The powerful jet of an off-nuclear intermediate-mass black hole in the spiral galaxy NGC 2276

0.69 MB·English

by  M. Mezcua

Mon.Not.R.Astron.Soc.000,1–7(2013) Printed10February2015 (MNLaTEXstylefilev2.2)
The powerful jet of an off-nuclear intermediate-mass black hole in
the spiral galaxy NGC 2276
M. Mezcua,1,2,3(cid:63) T.P. Roberts,4 A.P. Lobanov5† A.D. Sutton,4
5 1Harvard-SmithsonianCenterforAstrophysics(CfA),60GardenStreet,Cambridge,MA02138,USA
1 2InstitutodeAstrof´ısicadeCanarias(IAC),E-38200LaLaguna,Tenerife,Spain
0 3DepartmentAstrof´ısica,UniversidaddeLaLaguna,E-38206LaLaguna,Tenerife,Spain
2 4DepartmentofPhysics,UniversityofDurham,SouthRoad,DurhamDH13LE,UK
b 5MaxPlanckInstituteforRadioAstronomy,AufdemHu¨gel69,D-53121Bonn,Germany
e
F
8 Accepted2015January20

]
A ABSTRACT
G
Jet ejection by accreting black holes is a mass invariant mechanism unifying stellar
.
h and supermassive black holes (SMBHs) that should also apply for intermediate-mass black
p holes(IMBHs),whicharethoughttobetheseedsfromwhichSMBHsform.Wepresentthe
- detection of an off-nuclear IMBH of ∼5 × 104 M(cid:12) located in an unusual spiral arm of the
o
r galaxy NGC 2276 based on quasi-simultaneous Chandra X-ray observations and European
t VLBINetwork(EVN)radioobservations.TheIMBH,NGC2276-3c,possessesa1.8pcradio
s
a jet that is oriented in the same direction as large-scale (∼650 pc) radio lobes and whose
[ emissionisconsistentwithflattoopticallythinsynchrotronemissionbetween1.6and5GHz.

Itsjetkineticpower(4×1040ergs−1)iscomparabletoitsradiativeoutputanditsjetefficiency
2
((cid:62)46%)isaslargeasthatofSMBHs.Aregionof∼300pcalongthejetdevoidofyoungstars
v
7 couldprovideobservationalevidenceofjetfeedbackfromanIMBH.Thediscoveryconfirms
9 thattheaccretionphysicsismassinvariantandthatseedIMBHsintheearlyUniversepossibly
8 hadpowerfuljetsthatwereanimportantsourceoffeedback.
4
0 Keywords: accretion,accretiondiscs–blackholephysics–ISM:jetsandoutflows–Radio
. continuum:general–X-rays:binaries.
1
0
5
1
: 1 INTRODUCTION formation triggered by jets or outflows (Volonteri & Silk 2014)
v
i or supra-exponential accretion (Alexander & Natarajan 2014).
X Intermediate-massblackholes(IMBHs)constitutethemissinglink Cosmologicalsimulationsshowthattheradiativeandmechanical
betweenstellar-massandsupermassiveblackholes(SMBHs)and
r feedbackfromanaccretingIMBHregulateitsgrowthanddeprive
a are the potential long-sought seeds from which SMBHs grow.
theinnercoreofthegalaxyofcoldstar-forminggas(Duboisetal.
IMBHs could form either from the death of very massive and
2012;Kimetal.2011).However,thedetectionofIMBHsand,in
short-lived stars, the direct collapse of a pre-galactic gas disc, or
particular,ofjetemissionfromIMBHsisscarceandnoevidence
thecollapseofdensestellarclusters,andthengroweitherthrough
ofitsfeedbackeffectshashithertobeenobserved.
hierarchical merging or secular gas accretion (Volonteri 2010).
