959.8 +081045 17.25 0.0284
NGC1019 023827.4 +015428 15.02 0.0242
neighbour2 023825.4 +015807 16.28 0.0203
NGC1194 030349.1 −010613 15.38 0.0134
neighbour1 030341.2 −010425 16.99 0.0140
neighbour4 030412.5 −011134 15.75 0.0130
1H1142−178 114540.4 −182716 16.82 0.0329
neighbour1 114540.9 −182736 18.01 0.0322
neighbour2 114538.8 −182919 18.45 0.0333
MRK699 162345.8 +410457 17.21 0.0342
neighbour1 162340.4 +410616 17.59 0.0334
NGC7469 230315.5 +085226 14.48 0.0162
neighbour1 230318.0 +085337 15.58 0.0156
NGC526A2 012354.5 −350356 15.693 0.0191
neighbour1 012357.1 −350409 15.803 0.0188
neighbour2 012358.1 −350654 15.683 0.0189
neighbour3 012409.5 −350542 16.373 0.0185
neighbour4 012359.2 −350738 16.043 0.0185
NGC5548 141759.5 +250812 14.18 0.0172
neighbour1 141733.9 +250652 17.16 0.0172
NGC6104 161630.7 +354229 15.11 0.0279
neighbour1 161649.9 +354207 16.44 0.0264
(1)withinaprojecteddistanceof100h−1kpcs
(2)RegionNotCoveredbyMAPSCatalog
(3) CalculatedfromO ,usingrelation:O = 14.61(±1.25)+
USNO MAPS
0.11(±0.11)O ,derivedfromVe´ron-Cettyetal.(2004)Table2.
USNO
8
Koulouridisetal.:NeighboursofSeyfertGalaxies.
Table2.ThesampleofSy2galaxiesandtheirneighbours1
NAME RA DEC O z
MAPS
J2000.0 J2000.0 integrated
ESO545-G013 022440.5 −190831 14.41 0.0338
neighbour1 022450.9 −190803 16.19 0.0340
NGC3786 113942.5 +315433 13.88 0.0091
neighbour1 113944.6 +315552 13.53 0.0085
UGC12138 224017.0 +080314 15.93 0.0250
neighbour1 224011.0 +075959 18.77 0.0236
UGC7064 120443.3 +311038 15.11 0.0250
neighbour1(2)2 120445.6 +311127 16.68 0.0236
neighbour1 120445.2 +311133 16.33 0.0244
neighbour2 120445.1 +310934 16.33 0.0261
IRAS00160−0719 001835.9 −070256 15.73 0.0187
neighbour1 001833.3 −065854 17.80 0.0173
ESO417-G06 025621.5 −321108 15.54 0.0163
neighbour1 025640.5 −321104 17.43 0.0163
NGC1241 031114.6 −085520 13.56 0.0135
neighbour1 031119.3 −085409 15.41 0.0125
NGC1320 032448.7 −030232 14.59 0.0090
neighbour1 032448.6 −030056 15.07 0.0095
MRK612 033040.9 −030816 15.78 0.0207
neighbour1 033042.3 −030949 16.13 0.0205
NGC1358 033339.7 −050522 13.98 0.0134
neighbour2 033323.5 −045955 14.95 0.0131
IC4553 153457.1 +233016 14.43 0.0181
neighbour1 153457.3 +233005 15.68 0.0190
NGC7672 232731.4 +122307 15.23 0.0134
neighbour1 232719.3 +122803 14.67 0.0138
NGC7682 232903.9 +033200 14.88 0.0171
neighbour1 232846.6 +033041 14.64 0.0171
NGC7743 234421.1 +095603 12.16 0.0044
neighbour3 234405.5 +100326 16.95 0.0054
(1)withinaprojecteddistanceof100h−1kpcs
(2) In Koulouridis et al. 2006a neighbour 1 was an unresolved
merging galaxy. In this paper we were able to obtain two distinct
spectraforeachnucleus.
9
Koulouridisetal.:NeighboursofSeyfertGalaxies.
Table3.Observationaldata,emissionlineratiosandclassification.
