Quantifying the Coexistence of Massive Black Holes and Dense Nuclear Star Clusters

16 Pages·2009·0.47 MB·English

Mon.Not.R.Astron.Soc.000,000–000 (0000) Printed30July2009 (MNLATEXstylefilev2.2)
Quantifying the Coexistence of Massive Black Holes and
Dense Nuclear Star Clusters
Alister W. Graham1⋆ and Lee R. Spitler1
1 Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia.
9
Accepted 2009May19
0
0
2
ABSTRACT

l
u
J Inlargespheroidalstellarsystems,suchasellipticalgalaxies,oneinvariablyfindsa
0 106-109M⊙ supermassiveblackholeattheircentre.Incontrast,withindwarfelliptical
3 galaxies one predominantly observes a 105-107M⊙ nuclear star cluster. To date, few
galaxieshavebeenfoundwithbothtypeofnucleicoexistingandevenlesshavehadthe

] masses determined for both central components. Here we identify one dozen galaxies
O housing nuclear star clusters and supermassive black holes whose masses have been
C measured. This doubles the known number of such hermaphrodite nuclei — which
are expected to be fruitful sources of gravitational radiation. Over the host spheroid
.
h (stellar)massrange108–1011M⊙,wefindthatagalaxy’snucleus-to-spheroid(baryon)
p mass ratio is not a constant value but decreases from a few percent to ∼0.3 percent
o- suchthatlog[(MBH+MNC)/Msph]=−(0.39±0.07)log[Msph/1010M⊙]−(2.18±0.07).
r Once dry merging has commenced by Msph ≈ 1011M⊙ and the nuclear star clusters
t
s have disappeared, this ratio is expected to become a constant value.
a As a byproduct of our investigation, we have found that the projected flux from
[
resolvednuclearstarclusterscanbe wellapproximatedwithS´ersicfunctionshavinga

1 rangeofindices from∼0.5to ∼3,the latter index describingthe Milky Way’snuclear
v star cluster.
0
Key words: black hole physics — galaxies: nuclei — galaxies: structure
5
2
5
.
7
0 1 INTRODUCTION Delgado et al. (2008) simultaneously undertook a comple-
9 mentary approach and searched for the presence of NCs in
Ferrarese et al. (2006a) and Wehner & Harris (2006) have
0 galaxies with known AGN. While they both detected some
recently shown that the division between either a (black
v: hole)- or a (nuclear cluster)-dominated galaxy core occurs galaxies in the mass range 109 to 1011M⊙ which contain
both a NC and an AGN, they were not able to acquire the
Xi aroundagalaxymassof∼1010M⊙.Wehner&Harris(2006) BH masses of theAGN.
wrote that “dE,N nuclei themselves [the nuclear star clus-
r
a ters] show no evidence of harbouring massive black holes”. To explore not only how commonplace these systems
Indeed, in the contemporaneous investigation by Ferrarese are, but importantly the nature of the above mentioned
et al. (2006a), they identified only two galaxies (M32 and transition, we have searched for NCs in galaxies whose BH
theMilkyWay)aspotentiallyhostingbothtypesofnuclear masshasalreadybeendeterminedviadirectdynamicalmea-
component. While apparently rare, Filipenko & Ho (2003) surements. This is important because the nucleus-to-(host
had identified at least one galaxy with both a nuclear clus- spheroid) mass ratio, as a function of spheroid mass, may
ter (NC) and a massive black hole (BH) and Graham & provideusefulconstraintsforpotentialgalacticevolutionary
Driver(2007)subsequentlyreportedontheexistenceoftwo assembly processes. For example, some massive BHs may
additional such galaxies (NGC 3384 and NGC7457). growthroughtherunawaycollision ofNCstars(e.g.,Light-
To investigate this near dichotomy in the type of cen- man & Shapiro 1978; Kochanek et al. 1987; Lee 1993), or
tral massive object which galaxies house, Seth et al. (2008) converselytheBHmayevaporatethesurroundingNC(e.g.,
searched for evidence of active galactic nuclei (AGN), and Ebisuzakietal.2001;O’Learyetal.2006),orperhapssome
thus massive BHs, in galaxies with known NCs. Gonzalez other mechanism dominates. Curiously, the continuous re-
lations shown by Wehner & Harris (2006) and Ferrarese et
al. (2006a) involving either the BH or NC mass and the
⋆ [email protected] host galaxy mass are suggestive of, at some level, mutually
(cid:13)c 0000RAS
2 Graham & Spitler
common physics governing the two types of nuclei. More- globular clusters included hereare of interest for scenarioes
over, given that the main mechanism of galaxy growth is in which they may be the relic nuclei of stripped galaxies
thought to be through the process of hierarchical merging, (e.g.Freeman1993; Bassinoet al.1994; Meylan etal.2001;
modelling their dual nuclei may be important for properly Bekkiet al. 2003; Walcher et al. 2005).
understanding the growth of supermassive black holes. For TheAppendixcontainsreferences to,or derivationsof,
example, dense nuclear star clusters may, through N-body allquantitiesshowninTable1.Briefly,theblackholemasses
interactions, greatly facilitate the coalesence rate of binary havebeentakenfrom theindividual(usuallydiscovery)pa-
massive black holes. pers which reported these values, and, when necessary, ad-
The coexistence of NCs and massive BHs is of further justed to our updated distances which are also provided in
interestduetoassociatedphysicalphenomenon.Theinward the Appendix. While galaxy masses were used for the el-
spiralofstarsontoamassiveBHisalikelysourceofUV/X- liptical galaxies, bulge masses have been used for the disc
rayflaringevents(Komossa&Bade1999;Komossa&Mer- galaxies. From hereon weshallgenerically refer toan ellip-
ritt2008;Lodatoetal.2008;Rosswog etal.2008). Thedis- tical galaxy or the bulge of a disc galaxy as a “spheroid”.
ruptionofbinarystarsmayresultinthehigh-speedejection Thespheroidmasseswereprimarilyobtainedbymultiplying
of hypervelocity stars (Bromley et al. 2006). Rapid inspi- the observed spheroid luminosity by an appropriate stellar
ral events may also generate gravitational radiation (e.g., mass-to-light (M/L) ratio. The NC masses were also ob-
Quinlan 1996; Alexander 2008; O’Leary et al. 2008; Mer- tainedthisway,albeit usingadifferentstellarmass-to-light
ritt 2008). Assuch,galactic nucleiwith confirmedBHs and ratio from the spheroid’s value. We found that the uncer-
NCs may prove useful targets for experiments such as the tainty involved in this process is generally constrained to
Laser Interferometer Space Antenna (LISA, Danzmann et withinafactoroftwo.Whilecomparison withdynamically-
al. 1996) which are hoping to discover such as yet unde- determined masses, when available, supports this level of
tected radiation. Given that the amplitude of gravitational accuracy, detailed spectroscopy that establishes the mean
wavesdecayslinearlywithdistance,andX-rayflaringevents ages and metallicities of thestars (e.g. Walcher et al. 2005)
withdistancesquared,therelativeproximityofthegalaxies isdesirableforbetterconstrainingM/Lratios.Suchdetails,
listed here makes them particularly attractive compared to however, were not available for most systems and therefore
more distant, nucleated AGN. we usually adopted thesingle colour approach used by Fer-
In the following section we briefly describe our galaxy rareseetal.(2006a) andSethetal.(2008) todeterminethe
data set, with more detailed information contained within nuclear cluster masses. For five galaxies (M32, NGC 205,
theAppendix.InSection3wepresentatentativenewscal- NGC 2778, NGC 4697 and the Milky Way) we have mod-
inglawinvolvingdensestarclustersandmassiveblackholes, elled, in the Appendix, their observed light distribution to
plusa more robust scaling relation which involves themass derivetheir NCfluxes.