They should thus be present in the nuclei of low-mass galaxies, The best observational evidence for IMBHs has been found
aspredictedfromBHmass(MBH)-scalingrelations(e.g.Graham inthenucleioflow-massspiralanddwarfgalaxies(Greene&Ho
& Scott 2013); in dense star-forming regions in spiral galaxies; 2007;Dongetal.2007;Thorntonetal.2008;Reinesetal.2013;
and in the haloes of large galaxies (e.g. after tidal stripping of Schrammetal.2013;Yuanetal.2014)andinultraluminousX-ray
merging low-mass satellite galaxies). SMBHs with masses up to sources(ULXs;Farrelletal.2009).Evidenceismountingthatthe
109 M(cid:12) already existed when the Universe was less than ∼1 high X-ray luminosity of the majority of ULXs can be explained
Gyr old (e.g. Mortlock et al. 2011). To reach this mass in such by stellar-mass BHs accreting at around or above the Eddington
a short time, seed IMBHs must have undergone short phases limit (e.g. Gladstone et al. 2009; Sutton et al. 2013; Motch et al.
of growth above the Eddington rate coupled with enhanced star 2014).However,thoseULXswithL >5×1040ergs−1remain
X
difficulttoexplainassuper-Eddingtonaccretionontostellar-mass
BHs,anddisplayX-rayemissionpropertiesconsistentwithIMBHs
(cid:63) Email:[email protected] eitheraccretinginasub-Eddingtonhardstate(Suttonetal.2012)or
† Visiting Scientist, University of Hamburg / Deutsches Elektronen inanintermediatestate(Pashametal.2014).Onesuchobjectisa
SynchrotronForschungszentrum. ULXinNGC2276(NGC2276-3cforsimplicity),whichhadapeak
(cid:13)c 2013RAS
2 M.Mezcuaetal.
X-rayluminosityofupto∼6×1040ergs−1whenobservedwith ThisspectralluminosityisofthesameorderasthatoftheIMBHin
the XMM-Newton satellite (see Section 2.2 for further details). If NGC4395(Wrobel&Ho2006)andoneorderofmagnitudelower
theyareinthesub-Eddingtonhardorintermediatestates,accreting than the radio luminosity of parsec-scale jets in active galactic
BHs are expected to emit radio jets. Observational evidence for nuclei(AGN)residinginspiralgalaxies(e.g.NGC1097;Mezcua
compact or transient jet radio emission has been found in a few & Prieto 2014). The emission was fitted by a two-dimensional
ULXs (Mezcua & Lobanov 2011; Cseh et al. 2012, 2014; Webb elliptical Gaussian component, which is centred at RA(J2000) =
et al. 2012; Middleton et al. 2013; Mezcua et al. 2013a, 2014). 07h26m48s.092 ± 0s.001, Dec.(J2000) = +85◦45(cid:48)48(cid:48)(cid:48).9180 ±
Thebestevidenceforthepresenceofaradiojetinanoff-nuclear 0(cid:48)(cid:48).0006,hasamajoraxisdeconvolvedsizeof11.3mas(∼1.8pc)
IMBHcomesfromESO243-49HLX-1,whichshowsflaringradio orientedatapositionangleof78◦andabrightnesstemperatureT
B
emission when appearing to transition from the low/hard X-ray =1.5×105K.Thedeconvolvedsizeof11.3masislargerthanthe
state to the high/soft X-ray state (Webb et al. 2012; Cseh et al. resolutionlimit1of5.4mas,implyingthattheemissionisresolved
2015). However, observational evidence of extended, steady jet along the major axis. Its orientation, at a position angle of 78◦
radio emission from IMBHs has so far only been found in the north-through-east,isconsistentwiththatoftheVLAradiolobes.
nuclearIMBHsNGC4395(jetextent∼0.3pc;Wrobel&Ho2006) ThefindingofT > 105 rulesoutthermalfree–freeemission(of
B
andGH10(jetextent<320pc;Wrobeletal.2008).FortheULX typically104K)frome.g.HIIregions,suggestingthattheemission
NGC2276-3c,extendedradioemissionformedbytwolobesoftotal maybenon-thermal.Thesameresultisobtainedwhenperforming
size ∼650 pc was detected with the Karl G. Jansky Very Large hybrid imaging with CLEAN deconvolution plus self-calibration
Array (VLA). The X-ray source is located in between the two loops, which proves that the extended emission is not caused by
radio lobes, suggesting the presence of a central BH powering a phaseerrors.