NAME Neigh.No U.T.date startU.T. exp.time [OIII]/Hβ [NII]/Hα [SII]/Hα Stasin´ska1 BPT2
(dd/mm/yy) (sec)
Sy1galaxies
NGC863 N1 SDSS - - - - - normal normal
MRK1400 N1 06/10/07 07:31 4800 0.55±0.02 0.31±0.01 0.35±0.01 SFG SFG
NGC1019 N2 21/10/06 08:25 4800 0.54±0.01 0.46±0.01 0.32±0.01 TO TO
NGC1194 N1 SDSS - - 0.37±0.01 0.31±0.01 0.35±0.01 SFG SFG
NGC1194 N4 25/10/06 07:56 5400 0.33±0.01 0.38±0.01 0.36±0.01 SFG SFG
1H1142−178 N1 19/05/07 04:29 3000 - - - normal normal
1H1142−178 N2 21/05/07 04:16 6000 0.83±2.54 0.35±0.03 0.57±0.06 SFG SFG
MRK699 N1 18/05/07 10:27 2100 0.64±0.12 0.60±0.06 0.67±0.05 TO TO
NGC7469 N1 01/12/06 03:07 3600 0.30±0.05 0.37±0.01 0.29±0.01 SFG SFG
NGC526A N1 08/10/07 06:39 2400 3.60±0.37 1.37±0.19 1.32±0.18 AGN AGN
NGC526A N2 08/10/07 08:39 1500 - - - normal normal
NGC526A N3 08/10/07 09:34 3600 0.57±0.06 0.35±0.01 0.35±0.02 SFG SFG
NGC526A N4 08/10/07 07:33 3600 0.34±0.02 0.34±0.01 0.38±0.01 SFG SFG
NGC5548 N1 SDSS - - 0.50±0.05 0.36±0.01 0.52±0.01 SFG SFG
NGC6104 N1 18/05/07 09:37 1800 - - - normal normal
Sy2galaxies
ESO545-G013 N1 01/12/06 05:13 3600 - 0.37±0.01 0.37±0.02 SFG -
NGC3786 N1 06/03/06 06:34 3600 1.08±0.06 0.71±0.02 0.57±0.02 AGN TO
UGC12138 N1 08/10/07 02:49 3600 4.48±0.11 0.07±0.01 0.18±0.01 SFG SFG
UGC7064 N1(2) 18/05/07 07:11 4200 0.25±0.02 0.38±0.01 0.15±0.01 SFG SFG
UGC7064 N1 SDSS - - 3.55±0.34 1.34±0.4 0.83±0.03 AGN AGN
UGC7064 N2 06/03/06 08:40 2100 0.74±0.05 0.56±0.02 0.30±0.02 TO TO
IRAS00160−0719 N1 06/10/07 0.97±0.02 0.25±0.01 0.44±0.01 SFG SFG
ESO417-G06 N1 06/10/07 11:08 4200 1.29±0.01 0.21±0.01 0.29±0.01 SFG SFG
NGC1241 N1 30/11/06 08:00 3600 1.02±0.04 0.34±0.01 0.35±0.01 SFG SFG
NGC1320 N1 25/10/06 09:38 3600 - - - normal normal
MRK612 N1 29/11/06 09:44 3600 - - - normal normal
NGC1358 N2 21/10/06 11:08 3600 - - - normal normal
IC4553 19/05/07 09:14 5400 merger
NGC7672 N1 21/10/06 05:51 3600 - - - normal normal
NGC7682 N1 25/10/06 06:42 3600 1.25±0.01 0.45±0.01 0.27±0.01 TO TO
NGC7743 N3 20/10/06 07:00 5400 2.27±0.03 0.07±0.01 0.24±0.01 SFG SFG
(1)ClassificationbasedonStasin´skaetal.(2006)
(2)ClassificationbasedontheBPTdiagrams(Baldwin,Phillips&Terlevich1981)
Table4.XMMobservations.
Name Neigh.No 2XMMID Opt.Class logL (0.2-12keV) Flux(0.2-12keV) X/Ooffset HR
X
(ergs−1) (ergcm−2s−1) (arcmin)
NGC1194 N1 - SFG <39.59 <7.3×10−15 - -
NGC1194 N4 - SFG <39.72 <1.1×10−14 - -
NGC526A N1 J012357.0-350410 AGN 40.46 3.3×10−14 0.023 −0.28±0.09
NGC526A N2 J012358.1-350653 Normal 40.75 5.9×10−14 0.008 0.05±0.1
NGC526A N3 - SFG <39.65 <4.7×10−15 - -
NGC526A N4 J012359.0-350741 SFG 39.95 9.4×10−15 0.035 −0.61±0.29
UGC12138 N1 - SFG <40.63 <2.8×10−14 - -
NGC1320 N1 J032448.6-030057 Normal 39.46 1.2×10−14 0.020 −0.38±0.17
MRK612 N1 J033042.5-030949 Normal 39.59 3.4×10−15 0.060 −0.67±0.24
NGC1358 N2 J033323.3-045953 Normal 40.19 3.8×10−14 0.044 0.05±0.3
NGC7682 N1 J232846.7+033041 TO 42.04 1.30×10−12 0.026 −0.32±0.02
NGC7743 N3 - SFG <39.44 3.4×10−14 - -
NGC3786 N1 J113944.3+315547 TO 39.73 2.7×10−14 0.08 −0.46±0.30
10