of the host spheroid. Finally a discussion, including some Although roughly one dozen galaxies (including
of the implications of this work, and a brief summary are NGC3621andexcludingNGC4026) from thesampleof76
provided in Sections 4 and 5 respectively. galaxies in Graham (2008a) appear to have both a NC and
a BH, it would be inappropriate to conclude that roughly
16 per cent of galaxies contain both. This is because sam-
pleselection effectshavenot been considered. For example,
2 DATA
high mass galaxies tend not to have nuclear star clusters;
Graham (2008a) tabulated a sample of 50 (+26) predomi- a sample dominated by such galaxies would be biased to-
nantly inactive galaxies with useful (rough) measurements ward low percentages. In passing we note that because the
of their central BH mass. This compilation was acquired centralstellardensityinhighmass,brightellipticalgalaxies
by scouring the literature for published values which were decreasesasafunctionofincreasinggalaxyluminosity(e.g.,
then updated if new distances were available. Of these 76 Faberetal.1997),nuclearstarclustersareactuallyeasierto
galaxies, Table 1 lists those which additionally contain a detectinluminousgalaxies thaninintermediateluminosity
NC. Included in this list is our own galaxy the Milky Way elliptical galaxies. Attheotherendofthescale, thesphere-
(Burbidge 1970; Rubin 1974), M32 (Tonry 1984), the ac- of-influence of a 106M⊙ BH within a lower-mass spheroid
tive galaxies NGC 3621 and NGC 4395 (Barth et al. 2008; having a velocity dispersion of 100 km s−1 is only ∼0.01
Filippenko & Ho 2003), plus NGC 3384 and NGC 7457 arcseconds at thedistance of theVirgo galaxy cluster; such
whichwerepreviouslynotedtocontainbothtypesofnuclei. BHswouldthereforegoundetected.Galloetal.(2008)have
An additional seven predominantly inactive galaxies (NGC however reported that 3-44 per cent of early-type galaxies
1023,1399,2778,3115,40261,4564.4697)whichhouseboth less massive than 1010M⊙ have an X-ray active BH, while
typeofnuclearcomponenthavebeenidentified—although 49-87 per cent of more massive early-typegalaxies do.This
no NC mass is currently available for NGC 4564. In addi- mayinpartbeareflectionthatmassiveBHsarelesspreva-
tion,Table1includesanotherfourgalaxieswithknownNC lentin lower mass galaxies. Inanyevent,ourgalaxy identi-
masses but only upper limits on their BH masses (as is the ficationinTable1confirmsthatthecoexistenceofNCsand
case for NGC 3621 mentioned above), three globular clus- BHsisnotasrareaspreviouslythought.Table1effectively
terswithpossibleBHs,twelvecoregalaxieswithnoNC(in- doubles the numberof galaxies reported to contain a dense
cluded for reference) and one young star cluster (MGG-11) nuclear star cluster and having a direct supermassive black
with a probable intermediate mass black hole. The massive hole mass measurement.
Shownin Table1isthemorphological typeofeach ob-
ject.Notsurprisingly,thefirstdozengalaxieswithaBHbut
1 TakenfromGu¨ltekinetal.(2009). no signs of a NC are big elliptical galaxies. The next dozen
(cid:13)c 0000RAS,MNRAS000,000–000
Massive Black Holes in Dense Nuclear Star Clusters 3
Inthefollowingsectionweattempttoprobethenature
Table 1.Blackhole,hostspheroidandnuclearclustermass.
of thetransition from one typeof nuclei to theother.
Object Type MBH[M⊙] Msph[M⊙] MNC[M⊙]
Twelve“coregalaxies”withMBH butnodetectable NC
NGC3379 E 1.4+2.7×108 1.0×1011 ...
−1.0
NGC3608 E 1.9+−10..06×108 9.4×1010 ... 3 MASS RATIOS
NGC4261 E 5.2+1.0×108 3.7×1011 ...
−1.1
NGC4291 E 3.1+0.8×108 7.8×1010 ... 3.1 From star clusters to massive black holes
−2.3
NGC4374 E 4.6+−31..58×108 4.1×1011 ... Figure1showstheratiooftheBHmasstothecombinedBH
NGC4473 E 1.1+−00..48×108 7.1×1010 ... plus NC mass. It is plotted against the stellar mass of the
NGC4486 E 3.4+−11..00×109 3.7×1011 ... hostspheroid:eitheranelliptical galaxy,thebulgeofadisc
NGC4649 E 2.0+−00..46×109 4.5×1011 ... galaxy, or nothing in the case of the three globular clusters
NGC5077 E 7.4+−43..70×108 1.1×1011 ... andoneyoungstarcluster(seeTable1).Forspheroidswith
NGC5813 E 7.0+−11..11×108 1.4×1011 ... stellar masses below ∼108M⊙ there is a dearth of reliable
NGC6251 E 5.9+−22..00×108 9.4×1011 ... BHdetections,althoughthemajorityoflow-massspheroids
NGC7052 E 3.7+−21..65×108 1.7×1011 ... areknowntocontainNCs(e.g., Binggeli etal.1987; Fergu-
TwelvegalaxieswithMBH andaNC son1989;Carolloetal.1998;Stiavellietal.2001;Balcellset
MilkyWay SBbc 3.7+0.2×106 1.2×1010 3.0×107
−0.2 al. 2003; Graham & Guzm´an 2003; Cˆot´e et al. 2006). From
M32 cE 2.5+0.5×106 2.6×108 2.0×107
−0.5 Local Group dwarf galaxies, such as NGC 205, we know
NGC1023 SB0 4.4+0.5×107 3.2×1010 4.4×106
−0.5 thatanypotentialBHswhichtheselowmassgalaxiesmight
NGC1399a E 4.8+0.7×108 1.5×1011 6.4×106
−0.7 hostarelessmassivethantheirNCs.Thisisreflectedbythe
NGC2778b SB0 1.4+0.8×107 4.3×109 6.7×106
−0.9 upperlimitsonfiveofthedatapointsinFigure1.Thesitua-
NNGGCC33131854 SS0B0 19..16+−+902...918××110087 71..44××11001100 12..52××110077 tionisreversedforspheroidmassesgreaterthan∼1011M⊙,
−0.2 wheretheBHsdominateattheexpenseoftheNCs.Figure1
NGC4026 S0 1.8+0.6×108 9.6×109 5.6×106
−0.4 reveals that in between is mutual ground where both BHs
NGC4395c Sm 3.2+6.8×104 3.4×107 1.4×106
−2.2 and NCs appear to coexist within the same spheroid. For
NGC4564 S0 5.6+0.3×107 7.4×109 ?
−0.8 the first time we are able to gain some preliminary insight
NGC4697 E 1.7+0.2×108 1.5×1011 2.8×107
−0.1 into the nature of this transition as a function of mass, al-
NGC7457d S0 3.5+1.1×106 1.1×109 9.3×106
−1.4 though we recognise that more data is needed in Figure 1
GalaxieswithaNCbutonlyanupperlimitonMBH before any possible relation can bedefined with certainty.
NMG33C205 EScd <<23.4××110034 81..75××110088 12.4××110066 ThedemiseofNCsatahostspheroidmassof∼1011M⊙
NGC3621 Sd <3.6×104 1.4×108 1.0×107 (Figure 1, see also Ferrarese et al. 2006a and Wehner &
NGC4041e Sbc <2.4×107 6.4×108 2.9×107 Harris 2006) is interesting. The onset of partially depleted
VCC1254 dE <9×106 3.2×109 1.1×107 galaxy cores occurs at an absolute B-band magnitude of
StarclusterswithlesssecureMBH −20.5±1 mag (e.g., Faber et al. 1997; Graham & Guzm´an
G1f GC 1.8+0.5×104 ... 8.0×106 2003), which is also where the dynamical properties vary
−0.5
M15f GC 0.5+2.5×103 ... 7.0×105 (e.g.,Daviesetal.1983;Dressler&Sandage1983;Matkovi´c
−0.5
MGG-11c SC 1.0+4.0×103 ... 3.5×105 & Guzm´an 2005). For an old stellar population, this stel-
−0.8
ω Cenf GC 4.0+−01..80×104 ... 4.7×106 lar flux corresponds to a stellar mass of 6+−94×1010M⊙ —
References are available in the extended version of this Table, which has recently been noted by many studies as marking
provided in the Appendix. Uncertainties on the spheroid stellar thetransitionofseveralgalaxyproperties(e.g.Rogersetal.
massesandthenuclearclustermassesareroughlyafactoroftwo. 2008)andmayalsocoincidewiththeturnoverofthegalaxy
The three globular clusters and one young star cluster (MGG-
mass function (Li & White 2009). As noted byFerrarese et
11) have no associated spheroid mass as they arenot located at
al. (2006a), it may therefore be that coalescing BHs in dry
the centre of a spheroid. Notes: a NC detection weak; b NC &
mergerevents(Begelman,Blandford,&Rees1980;Merritt,
BHdetectionweak;c indirectBHmassestimate;d BHdetection
weak;e discmightbedynamicallydecoupled; f maybenoBH. Mikkola & Szell 2007; Berentzen et al. 2009) preferentially
destroy their shroud of NC stars prior to the creation of
the galactic loss cones observed in spheroids brighter than
−20.5±1B-mag.Alternatively,perhapsthelifespanofaNC
objects, those with evidence for both a BH and a NC, are
is simply short once the mass of the BH dominates, hence
predominantly disc galaxies; the exceptions are the ellipti-
cal galaxy NGC 4697, the “compact elliptical” galaxy M32 thescarcity of NCs around BHs with Mbh >∼5×107M⊙.