two-sided radio jet (Mezcua et al. 2013c). Using the X-ray peak The radio map at 5 GHz was produced using the data from
luminosity, a M (cid:62) 4700 M(cid:12) was estimated assuming 10% baselines shorter than 20 Mλ (1200 km) in order to improve the
BH
Eddington accretion (Sutton et al. 2012). The VLA observations detectionofextendedemission.Arestoringbeamof16.4×13.1
couldthushaverevealedthelargestradiojeteverdetectedfroman mas full width at half-maximum (FWHM) was used in order to
IMBH. matchthatofthe1.6GHzradiomapandderivethespectralindex.
Inthispaper,wereportquasi-simultaneousverylongbaseline A compact component of flux density 39 ± 18 µJy and radio
interferometry(VLBI)radioobservationswiththeEuropeanVLBI luminosityofL =2.6×1035ergs−1isdetectedatthe∼8σ
5GHz
Network (EVN) and X-ray observations with the Chandra X-ray level.ThecomponentiscentredatRA(J2000)=07h26m48s.093±
observatory of NGC2276-3c, which reveal the detection of a 0s.001,Dec.(J2000)=+85◦45(cid:48)48(cid:48)(cid:48).912±0(cid:48)(cid:48).001.Ithasasizeof
parsec-scale radio jet with an orientation consistent with that of 13.3×12.7masorientedatapositionangleof11◦andT =1.1×
B
theVLAradiolobesandallowustoestimateaM intheIMBH 104K.Thetotalpositionalerrorsofthetargetsourceareestimated
BH
range.Adescriptionoftheobservationsandresultsisprovidedin at both frequencies as a quadratic sum of the positional error of
Section 2. The discussion of the results and final conclusions are thetargetsourceinthephase-referencedmap,thepositionalerror
giveninSections3and4,respectively. ofthephase-referencecalibratorandtheerrorofphasereferencing
due to ionospheric effects. This yields a total positional error for
NGC2276-3c of 3.5 mas at 1.6 GHz and 2.4 mas at 5 GHz. The
positionofthecomponentsidentifiedateachfrequencycoincides
2 OBSERVATIONS,ANALYSISANDRESULTS within these positional errors. A spectral index α = −0.5 ± 0.2,
S ∝ να,isderivedfromtheintegratedfluxdensitiesat1.6and
ν
2.1 EVNobservations
5GHz.Usingthepeakdensities,anevenflatterspectralindexα=
EVN observations of NGC2276-3c were carried out during two −0.3±0.1wouldbeobtained.Inbothcases,thespectralindexis
6-h observing runs on 2013 May 23 and June 3 at 5 and consistentwithflattoopticallythinsynchrotronemission.
1.6 GHz, respectively (experiments EM106A and EM106B).
The observations were carried out in the phase-reference mode,
2.2 Chandraobservations
alternating scans between the target source and a nearby (∆θ ∼
3◦.2) compact source used as phase calibrator (J0508+8432). In A Chandra X-ray observation (ID 15648) was carried out
total, ∼3 h were spent on NGC2276-3c at both frequencies. The quasi-simultaneously with (i.e. started within one day of the
brightradiosource4C+39.25wasalsoobservedtobeusedasfringe completionof)the5-GHzradioobservations,withthetargetplaced
finder and bandpass calibrator. The data were recorded at a rate at the nominal aim-point on the S3 chip of the ACIS-S array.
of1024Mbpsindualcircularpolarization,usingeightsub-bands The source was observed for an exposure time of 25 ks, and the
(each 16 MHz in width and split into 32 spectral channels), and observationwasprocessedusingstandardtoolsinCIAOv4.6,with
correlatedattheEVNCorrelatorFacilityoftheJointInstitutefor calibrationfilesfromCALDBv4.6.1.1.Intotal,188countswere
VLBIinEurope(JIVE)withanaveragingtimeof1s.Amplitude detectedfromtheULX,yieldinga0.3–10keVcountrateof(7.5
calibration (using system temperatures and antennas gains) and ±0.6)×10−3counts−1.Thesourcewasalsopreviouslydetected
fringefittingwasperformedusingtheAIPSsoftware.Ionospheric byChandraon2004June23(ID4968),whenithadacountrate
calibrationwasalsoappliedatbothfrequencies.Radiomapsat1.6 of∼2.6×10−3counts−1(Wolteretal.2011;Suttonetal.2012).