(whichmaybeadiscgalaxyundergoingtransformation,e.g. Figure 2 shows the same mass ratio as seen in Figure
Bekki et al. 2001; Graham 2002) and the elliptical galaxy 1, but plotted against the BH mass. Plotting it like this
NGC 1399 with only tentative evidence for a NC (possibly reveals, without recourse to the host spheroid, the nature
aswallowed GC, Lyubenovaet al. 2008, which isoneof the of the coexistence of black holes and dense star clusters.
propsedmechanismsforbuildingNCs).However,giventhat The line shown in Figure 2 has simply been marked by eye
almost every galaxy with a reliable BH mass measurement to roughly capture the behaviour of the points and is such
thatislessthan5×107M⊙ isadiscgalaxy,theirprevalence that
tishneoltevseulrpofrirsointagt.iFoninaalllvye,rlsaucskipnrgesksiunreemsautpicpaolritnifnortmhaetbiounlgoens loghMBHM+BHMNCi= 32log(cid:20)5×M10B7HM⊙(cid:21) (1)
of our sample, we are unable to comment on the role that
pseudobulges versusclassical bulges may play. forMBH <5×107M⊙ andequalszeroforlargerBHmasses
(cid:13)c 0000RAS,MNRAS000,000–000
4 Graham & Spitler
and when MNC = 0. Given the somewhat sparse nature of (Forbes et al. 2008, their Equation 1), one has the expres-
the data, a more sophisticated regression analysis for this sion log(M/L ) = 0.01(9.18+1.55M ). Applying this to
K K
new(black hole)-(nuclearcluster) mass ratio relation is not the K-band expression MBH ∝ L1sp.0h0,±st0el.l0a5r from Graham
performed here. (2007, his section 5.2) gives MBH ∝ Ms1p.0h4,s±te0l.l0a5r. For com-
parison, Marcomi & Hunt (2003) report MBH ∝Ms0p.9h6,v±ir0i.a0l7
for their “Group 1” galaxies, while H¨aring & Rix (2004)
3.2 Nuclei-to-spheroid mass ratios
report MBH ∝ Ms1p.1h2,d±y0n.06 for a slightly larger galaxy sam-
ItisgenerallyacceptedthatmassiveBHsareassociatedwith ple.Itshouldhoweverbenotedthattheselattertwostudies
the host spheroid rather than the host galaxy (e.g., Kor- have,atsomelevel,alsoaccountedforthecontributionfrom
mendy& Gebhardt 2001). Given this, wehavedisplayed in dark matterin thespheroid mass, an isue wediscuss in the
Figure3thecombinedmassoftheBHandtheNC,divided following Section.
by the stellar mass of the host spheroid. For high spheroid
masses, where supermassive BHs dominate the core region,
onecanseethatthisratioscattersbetweenvaluesfrom10−3
4 DISCUSSION
to 10−2. One can also see that this mass ratio is greater in
the lower mass spheroids whose cores are dominated by a Onlyafew yearsago it wasgenerally believed thatmassive
NC2.Fromanorthogonalregressionanalysis,usingthecode blackholesandnuclearstarclustersdidnot(frequently)co-
BCES(Akritas&Bershady1996),andassumingafactorof existatthecentresofgalaxies.Here,asinSethetal.(2008)
two uncertainty on each data point in both directions, one and Delgado et al. (2008), we present evidence suggesting
obtains therelation3 thecontraryforgalacticspheroidswithstellarmassesrang-
log MBH+MNC =−(0.39±0.07)log Msph −(2.18±0.07).(isn2te)gpfrfoomrw∼ard10b8y–1re0p11oMrti⊙n.gFounrtshyesrtmemosref,owrewthaikcheawneihmapveorbteaennt
(cid:20) Msph (cid:21) (cid:20)1010M⊙(cid:21) abletoacquirethe(blackholeandstellar)massesofthenu-
Repeating the analysis while assigning a factor of 5 uncer- clearcomponentsandthe(stellar)massofthehostspheroid.
taintytotheordinate(andafactorof2intheabscissa)does This has enabled us to present mass relations defining this
not change this result by more than the quoted 1σ uncer- exciting coexistence.
tainties. Setting MBH = 0 for the systems which only have From Equation 2, when Msph = 108M⊙ one has a
upper limits on their BH masses also does not significantly nucleus-to-spheroid mass ratio of 0.04, and when Msph =
alter these results. The Pearson and Spearman correlation 1011M⊙ onehasaratio of0.0027. Perhapsnot surprisingly,
coefficientsare-0.73and-0.65,andtheprobabilityofsucha the latter value is in excellent agreement with the mean
strong correlation occurring by chance is less than 0.02 per MBH/Msph ratio from studies of galaxies at the high-mass
cent. The vertical scatter (i.e. in the logMnuclear direction) end with MBH ∼ 108±1M⊙, and which excluded any NC
is 0.41 dex, and 0.36 dex without NGC 205. mass component (e.g. Merritt & Ferrarese 2001; H¨aring &
While equation 1 expressed the relevant dominance of Rix 2004). At the low-mass end, from an analysis of nu-
theBHcomparedtotheNCasafunctionofBHmass,equa- clearstarclustersindwarfelliptical galaxiesandthebulges
tion2revealstheircombinedimportance(intermsofmass) of early-type disc galaxies, the MNC/Msph stellar mass ra-
relativetothehostspheroid’sstellarmass.Italsoeffectively tio has been observed to be both higher and to increase
providesanewmeanstopredictthecentralmassinsystems (decrease) as one samples lower (higher) mass spheroids.
where one is unable todirectly measure this quantity. Balcells et al. (2003) find a value of ∼2 per cent when
OncedrymergingcommencesatMB ≈−20.5mag(e.g. Msph =108M⊙ andfrom asample ofdwarfelliptical galax-
Graham&Guzm´an2003,andreferencestherein),orroughly ies a value of 1 per cent when Msph = 108M⊙ is readily
MK ≈ −24 mag (or 5×1010 to 1011M⊙), the MBH/Msph derived from Graham & Guzm´an (2003, their Eq.3 assum-
mass ratio should remain constant (or decrease if BHs can ing an F606W filter mass-to-light ratio of 3).
be ejected, e.g., Merritt et al. 2004; Gualandris & Mer- As noted above, in low-mass spheroids it has been
ritt 2008). The one-to-one MBH-LK relation given by Gra- knownforsomeyearshowthestellarfluxratioofthenucleus
ham (2007, which is dominated by systems with MBH > andhostspheroidvary(seealsoLotzetal.2004,theirFigure
5×107M⊙), coupled with a near constant stellar M/LK 7).Grantetal.(2005),forexample,reportthattheirB-band
ratio for massive elliptical galaxies, supports the scenario data for dwarf elliptical galaxies yields LNC ∝L0sp.6h8, which
in which the MBH/Msph baryon4 mass ratio is a roughly implies a nine-fold variation in the nuclear-to-spheroid flux
constantvalue.UsingtheK-bandstellarmass-to-lightratio ratio overahost spheroid fluxrange of 1000. (Forcompari-
log(M/LK)=0.1−0.1(B−K), for (B−K)>2.3 (Forbes son,giventhatMBH/MNC≈0atthelowmassendofequa-
et al. 2008, their Figure 10), and the colour-magnitude re- tion2,onehasthe(stellarmass)relationMNC∝Ms0p.6h1±0.07
lation (B −K) = 0.082−0.155M , for M < −18 mag whentheNCsdominate. From Shenetal.’s (2008) analysis
K K
of900broadlineAGN,theyreportthatMBH ∝L0ga.7la3x±y0.05.)