GHz(Fig.1d)and5GHzwereobtainedusinghybridimagingwith Hence,NGC2276-3cwas∼3timesbrighterinthenewobservation
CLEAN deconvolutionand thenaturalweightingofthedata. At
1.6GHz,radioemissionata9σlevelisdetectedinbetweenthetwo
peaksoftheradiolobespreviouslydetectedwiththeVLA(Fig.1, 1 The resolution limit is given by θlim =
bd)e.riTvheeaneminitsesgiorantehdasraadfliouxludmeinnsoitsyityofL615.6±GH1z6=µ1J.y4,×fro1m03w5heircghsw−1e tπ2he(cid:2)bπebammamjbamjoirnalnnd((mSiNnoRr+axi1s),/reSsNpeRc)ti(cid:3)v1e/ly2,,awndheSrNeRbmthaejsaignndalb-mtoi-nnoaisree
assumingadistancetoNGC2276of33.3Mpc(Suttonetal.2012). ratio(Lobanov2005).
(cid:13)c 2013RAS,MNRAS000,1–7
RadiojetofIMBH 3
Figure1.a)Three-colourimageofthewesternarmofNGC2276wheretheULXislocated.TheimagehasbeencreatedusingthreefiltersoftheWide
FieldPlanetaryCamera2ontheHST:F814Winred(∼0.8µm),F606Wingreen(∼0.6µm)andF550Winblue(∼0.55µm).b)I-band(∼0.8µm)HST
imageoftheregionwheretheULXNGC2276-3cislocated.The5GHzVLAradiolobes(Mezcuaetal.2013c)areoverplottedas(5,6,7,8,9,10)timesthe
off-sourcermsnoiseof0.04mJybeam−1.TheChandraX-raypositionismarkedwithagreencircleof∼1arcsecdiameter.TheVLAbeamsizeis4.6arcsec
×3.5arcsec.c)Chandrared(0.2–1.5keV),green(1.5–2.5keV)andblue(2.5–8keV)imageconvolvedwithan∼2arcsecFWHMGaussian.Theposition
ofNGC2276-3cismarkedwithabluecircleof∼1arcsecdiameter.d)EVN1.6GHzimageofNGC2276-3c.Thesynthesizedbeamsizeis16.4×13.1mas
orientedataP.A.of−52◦.Theoff-sourcermsnoiseis8µJybeam−1.
(cid:13)c 2013RAS,MNRAS000,1–7
4 M.Mezcuaetal.
thanintheonlypreviousobservationinwhichitwasresolvedfrom with a doubly absorbed power law, by minimizing the modified
itsneighbouringULXs. Cash-statistic(whichallowsforabackgroundspectrum)inXSPEC.
We used XSPEC v12.8.1 to fit models to the binned X-ray Wefindthatsource3aiswellmodelledbyapower-lawspectrum
spectrum. Both an absorbed power-law and a multicolour-disc withN = (5±2)×1021 cm−2,Γ ∼ 1.7±0.2andobserved
H
(MCD) model were used to fit the data. In both cases, we 0.3–10keVluminosityL =(1.3±0.2)×1040ergs−1,whereas
X
included two absorption components: the first was set equal to source3bhasN = (6±2)×1021 cm−2,Γ = 2.2±0.4and
H
theline-of-sightGalacticcolumndensityinthedirectionofNGC observed 0.3-10keV luminosity L = 5.5+1.1 ×1039 erg s−1.
X −0.9
2276 (5.52 × 1020 cm−2; Dickey & Lockman 1990), whilst the Thesemodelparametersandfluxesareconsistent(withinthe90per
other was free to vary, to model any intrinsic absorption in the cent uncertainty regions) with the previous values from the 2004
sourceand/orthehostgalaxy.Thefitstatisticsforthepower-law observation.
and MCD were χ2/degrees of freedom = 13.7/15 and 13.2/15
respectively. We extracted the best-fitting model parameters from
the absorbed power-law and MCD. These were N = 1.3+0.5 ×
H −0.4
1022 cm−2 andΓ = 1.4±0.3forthepower-law,andN =(0.9
H
± 0.3) × 1022 cm−2 and k T = 2.4+1.0 keV for the MCD.
B in −0.5
All of these parameters are consistent with the values reported
bySuttonetal.(2012)forthepreviousChandraobservation,and
3 DISCUSSION
onlythenormalizationsdiffersignificantly.Theparametersarealso
consistent,withintheerrors,withthoseobtainedwhenfittingthe
3.1 ThenatureofNGC2276-3c
unbinneddatausingtheC-statisticinXSPECforPoissondatawith
a Poisson background2. Interestingly the high disc temperature The spatial coincidence of the EVN non-thermal radio emission
from the MCD fit is similar to that seen in Galactic black holes with the location of unresolved, hard X-ray emission constitutes
in the steep power-law state (bright intermediate states), and so oneofthemostcompellingradiativesignaturesofanaccretingBH.
appears somewhat too high for an IMBH. However, we note that ItalsopermitsameasurementoftheM usingtheFundamental
BH
the moderate data quality may simply be insufficient to reject an Plane of BH accretion, which is a correlation between core
MCDfittoapower-lawspectruminthiscase. radioluminosity,X-rayluminosityandM validfromstellarto
BH
We estimated observed X-ray fluxes from the absorbed SMBHsinthehardX-rayspectralstate,wheresteadyjetemission
power-law model, which we converted to luminosities. These isubiquitous.Forthispurpose,weusetheempiricalFundamental
are: L0.3−10keV = 1.8+−00..32 × 1040 ergs−1 and L2−10keV = PlanedependenceofGu¨ltekinetal.(2009,eq.3),whichistheonly
1.6+0.3 × 1040 ergs−1. This corresponds to an increase in onethathasbeentestedintheIMBHrange(Gu¨ltekinetal.2014).
−0.2
luminosity of ∼ 3.5 from the 2004 Chandra observation. UsingtheEVNradioluminosityat5GHzandtheChandraX-ray
Although NGC2276-3c has increased in X-ray luminosity, the luminosityinthe2-10keVband,weobtainM =5×104 M(cid:12)
BH
total luminosity of this and the other nearby point sources (see with a scatter of 0.7 dex, which confirms that the ULX harbours
below) is still less than that of the extreme luminosity ULX an IMBH (Fig. 2). The mass obtained using other Fundamental
observed by XMM–Newton (2XMM J072647.9+854550; 0.3–10 Plane correlations (e.g. Merloni et al. 2003; Miller-Jones et al.
keV luminosity of (6.1 ± 0.3) × 1040 ergs−1), meaning that 2012;Plotkinetal.2012)isalsoconsistentwithanIMBH,though
at least one of the point sources detected by Chandra within wenotethatthesehavenotbeenproventobevalidintheIMBH
the XMM-Newton beam must have previously been even more range and sometimes assume certain accretion physics that do
luminous in X-rays. Given the location of the centroid of the not necessarily apply to NGC2276-3c. Further uncertainties also
XMM–Newtondetection(cf.Fig.1ofSuttonetal.2012),itislikely arise from the discovery of a second track in the Fundamental
thatthebrightersourcewaseither3bor3c. Plane(Galloetal.2012)andofnodependenceoftheradio/X-ray
WealsoexaminedtheChandralightcurveoftheULXtotest correlation with BH mass in X-ray binaries (XRBs; Gallo et al.
forevidenceofvariability.However,the3σupperlimitontherms 2014).
fractionalvariabilityisveryunconstrainedat∼ 0.6.Althoughthe Tofurtherprobethenatureofthesource,wealsocalculatethe
dataarerelativelypoor,wenotethatthecharacteristicsofahard R ratio of 5 GHz radio emission to 2–10 keV X-ray emission:
X
accretion state – a power-law spectral form with Γ ∼ 1.7 and R = νL (5 GHz)/L (2–10 keV) (Terashima & Wilson 2003).