In Figure 3 we have revealed, over a host spheroid stellar
2 If apopulation of yet-to-be-detected, low-massspheroids with mass range of 104, how the combined central object mass
MBH >MNC exists, they would act to increase the distribution (black hole plus nuclear star cluster) divided by the stellar
of points at the low-mass end of Figure 3 to higher values and
mass of the host spheroid varies with the latter quantity.
therebysteepentherelationfurther.
3 ExcludingNGC205fromFigure3givesaconsistentslopeand This ratio increases by more than an order of magnitude
interceptof−0.41±0.06and−2.13±0.07,respectively. from∼0.1percentingiant elliptical galaxiesdominatedby
4 Thisterminologyassumesthattheblackholeshavebeenbuilt massive black holes, to 5–9 percent in dwarf galaxies and
bybaryons(e.g.Shankareta.2004). thebulgesoflate-typediscgalaxies whoseinnerregions are
(cid:13)c 0000RAS,MNRAS000,000–000
Massive Black Holes in Dense Nuclear Star Clusters 5
dominated by a nuclear star cluster (see also Balcells et al. NCs may grow through the accretion of globular clusters,
2007). and/or super star clusters in spiral galaxies, via dynami-
Atfirstglance, thisresultmayappeartocontradict re- calfriction(Tremaineetal.1975;Quinlan&Shapiro1990).
cent claims of a constant (central massive object)-to-(host Darkmattermaythusalsohavearoletoplay,therebymoti-
galaxy)massratio,wherethecentralmassiveobjectinsuch vatingthepursuitofreliabletotalmasses.Ithasadditionally
works was either a nuclear star cluster or a massive black been suggested that some BHs may be built through the
hole.InthecaseofWehner&Harris(2006),theyeffectively runaway collision of the NC stars (Kochanek et al. 1987)
tooktheabovefluxrelationfordwarfellipticalgalaxiesfrom or that, alternatively, a massive BH may effectively evapo-
Grant et al. and used the expression (M /L) ∝L−0.3 rate thesurroundingNC(Ebisuzaki et al. 2001; O’Leary et
total sph sph
toobtainLNC∝Msph,total.Ifthenuclearclustershavesim- al. 2006). It has also been proffered that NCs and massive
ilar stellar M/L ratios, this leads to the result that the nu- BHs may have developed from the same initial formation
clear cluster mass islinearly proportional tothe total (dark process (Wehner & Harris 2006), such that a slower gas in-
matter plusstellar) mass of thehost spheroid; i.e. that this fall rate in smaller spheroids allows time for star formation
massratioisconstantwithvaryingspheroidmass.Themass and thus produces nuclear star clusters rather than mas-
ratios presented by Ferrarese et al. (2006a), while also ac- sive black holes. Our larger (stellar) mass ratios in smaller
counting for dark matter at some level, are slightly differ- mass spheroids may have implications for the required effi-
ent dueto their inclusion/treatment of disc galaxies from a ciencyoffeedbackmechanismsinwhichsupernovaandstel-
sample of early-type,Virgo cluster galaxies. Althoughtheir lar winds from NC stars regulate the nuclear-to-spheroid
application of the virial theorem (M ∝Reσ2) using the ve- massratio(McLaughlin etal.2006b).Moreover,itishoped
locitydispersionσofthe(pressuresupported)bulgecompo- thatthenuclear-to-spheroid(baryonic)massratiosprovided
nenttogetherwith theeffectivehalf-light radiiof thewhole heremayprovideusefulconstraintsforanypotentialevolu-
galaxy —which areeffected by thesize of the(rotationally tionary scenarios.
supported) disc component — is a questionable meausure In future work we intend to present a new diagram
of a lenticular galaxy’s mass, it is clear that these virial showing (MBH +MNC) versus velocity dispersion, σ. This
productsarelargerthanthespheroid massesthatwould be willbeachievedviaacarefulanalysisofhigh-resolutionHST
obtainedfromtheuseofRe,sphinsuchaformula.5Theaver- images for as many of the 50 (+26) galaxies as possible.
agenucleus-to-spheroidmassratiowouldthereforebelarger While barred galaxies can deviate from the M-σ relation
than the reported value of 0.2 per cent for the nucleus-to- defined by non-barred galaxies (e.g. Graham 2008b; Gra-
galaxy total mass ratio. ham & Li 2009), and one expects them to similarly deviate
While an investigation of whether the use of Reσ2 is inthenew(M+M)-σ diagram duetotheirelevatedvalues
an appropriate tracer of total mass is beyond the intended ofsigma,itmayproveinsightfultoinvestigatethisfurther.
scopeofthispaper,itdoesseemapttoremindreadersthat IfbarredgalaxieshavepreferentiallylargerMNC/MBH mass
this question has a long history, often discussed in associa- ratios thannon-barredgalaxies ofthesamevelocity disper-
tionwiththeFundamentalPlane(Djorgovski&Davis1987; sion,thenonemayfindlessscatterinthenewdiagram and
Faber et al. 1987; Djorgovksi, de Carvalho & Han 1988). furtherpotential clues to theirevolution.
For example, even within elliptical galaxies, luminosity- Within galaxy clusters, dwarf galaxies are the most
dependent dynamical non-homology may severely bias the commontypeofgalaxy(e.g.Binggelietal.1985) andmany
applicability of aperture velocity dispersion measurements of these are nucleated. Within the field environment, the
when deriving such quasi-”virial masses” from Reσ2, and mostcommontypeofgalaxiesarespiralgalaxies(e.g.Allen
thus also bias any Mtotal/L trends with luminosity (e.g. et al. 2006; Baldry et al. 2006) and many of these are also
Hjorth & Madsen 1995; Ciotti et al. 1996; Busarello et al. known to be nucleated (e.g. Carollo et al. 1998; B¨oker et
1997; Graham & Colless 1997; Prugniel & Simein 1997). al. 2002; Balcells et al. 2003). Dueto thedifficulties associ-
Asidefromconcernsaboutmeasuringtotalspheroidmasses, ated with thedetection oflow mass black holes (<106M⊙)
baryonic fuelling and feedback, albeit within a dark matter in external galaxies, we speculate that androgynous nuclei
halo, arecommonly thoughttoberesponsiblefor establish- might be far more common than currently recognised. Fur-
ing the (bulk of the) nuclei mass and setting the observed thermore, given that nuclear star clusters are among the
nuclear-to-(hostspheroid)massratios(e.g.Silk&Rees1998; highest stellar density objects in the Universe, such a com-
Kauffmann&Haehnelt 2000; Benson et al.2003; Croton et monplace coexistence of nuclear star clusters and massive
al. 2006; Booth & Schaye 2009). It therefore seems reason- black holes may open up the prospect for numerous detec-
able to construct a baryonic rather than (only) total mass- tions of low frequency gravitational radiation (e.g. Ju et al.
ratio relation. Moreover, wehavebeen able todoso for the 2000, and references therein) with the Laser Interferometer
first time when including the mass of both nuclear compo- SpaceAntenna(LISA,Danzmann et al. 1996) from rapidly
nentsfrom thesame galaxy. inspiralling stars, white dwarfs, neutron stars and stellar
Whileinwardgasflowmayresultingalactic-centricstar mass black holes about these massive black holes. Due to
formation or fuelling much of the growth of massive BHs the substantially higher density of stars in NCs, compared
(e.g. Shankar et al. 2004), it has also been suggested that totheunderlyinghostgalaxy(seeFigure7.5),pastestimates
of LISA-detectable gravitational radiation events (e.g. Sig-
urdsson 1997; Freitag 2001; Gair et al. 2004; Hopman &
5 Amoresubtleissueisthatthenuclearclusterfluxes(andthus Alexander 2005, 2006a,b) may need to be revised upwards.
masses)may have been underestimated due to the steeper inner The sense of the correction is of course welcome given the
S´ersicprofilesobtainedfromsinglehigherS´ersicindexfitstoeach significance a direct detection could have by not only sup-
discgalaxyratherthanfromaS´ersicbulge+exponentialdiscfit. portingEinstein’sconceptofspaceandtimebutopeningan
(cid:13)c 0000RAS,MNRAS000,000–000
6 Graham & Spitler
Figure2. ThemassratiofromFigure1isshownhereversusthe Figure3. Theimportance,intermsofmass,ofthenuclearcom-
massoftheblackhole.SymbolsareasinFigure1.Anexpression ponentsrelativetothehostspheroid,astracedbythemassratio
for the solid line is given in Equation 1. Galaxies with only an (MBH+MNC)/Mspheroid, is shown as a function of Mspheroid.