X ν X
rms variability at the 10-20 per cent level – are well within the XRBshavetypicalvaluesoflogR <−5.3,low-luminosityAGN
X
rangeofallowedparameterswehavederived.Giventhedetection have−3.8 <logR < −2.8,andsupernovaremnantshavelog
X
of a radio jet characteristic of the hard state, a thermal-dominant R ∼ −2(e.g.Mezcuaetal.2013b;seealsotable3inMezcua
X
stateappearsunlikely,althoughwecannotstatisticallyruleoutan etal.2013aandreferencestherein).NGC2276-3chaslogR =
X
accretion disc-like spectrum (but we again note that the derived −4.9,whichfallsintherange−5.3to−3.8expectedforIMBHs
temperatureissomewhathigherthantypicalforathermaldominant (Mezcuaetal.2013a)andrulesoutaSMBHnature.Abackground
state).Wecanthereforeconcludethatthedataareconsistentwith AGNisalsodisfavouredbythelocationoftheULXwithinaspiral
ahardaccretionstate. arminwhichtheVLAradiolobesandtheX-raysourceappearto
Motivated by the large change in the X-ray luminosity of lineupwithacavityintheopticalemission(Fig.1,b),andbythe
NGC2276-3c,wealsotestedwhethertheneighbouringsources(3a low probability of a chance alignment with a background source
and3binSuttonetal.2012)hadvariedtoo.Todothis,weextracted calculated from the VLA field (Mezcua et al. 2013c). It should
energyspectrafromthelatestChandraobservationusingthesame be noted that the fact that this ULX has a large-scale (lobe-like)
method as above. We then fitted both of the ungrouped spectra structure and a small-scale jet indicates that other ULX masses
derivedforsimilarsourcesusingVLAfluxdensitymeasurements
may be biased towards higher values (since the radio luminosity
2 https://heasarc.gsfc.nasa.gov/xanadu/xspec/manual/XSappendixStatistics.html obtainedfromVLAmeasurementscanbeoverestimated).
(cid:13)c 2013RAS,MNRAS000,1–7
RadiojetofIMBH 5
a typical efficiency (η) of conversion of rest-mass into energy of
10%. Taking k = 5 yields M˙ = 1.5 × 10−5 M(cid:12) yr−1,
bol BH
which is close to the Eddington accretion rate of a 1000 M(cid:12)
BH (2×10−5 M(cid:12) yr−1). The accretion rate is also one to two
orders of magnitude higher than that of sub-Eddington to nearly
Eddingtonaccretingstellar-massBHswithradiojetemission(e.g.
IC10X-1,Csehetal.2012;GRS1915+105,Rushtonetal.2010),
which favours an IMBH nature for NGC2276-3c. However, we
notethatitisalsooneorderofmagnitudesmallerthanthatofthe
super-Eddington accreting galactic microquasar SS433 (M˙ =
BH
10−4M(cid:12)yr−1;Fabrika2004).