upper BH mass limit have been circled. An error in MBH will SystemswithonlyanupperBHmasslimithaveaconnectedcir-
move the data points nearly parallel to this relation, insuring clewhichshowstheirlocationifMBH=0.Thefittedlineisgiven
that itispreservedinthepresence ofMBH measurement errors. by equation 2. The bi-directional arrow in the top right reveals
The lines emanating from each data point show how much each how the points wouldmove if the value of Mspheroid changes by
point would move if the black hole mass changed by a factor a factor of ±2. No arrow is provided for changes in the nuclear
of ±2 (upper points) or ±5 (lower seven points). Reflecting the componentmassesbecausetheresult/movementisobvious,with
increasing uncertainty on the smaller BH mass measurements, asimpleshiftintheordinate.
theshadedareahasahorizontalwidthoflog(2.0)atthetopand
log(5.0)atthemassratio5×10−3.
6 ACKNOWLEDGMENT
entire new window through which to view, or rather listen We thank Kenji Bekki for motivating us in 2007 to under-
to, our Universe. takethisproject.A.G.thankstheorganisersoftheFebruary
2008 “Nuclear Star clusters across the Hubble Sequence”
workshop held at the Max-Planck-Institut fu¨r Astronomie
inHeidelberg,Germany,andtheorganisersoftheJuly2008
5 SUMMARY
LorentzCentreWorkshop “Central Mass Concentrations in
WehaveidentifiedroughlyadozengalaxieswithadirectBH Galaxies” held in Leiden, The Netherlands, where prelimi-
mass measurement and a nuclear star cluster, doubling the nary versions of Figure 1 were presented.
previoussamplesizeforwhichthesemeasurementsareavail-
able.Wespeculatethattheexistenceofsuchhermaphrodite
nuclei may be a rather common event for spheroids with
stellar-massesrangingfrom108 to1011M⊙ (seealsoGonza- REFERENCES
lez Delgado et al. 2008 and Seth et al. 2008). Akritas,M.G.,&Bershady,M.A.,1996,ApJ,470,706
We have shown that the mass of the nuclear compo- Alexander, T. 2008, in the 2nd Kolkata conference on Ob-
nent(s) increases from ∼0.1 per cent of the host spheroid’s servational Evidence for Black Holes in the Universe, ed.
stellarmassinlargeellipticalgalaxieswhosecoresaredom- S.Chakrabarti,AIPConf.Ser.1053,79
inated by a massive black hole, to several per cent in low Allen,P.D.,Driver,S.P, Graham, A.W.,Cameron, E., Liske, J.,
stellar mass (∼ 108M⊙) spheroids whose cores are domi- &dePropris,R.2006, MNRAS,371,2
Argon,A.L.,etal.2004,ApJ,615,702
nated by a nuclear star cluster (see Figures 1 and 3, and
Balcells,M.,Graham,A.W.,Dom´ınguez-Palmero,L.,&Peletier,
also Balcells et al. 2007).
R.F.2003,ApJ,582,L79
We have derived a linear relation between the nuclear
Balcells,M.,Graham,A.W.,Peletier,R.2007, ApJ,665,1084
mass(BHandNCcombined)andthestellarmassofthehost
Baldry,I.K.,Balogh,M.L.,Bower,R.G.,Glazebrook,K.,Nichol,
spheroid.Giveninequation2,therelationcanbeexpressed R.C.,Bamford,S.P.,&Budavari,T.2006,MNRAS,373,469
as (MBH +MNC) ∝ M∗0,.s6p1h±0.07 (see Figure 3). We hereby Barth,A.J.,Strigari,L.E.,Bentz,M.C.,Greene,J.E.,&Ho,L.C.
suggest that this baryonic, nuclear-to-spheroid mass ratio 2009,ApJ,690,1031
relation is applicable to spheroids with eith either nuclear Bash, F.N., Gebhardt, K., Goss, W.M., & Vanden Bout, P.A.
clusters or dual nuclei type, noting that the exponent may 2008,AJ,135,182
equal a value of 1 once dry merging commences (assuming Bassino,L.P.,Muzzio,J.C.,&Rabolli,M.1994,ApJ,431,634
no loss of nuclear components). Baumgardt, H., Hut, P., Makino, J., McMillan, S., & Portegies
Zwart,S.2003a,ApJ,582,L21
Wehavealsoidentifiedanew(blackhole)-(nuclearclus-
Baumgardt, H., Makino, J., Hut, P., McMillan, S., & Portegies
ter)massratiorelationpertainingtothecoexistenceofthese
Zwart,S.2003b,ApJ,589,L25
entities. We provide a preliminary quatification of this re-
Becklin,E.E.,&Neugebauer, G.1968,ApJ,151,145
lation in equation 1, which is such that (MBH +MNC) = Begelman, M.C., Blandford, R.D., & Rees, M.J. 1980, Nature,
(5×107M⊙)2/3MB1/H3 forMBH <5×107M⊙ (seeFigure2). 287,307
(cid:13)c 0000RAS,MNRAS000,000–000
Massive Black Holes in Dense Nuclear Star Clusters 7
Figure1. Theincreasingdominanceofthecentralblackholeoverthenuclearclusterofstars,tracedbythemassratioMBH/(MBH+
MNC),isshowntodependonthehostspheroidmassMsph —whichiszeroforGCs.ThehighestmassspheroidsdonotcontainaNC,
but are included forillustrativepurposes (see Table1). The five circled points have only anupper limitto their BH mass. The arrows
ontherighthandsideofthefiguredenotetheupanddownmovementifeitherofthetwonuclearmassesareinerrorbyafactorof±2.
Bekki,K.,Couch, W.J.,Drinkwater,M.J.,&Gregg, M.D.2001, Cappellari, M., et al. 2008, in the proceedings of IAU Sympo-
ApJ,557,L39 sium245,“FormationandEvolutionofGalaxyBulges”,eds.
Bekki,K.,Couch,W.J.,Drinkwater,M.J.,Shioya,Y.2003,MN- M.Bureau,E.Athanassoula,andB.Barbuy,p.215
RAS344,399 Cardone,V.F.,&Sereno,M.2005,A&A,438,545
Bekki,K.,&Norris,J.E.2006,ApJ,637,L109 Carollo,C.M.,Stiavelli,M.,&Mack,J.1998, AJ,116,68
Bellazzini,M.,etal.2008,AJ,136,1147 Carollo,C.M.,Franx,M.,Illingworth,G.D.,&Forbes,D.A.1997,
BensonA.J.,Bower R.G.,FrenkC.S.,Lacey C.G.,Baugh C.M., ApJ,481,710
ColeS.2003,MNRAS,599,38 Carter,D.,&Sadler,E.1990, MNRAS,245,12P
Berentzen,I.,Preto,M.,Berczik,P.,Merritt,D.,&Spurzem,R. Catchpole, R.M., Whitelock, P.A., & Glass, I.S. 1990, MNRAS,
2009,ApJ,695,455 247,479
Berghea, C.T., Weaver, K.A., Colbert, E.J.M., Roberts, T.P. Chabrier,G.2003,AJ,586,L113
2008,ApJ,687,471 Ciotti,L.,Lanzoni,B.,&Renzini,A.1996,MNRAS,282,1
Bessell,M.S.,Castelli,F.,&Plez,B.1998, A&A,333,231 Coelho, P., Mendes de Oliveira, C., & Cid Fernandes, R. 2009,
Binggeli,B.,Sandage, A.,&Tammann,G.A.1985,AJ,90,1681 MNRAS,inpress(arXiv:0903.0448)
Binggeli,B.,Tammann,GA.,&Sandage, A.1987, AJ,94,251 Colina,L.,Gonzalez Delgado, R.,Mas-Hesse,J.M.,&Leitherer,
Bo¨ker,T.,Laine,S.,vanderMarel,R.P.,Sarzi,M.,Rix,H.-W., C.2002,ApJ,579,545
Ho,L.C.,&Shields,J.C.2002, AJ,123,1389 Condon, J.J., Cotton, W.D., Greisen, E.W., Yin, Q.F., Perley,
Bo¨ker,T.,Stanek,R.,&vanderMarel,R.P.2003,AJ,125,1073 R.A.,Taylor,G.B.,&Broderick,J.J.1998,AJ,115,1693
Booth, C.M., & Schaye, J. 2009, MNRAS, in press Coˆt´e,P.,etal.2006,ApJS,165,57
(arXiv:0904.2572) Cox,D.P.2000,Allen’sAstrophysicalquantities,NewYork:AIP
Bothun,G.1992,AJ,103,104 Press;Springer
Bower,G.A.,etal.2001,ApJ,550,75 Croton,D.J.,etal.2006, MNRAS,365,11
Bromley, B.C., Kenyon, S.J., Geller, M.J., Barcikowski, E., Dabringhausen, J., Hilker, M., Kroupa, P. 2008, MNRAS, 386,
Brown,&W.R.,Kurtz,M.J.2006, ApJ,653,1194 864
Brown,T.M.2009,in“GalaxyEvolution:EmergingInsightsand Danzmann, K., et al. 1996, Classical and Quantum Gravity, 13,
FutureChallenges”(arXiv:0901.2577) A247
Bruzual,G.,&Charlot,S.2003, MNRAS,344,1000 Davies,R.L.,etal.1983,ApJ,266,516
Burbidge,G.R.1970, ARA&A,8,369 de Francesco, G., Capetti, A., & Marconi, A. 2008, A&A, 479,
Busarello, G., Capaccioli, M., Capozziello, S., Longo, G., & 355
Puddu,E.1997,A&A,320,415 dePaolis,F.,Gurzadyan, V.G.,&Ingrosso,G.1996, A&A,315,
Butler,D.J.,&Mart´ınez-Delgado,D.2005,AJ,129,2217 396
Byun,Y.-I.,etal.1996,AJ,111,1889 De Rijcke, S., Prugniel, P., Simien, F., & Dejonghe, H. 2006a,
Caon, N., Capaccioli, M., D’Onofrio, M., 1993, MNRAS, 265, MNRAS,369,1321
1013 De Rijcke, S., Buyle, P., & Dejonghe, H. 2006b, MNRAS, 368,
Capaccioli,M.1989,inTheWorldofGalaxies,ed.H.G.Corwin, L43
L.Bottinelli(Berlin:Springer-Verlag),208 deVaucouleurs, G.,deVaucouleurs,A.,Corwin,H.G.,Jr.,Buta,
Cappellari, M., Verolme, E.K., van der Marel, R.P., Kleijn, R.J.,Paturel,G.,&Fouque, P.1991, Springer-Verlag(RC3)
G.A.V., Illingworth, G.D., Franx, M., Carollo, C.M., & de Djorgovski,S.,&Davis,M.1987,ApJ,313,59
Zeeuw,P.T.2002,ApJ,578,787 Djorgovski, S., de Carvalho, R., & Han, M.-S.1988, The Extra-
(cid:13)c 0000RAS,MNRAS000,000–000
8 Graham & Spitler
galacticDistanceScale,4,329 Graham,A.W.2008a,PASA,25,167Hu,J.2008,MNRAS,386,
D’Onofrio, M., Capaccioli, M., & Caon, N. 1994, MNRAS, 271, 2242
523 Graham,A.W.2008b, ApJ,680,143
Dressler,A.,&Sandage, A.1983,ApJ,265,664 Graham,A.W.,&Colless,M.M.1997,MNRAS,287,221
Driver, S. P., Popescu, C. C., Tuffs, R. J., Liske, J., Graham, Graham,A.W.,&Driver,S.P.2005,PASA,22,118
A. W., Allen, P. D., & de Propris, R. 2007, MNRAS, 379, Graham,A.W.,&Driver,S.P.2007,ApJ,655,77
1022 Graham,A.W.,&Guzma´n, R.2003,AJ,125,2936
Dull, J.D., Cohn, H.N., Lugger, P.M., Murphy, B.W., Seitzer, Graham,A.W.,&Li,I.2009,ApJ,698,812
P.O., Callanan, P.J., Rutten, R.G.M., & Charles, P.A 1997, Graham,A.W.,&Worley,C.C.2008,MNRAS,388,1708
ApJ,481,267 Graham, A. W., Merritt, D., Moore, B., Diemand, J., & Terzi´c,
Dull, J.D., Cohn, H.N., Lugger, P.M., Murphy, B.W., Seitzer, B.2006,AJ,132,2701
P.O., Callanan, P.J., Rutten, R.G.M., & Charles, P.A 2003, Grant, N.I., Kuipers, J.A., & Phillipps, S. 2005, MNRAS, 363,
ApJ,585,598 1019
Dwek,E.,etal.1995,ApJ,445,716 Gualandris,A.,&Merritt,D.2008,ApJ,678,780
Ebisuzaki,T.,etal.2001, ApJ,562,L19 Gu¨ltekin,K.etal.2009,ApJ,695,1577
Emsellem,E.,Dejonghe,H.,&Bacon,R.1999,MNRAS,303,495 Ha¨ring,N.,&Rix,H.-W.2004,ApJ,604,L89
Fabbiano,G.,etal.2003,ApJ,588,175 Harris,W.E.1996.AJ,112,1487
Faber,S.M.,Dressler,A.,Davies,R.L.,Burstein,D.,&Lynden- Ho, L.C.,Filippenko, A.V.,& Sargent, W.L.W. 1995, ApJS, 98,
Bell,D.1987,NearlyNormalGalaxies.FromthePlanckTime 477
tothePresent,175
Hopman,C.,&Alexander,T.2005, ApJ,629,362
Faber,S.M.,etal.1997,AJ,114,1771
Hopman,C.,&Alexander,T.2006a, ApJ,645,1152
Ferguson,H.C.1989, AJ,98,367
Hopman,C.,&Alexander,T.2006b, ApJ,645,L133
Ferrarese,L.,Ford,H.C.,Jaffe,W.1996,ApJ,470,444
Houghton,R.C.W.,Magorrian,J.,Sarzi,M.,Thatte,N.,Davies,
Ferrarese,L.,etal.2006a, ApJ,644,L21
R.L.,&Krajnovi´c,D.2006,MNRAS,367,2
Ferrarese,L.,etal.2006b, ApJS,164,334
Hu,J.2008, MNRAS,386,2242
Ferrarese,L.,&Ford,H.C.1999,ApJ,515,583
Hughes,M.A.,etal.2005,AJ,130,73
Ferraro, F. R., Possenti, A., Sabbi, E., Lagani, P., Rood, R. T.,
Jarrett,T.H.,Chester,T.,Cutri,R.,Schneider,S.E.,Huchra,J.P.