3.3 Jetkinematics
ThetwoVLAradiolobeshavethesamepeakintensity(0.35±0.06
mJybeam−1 and0.34±0.06mJybeam−1;Mezcuaetal.2013c)
anddistancetothecompactsourceorX-rayposition(arm-length
ratio R=1.2). This argues against nebular radio emission and
Figure2.FundamentalPlaneofBHaccretionfromGu¨ltekinetal.(2009)
indicatesthattheVLAradiolobesmostlikelybelongtoaradiojet
extending from XRBs hosting stellar-mass BHs (green dots) to AGN
thatiseithernotsignificantlyaffectedbyDopplerboostingeffects
hostingSMBHs(bluedots).ThesampleofXRBsandAGNistakenfrom
Merlonietal.(2003).AcleargapinthecorrelationisobservedintheIMBH orwhosevelocityvectoroftheemittingplasmaisveryclosetothe
range,whereNGC2276-3cislocated. planeofthesky.Fromthearm-lengthratioandtheangle-to-lineof
sight(θ),anestimateofthevelocityinunitsofthespeedoflight
(β)ofablobofplasmamovingalongthejetcanbeobtainedfrom
3.2 Eddingtonratioandaccretionrate βcosθ = (R−1)/(R+1)(Csehetal.2014).Basedonthelobe
flux ratio and arm-length ratio, we assume θ = 80◦ and find β =
The Fundamental Plane has been applied under the assumption
0.4,whiletheconstraintofcosθ(cid:54)1yieldsβ (cid:62)0.1.Thereforethe
thatthesourceisaccretingatasub-Eddingtonrate.TheEddington
ratioisgivenbyk L /[1.38×1038×(M /M )]wherek velocityoftheradiojetisconstrainedto2.3×104–1.2×105km
bol X BH (cid:12) bol s−1.
is the 2–10 keV bolometric correction factor and ranges from 5
(Marcheseetal.2012)to∼30forAGNwithEddingtonratios(cid:54) The localization of the IMBH in the spiral arm of a nearby
galaxy suggests that it might have formed in situ, as IMBHs of
0.1 (Vasudevan & Fabian 2009) and k ∼ 1 for XRBs (e.g.
bol 102–104M(cid:12)areexpectedtoformindenseyoungstellarclusters
Migliari & Fender 2005). Assuming k = 5 for NGC2276-3c
bol (Devecchi&Volonteri2009).However,theM =5×104 M(cid:12)
(i.e.inbetweenthoseofAGNandXRBs),andusingM =5× BH
BH
104 M(cid:12)andthe2-10keVX-rayluminosity,yieldsanEddington of NGC2276-3c appears too large for that. The main alternative
ratio ∼ 10−2. The possibility that NGC2276-3c is accreting at is that NGC2276-3c is the nucleus of an accreted stripped dwarf
galaxy,asislikelythecaseforESO243-49HLX-1(Farrelletal.
a higher Eddington rate should also be considered. If the source
is accreting at the Eddington limit, it has M (cid:62) 725 M(cid:12), 2012; Soria et al. 2013) and for the SMBH in the ultra-compact
BH
dwarfgalaxyM60-UCD1(Sethetal.2014).Thisissupportedby
which still puts NGC2276-3c in the IMBH range. Even in the
the unusual morphology of the host spiral arm, its very high star
most conservative scenario that all the luminosity is emitted in
formationrate(5–15M(cid:12)yr−1;Wolteretal.2015)andthelocation
the 2-10 keV X-ray band (k = 1) and there is no intrinsic
bol
absorption, the source would have M (cid:62) 145 M(cid:12). Therefore, ofNGC2276inagalaxygroup(Davisetal.1997).Interestingly,
BH
in this scenario the presence of the two nearby luminous ULXs
NGC2276-3cwouldqualifyasanIMBHevenifitwereaccreting
is readily explained as a result of the star formation triggered by
at its Eddington rate. However, we now know that ULXs can be
thepassageofthestrippeddwarfgalaxynucleusthroughthespiral
poweredbysuper-Eddingtonemissionfromastellar-massBH(e.g.
arm. The symmetrical VLA radio jet lobe structure implies that
Middleton et al. 2013: Liu et al. 2013). Given that the maximal
NGC2276-3c must be travelling at a slower rate than it takes to
emission from a super-Eddington accretion disc is a factor ∼20
abovetheEddingtonluminosity(e.g.Mineshige&Ohsuga20113), inflatethelobessothatwhenpassingthroughNGC2276thelobes
retaintheirshapeandarenotdistortedbythegalaxy’sinterstellar
in the case of NGC 2276-3c we would require a massive stellar
medium(ISM).Typicaltime-scalesforminormergerpassagesare
BH (> 31 M(cid:12)) to power the peak X-ray luminosity. We note,
∼1 Gyr (e.g. Lotz et al. 2010), with relative velocities between
though, that the presence of such a powerful, extended radio jet
thetwomerginggalaxiesof100–500kms−1 (e.g.∼200kms−1
is unprecedented for a ULX accreting at super-Eddington rates.
forHLX-1;Mapellietal.2012).Fromthejetexpansionvelocity
EmpiricalevidencethatBHswithsupercriticalaccretioncanhave
andconsideringadistancebetweenthecompactcoreandtheouter
powerfuljetshassofaronlybeenfoundintwostellar-massBHs
VLAradiolobesof∼322pc,arangeofdynamicjetagesof∼2500
(S26,inthenearbygalaxyNGC7793,Pakulletal.2010;andMQ1,
–14000yrisobtained.Thismeansthatthetime-scaleforforming
inthespiralgalaxyM83,Soriaetal.2014).
thelobes(t )ismuchlowerthanthetime-scaleofadwarfgalaxy
Anestimateoftheabsoluteaccretionrate(i.e.intermsofmass lobe
accreted) can be obtained as M˙ = k L /(ηc2) assuming passing through NGC 2276 (tpass). The jet radio morphology is
BH bol X
expectedtobedistortedortopresentanon-linearshapeduringthe
courseofamergerevent(e.g.asitissuggestedinX-shapedradio
3 ThisfactormightbefurtherexceededinULXsthatareabletostrongly galaxies;Merritt&Ekers2002;Mezcuaetal.2011,2012)unless
beamtheiremission,forexamplemagneticneutronstarssuchastheobject tlobe <<tpass.Hence,thefindingthattlobe <<tpassandthatthe
underlyingM82-X2(Bachettietal.2014). jetvelocityismuchlargerthantheIMBH/dwarfgalaxy’svelocity
(cid:13)c 2013RAS,MNRAS000,1–7
6 M.Mezcuaetal.
favoursthescenarioinwhichtheIMBHisthenucleusofastripped candidates.ThefindingthatthejetkineticpowerofNGC2276-3c
dwarfgalaxy,andmightexplaintheunusualmorphologyandhigh iscomparabletoitsradiativeoutputandthatitsjetefficiencyisas
starformationinthespiralarmofNGC2276. large as those of radio galaxies indicates that, if seed BHs in the
early Universe had powerful jets similar to that of NGC2276-3c,
their jet mechanical feedback must be taken into account in
3.4 Jetpower,efficiencyandfeedback cosmologicalsimulationsandstudiesofSMBH/galaxygrowth.
An estimate of the total jet power, Q , can be derived from the
jet
radioluminosityofthecompactjet.Usingthecorrelationbetween
core radio luminosity and kinetic power from Merloni & Heinz ACKNOWLEDGEMENTS
(2007),whichhastheminimumscatter(0.4dex)andisbasedon
a sample of sub-Eddington accreting SMBHs with flat-spectrum MM acknowledges support from the Spanish Grant
compactjetcores,weobtainQ = 4×1040 ergs−1,whichis PNAYA2011-25527. TPR and ADS were funded as part of
jet
of the same order as the bolometric luminosity of NGC2276-3c theSTFCconsolidatedgrantsST/K000861/1andST/L00075X/1.
(∼1040−1041ergs−1). The authors thank Eduardo Ros and Rodrigo Nemmen for
From the jet power and accretion rate, we can estimate the insightful discussion. We thank the Chandra X-ray Center for
jet efficiency, or fraction of the accretion power that is used granting us a Director’s Discretionary Time observation of NGC
in the kinetic motion of the jet, η = Q /(ηM˙ c2), where 2276. The EVN is a joint facility of European, Chinese, South
j jet BH
η=10%. Jet efficiencies of ∼10% or higher (e.g. Nemmen & African and other radio astronomy institutes funded by their
Tchekhovskoy 2014) are estimated for massive radio galaxies, nationalresearchcouncils.
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