D’Amico,N.,&Origlia,L.2003,ApJ,595,179
2000,AJ,119,2498
Filippenko,A.V.,&Ho,L.C.2003,ApJ,588,L13
Hjorth,J.,&Madsen,J.1995,ApJ,445,55
Forbes,D.A.,Franx,M.,&Illingworth,G.D.1995,AJ,109,1988
Jerjen,H.,Binggeli,B.,&Barazza,F.D.,2004,AJ,127,771
Forbes,D.A.,Lasky,P.,Graham,A.W.,Spitler,L.R.2008,MN-
Jones,D.H.,etal.1996,ApJ,466,742
RAS,389,1924
Ju,L.,Blair,D.G.,Zhao,C.2000,Rep.Prog.Phys.63,1317
Freeman,K.1993,in:GraemeH.Smith&JeanP.Brodie(eds.),
Kauffmann,G.,&Haehnelt, M.2000,MNRAS,311,576
The Globular Cluster-Galaxy Connection, ASP-CS, vol.48,
Kent,S.M.1985, ApJS,59,115
p.608
Kent,S.M.1987, AJ,94,306
Freitag,M.2001, ClassicalandQuantumGravity,18,4033
Kent,S.M.,Dame,T.,&Fazio,G.,1991, ApJ,378,131
Fukugita, M., Shimasaku, K., & Ichikawa, T. 1995, PASP, 107,
King,I.1962, AJ,67,471
945
Knapen,J.H.,deJong,R.S.,Stedman,S.,&Bramich,D.M.2003,
Gair, J.R., Barack, L., Creighton, T., Cutler, C., Larson, S.L.,
MNRAS,344,527
Phinney,E.S.,&Vallisneri,M.2004,ClassicalandQuantum
Kochanek,C.S.,Shapiro,S.L.,&Teukolsky,S.A.1987,ApJ,320,
Gravity,21,1595
73
Gallo, E., Treu, T., Jacob, J., Woo, J.-H., Marshall, P.J., An-
Komossa,S.&Bade,N.1999,A&A,343,775
tonucci, R.2008,ApJ,680,154
Komossa,S.,&Merritt,D.2008,ApJ,683,L21
Gebhardt, K.,etal.1996,AJ,112,105
Kormendy, J., & Gebhardt, K., 2001, in The 20th Texas Sym-
Gebhardt, K.,etal.2001,AJ,122,2469
Gebhardt, K.,Rich,R.M.,&Ho,L.C.2002,ApJ,578,L41 posium on Relativistic Astrophysics, ed. H. Martel, & J.C.
Wheeler,AIP,586,363
Gebhardt, K.,etal.2003,ApJ,583,92
Gebhardt, K.,Rich,R.M.,&Ho,L.C.2005,ApJ,634,1093 Kormendy,J.,&McClure,R.D.1993,AJ,105,1793
Gebhardt, K.,etal.2007,ApJ,671,1321 Kormendy,J.,&Richstone,D.1995,ARA&A,33,581
Geha, M., Guhathakurta, P., & van der Marel, R.P. 2002, AJ, Kormendy,J.,etal.1996,ApJ,459,L57
124,3073 Lanzoni,B.,Dalessandro,E.,Ferraro,F.R.,Miocchi,P.,Valenti,
Geha, M., Guhathakurta, P., & van der Marel, R.P. 2003, AJ, E.,&Rood,R.T.2007, ApJ,668,L139
126,1794 Laor,A.2001,ApJ,553,677
Georgiev, I.Y., Hilker, M., Puzia, T.H., Goudfrooij, P., Baum- Lauer,T.R.,Faber,S.M.,Ajhar,E.A.,Grillmair,C.J.,&Scowen,
gardt,H.2009,MNRAS,396,1075 P.A.1998,AJ,116,2263
Gerssen, J., van der Marel, R.P., Gebhardt, K., Guhathakurta, Lauer,T.R.,etal.2005,AJ,129,2138
P.,Peterson,R.C.,&Pryor,C.2002,AJ,124,3270 Launhardt,R.,Zylka,R.,&Mezger,P.G.2002,A&A,384,112
Gerssen, J., van der Marel, R.P., Gebhardt, K., Guhathakurta, Lee,M.-H.1993, ApJ,418,L147
P.,Peterson,R.C.,&Pryor,C.2003,AJ,125,376 Lee,M.-H.1995, MNRAS,272,605
Ghez, A. M., Salim, S., Hornstein, S. D., Tanner, A., Lu, J. R., Li, C., & White, S.D.M. 2009, MNRAS, submitted
Morris, M., Becklin, E. E., & Duchˆene, G. 2005, ApJ, 620, (arXiv:0901.0706)
744 Li, J., Ma, J., Zhou, X., Jiang, Z., Yang, Y., & Chen, J. 2004,
Ghosh, K.K., Suleymanov, V., Bikmaev, I., Shimansky, S., & A&A,420,89
Sakhibullin,N.2006,MNRAS,371,1587 Lightman, A.P., & Shapiro, S.L. 1978, Reviews of Modern
Gonza´lezDelgado,R.M.,P´erez,E.,CidFernandes,R.,&Schmitt, Physics,50,437
H.2008,AJ,135,747 Lodato,G.,King,A.R.,Pringle,J.E.2008, MNRAS,392,332
Graham,A.W.2002,ApJ,568,L13 Lotz,J.M.,Miller,B.W.,Ferguson,H.C.2004,ApJ,613,262
Graham,A.W.2007,MNRAS,379,711 Lyubenova,M.,Kuntschner,H.,Silva,D.R.2008,A&A,485,425
(cid:13)c 0000RAS,MNRAS000,000–000
Massive Black Holes in Dense Nuclear Star Clusters 9
Ma,J.,deGrijs,R.,Fan,Z.,Rey,S.-C.,Wu,Z.,&Zhou,X.2009, Satyapal, S., Vega, D., Heckman, T., O’Halloran, B., & Dudik,
RAA,inpress(arXiv:0904.0674) R.2007,ApJ,663,L9
Maccarone,T.J.,&Servillat,M.2008,MNRAS,389,379 Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500,
Macchetto,F.,Marconi,A.,Axon,D.J.,Capetti,A.,Sparks,W., 525
&Crane,P.1997,ApJ,489,579 Schmidt,A.A.,Bica,E.,&Alloin,D.1990,MNRAS,243,620
Maciejewski,W.,&Binney,J.2001,MNRAS,323,831 Scho¨del,R.,etal.2007,A&A,469,125
Marconi,A.,etal.2003,ApJ,586,868 Scho¨del, R.,Merritt,D.,& Eckart, A.2009a, Journal of Physics
Marconi,A.,&Hunt,L.K.,2003,ApJ,589,L21 ConferenceSeries,131,012044
MateoM.L.1998,ARA&A,36,435 Scho¨del, R., Merritt, D., & Eckart, A. 2009b, A&A, in press
Matkovi´c,A.,&Guzma´n, R.2005,MNRAS,362,289 (arXiv:0902.3892)
McConnachie,A.W.,Irwin,M.J.,Ferguson,A.M.N.,Ibata,R.A., Seigar, M., Carollo, C.M., Stiavelli, M., de Zeeuw, P.T., & De-
Lewis,G.F.,Tanvir,N.2005,MNRAS,356,979 jonghe,H.2002,AJ,123,184
McCrady,N.,Gilbert,A.M.,Graham,J.R.2003, ApJ,596,240 Seth,A.,Agu¨eros,M.,Lee,D.,Basu-Zych,A.2008,678,116
McLaughlin, D.E., Anderson, J., Meylan, G., Gebhardt, K., Shankar, F.,Salucci, P.,Granato, G.L.,DeZotti, G.,&Danese,
Pryor,C.,Minniti,D.,&Phinney,S.2006a, ApJS,166,249 L.2004,MNRAS,354,1020
McLaughlin, D.E., King, A.R., Nayakshin, S. 2006b, ApJ, 650, Shapiro, K.L., Cappellari, M., de Zeeuw, T., McDermid, R.M.,
L37 Gebhardt, K., van den Bosch, R.C.E., & Statler, T.S. 2006,
Miller,N.A.,Mushotzky,R.F.,&Neff,S.G.2005,ApJ,623,L109 MNRAS,370,559
Merritt,D.2008,ApJ,694959 Shen,J.,VandenBerk,D.E.,Schneider,D.P.,&Hall,P.B.2008,
Merritt,D.,&Ferrarese,L.2001,MNRAS,320,L30 AJ,135,928
Merritt,D.,Ferrarese,L.,&Joseph,C.L.2001,Science,293,1116 Shields,J.C.,Walcher,C.J.,Boeker,T.,Ho,L.C,Rix,H.-W.,van
Merritt,D.,Mikkola,S.,&Szell,A.2007,ApJ,671,53 derMarel,R.P.2008,ApJ,682,104
Merritt,D.,Milosavljevi´c,M.,Favata,M.,Hughes,S.A.,&Holz, SilkJ.,&ReesM.J.1998,A&A,331,L1
D.E.2004,ApJ,607,L9 Sigurdsson,S.1997, ClassicalandQuantumGravity,141425
Meylan,G.,Mayor,M.,Duquennoy,A.,&Dubath,P.1995,A&A, Sil’chenko,O.K.1999,AJ,117,2725
303,761 Sil’chenko, O.K., Afanasiev, V.L., Chavushyan, V.H., & Valdes,
Meylan, G., Sarajedini, A., Jablonka, P., Djorgovski, S.G., J.R.2002,ApJ,577,668
Bridges,T.,&Rich,R.M.2001,AJ,122,830 Smith,S.1935, ApJ,82,192
Mighell,K.J.,&Rich,R.M.1995,AJ,110,1649 Soria,R.,Graham,A.W.,Fabbiano,G.,Baldi,A.,Elvis,M.,Jer-
Minniti,D.,Olszewski,E.W.&Rieke,M.1993, ApJ,410,L79 jen, H., Pellegrini, S., & Siemiginowska, A. 2006, ApJ, 640,
Miniutti, G., Ponti, G., Dadina, M., Cappi, M., Malaguti, G., 143
Fabian,A.C.,&Gandhi,P.2006,MNRAS,373,L1 Stanford, L.M., Da Costa, G.S., Norris, J.E., & Cannon, R.D.
Noyola,E.,Gebhardt, K.,Bergmann,M.2008,ApJ,676,1008 2007,ApJ,667,911
Nucita, A.A., De Paolis, F., Ingrosso, G., Carpano, S., & Stiavelli,M.,Miller,B.W.,Ferguson,H.C.,Mack,J.,Whitmore,
Guainazzi, M.2008, A&A,478,763 B.C.,&Lotz,J.M.2001, AJ,121,1385
Oh,S.,Kim,S.S.,&Figer,D.F.2009,JKAS,42,17 Thim,F.,Hoessel,J.G.,Saha,A.,Claver,J.,Dolphin,A.,&Tam-
O’Leary,R.M.,Kocsis,B.,Loeb,A.2009,MNRAS,395,2127 mann,G.A.2004, AJ,127,2322
O’Leary, R.M., Rasio, F.A., Fregeau, J.M., Ivanova, N., & Tomita,A.,Aoki,K.,Watanabe,M.,Takata,T.,&Ichikawa,S.-I.
O’Shaughnessy, R.2006,ApJ,637,937 2000,AJ,120,123
Pastorini,G.,etal.2007, A&A,469,405 Tonry,J.L.,1984,ApJ,283,L27
Patruno, A.,Portegies Zwart, S.,Dewi, J., & Hopman, C. 2006, Tonry, J.L., Dressler, A., Blakeslee, J.P., Ajhar, E.A., Fletcher,
MNRAS,370,L6 A.B., Luppino, G.A., Metzger, M.R., & Moore, C.B. 2001,
Peterson,B.M.,etal.2005, ApJ,632,799 ApJ,546,681
Pinkey,J.,etal.2003,ApJ,596,903 Tremaine, S.D., Ostriker, J.P., & Spitzer, L.Jr. 1975, ApJ, 196,
Pooley,D.,&Rappaport,S.2006,ApJ,644,L45 407
Portegies Zwart, S.F., Baumgardt, H., Hut, P., Makino, J., & van de Ven, G., van den Bosch, R.C.E, Verolme, E.K., & de
McMillan,S.L.W.2004,Nature,428,724 Zeeuw,P.T.2006,A&A,445,513
Prugniel,P.,&Simien,F.1997,A&A,321,111 van den Bosch, R., de Zeeuw, T., Gebhardt, K., Noyola, E., &
Quinlan,G.D.1996,NewAstronomy,1,35 vandeVen,G.2006, ApJ,641,852
Quinlan,G.D.,&Shapiro,S.L.1987, ApJ,321,199 Valluri,M.,Ferrarese,L.,Merritt,D.,&Joseph,C.L.2005,ApJ,
Quinlan,G.D.,&Shapiro,S.L.1990, ApJ,356,483 628,137
Ravindranath,S.,Ho,L.C.,Peng,C.Y.,Filippenko,A.V.,&Sar- Verdoes Kleijn, G. A., Baum, S. A., de Zeeuw, P. T., & O’Dea,
gent, W.L.W.2001,AJ,122,653 C.P.2002,AJ,123,1334
Regan,M.W.,&Vogel,S.N.1994, ApJ,434,536 Verolme,E.K.,etal.2002,MNRAS,335,517
Renzini, A., Greggio, L., di Serego Alighieri, S., Cappellari, M., Walcher,C.J.,etal.2005,ApJ,618,237
Burstein,D.,&Bertola,F.1995,Nature,378,39 Wehner,E.H.,&Harris,W.E.2006,ApJ,644,L17
Rest, A., van den Bosch, F. C., Jaffe, W., Tran, H., Tsvetanov, Wolter,A.,Trinchieri,G.,&Colpi,M.2006,MNRAS,373,1627
Z.,Ford,H.C.,Davies,J.,&Schafer,J.2001,AJ,121,2431 Worthey, G.2004,AJ,128,2826
Rix,H.-W.,Carollo,C.M.,&Freeman,K.,1999, ApJ,513,L25 Wyse,R.F.G.,Gilmore,G.,&Franx,M.1997,ARA&A,35,637
Rogers,B.,Ferreras,I.,Peletier,R.F.2008,Silk,J.2008,MNRAS, Yanny, B., Guhathakurta, P., Bahcall, J.N., & Schneider, D.P.
submitted(arXiv:0812.2029) 1994,AJ,107,1745
Rose,J.A.,Arimoto,N.,Caldwell,N.,Schiavon,R.P.,Vazdekis, Zepf,S.E.,etal.2008,ApJL,683,L139
A.,&Yamada,Y.2005,AJ,129,712
Rosswog,S.,Ramirez-Ruiz,E.,&Hix,R.2009,ApJ,695,404
Rubin, V. 1974, Nuclei of Galaxies, Black Holes, and Collapsed 7 APPENDIX: THE DATA
Matter,3rd,Erice,Italy,May8-20,1974, Lecture.11p.
Rubin, V.C., Peterson, C.J., & Ford, W.K., Jr. 1980, ApJ, 239, Here we provide sufficient information for the reader to re-
50 constructourdataset,showninTables1and2,andinclude
(cid:13)c 0000RAS,MNRAS000,000–000
10 Graham & Spitler
the necessary references to relevant sources of literature. lightusingastandarddecompositiontechnique.Forperhaps
When noclear (refereed) literature data was available for a thefirsttime,wereportthatthenuclearclusteroftheMilky
nuclear cluster’s magnitude, we have modelled the galaxy’s WayisreasonablywellrepresentedbyaS´ersicfunctionwith
light profile ourselves to determine the stellar flux. Select indexn=3; ithasan effectivehalf-light radiusRe equalto
notes on individual objects are provided below, along with 80arcseconds(3.2pc).Itisperhapsworthemphasisingthat
a brief mention of some objects that were excluded. thisis far from aGaussian distribution which would havea
Columns1and2ofTable2providetheobjectnameand S´ersic index of 0.5. Determination of whether alternative
its morphological type.Theblack hole masses (column 4 of models, such as theKing(1962) model, provide abetter fit
Table 2) havebeen adjusted according toupdated informa- will beleft for a separate study.
tion on their distances, which are provided in column 3. Due to the uncalibrated nature of the published light
Galaxymagnitudeswereusedfortheellipticalgalaxies, profile,wearenotabletoreportamagnitudeforthenuclear
whilebulgemagnitudeswereusedforthelenticular(andthe cluster.However,eventhoughnoextinctioncorrectionswere
fewspiral)galaxies.Thesearelistedincolumn4ofTable2. applied, having accounted for the background bulge flux in
These “spheroid” magnitudes were then converted into so- Figure 7.5, we can provide rough estimates to the negative
lar masses. This first required that we calibrate them in logarithmic slope of the nuclear cluster’s projected density
terms of solar luminosity, and we used the following abso- profile γ(R) ≡ −d[logI(R)]/dlogR = (bn/n)(R/Re)1/n,
lute solar magnitudes (Cox 2000): MB =5.47; MV = 4.82; with bn ≈ 1.9992n −0.3271 (Capaccioli 1989; Graham &
MK = 3.33; supplemented with MH = 3.32 (Bessell et Driver 2005). At R = Re/4 = 20′′, γ = 1.19, and at
al. 1998); MF814W = 4.14 and MF850LP = 3.996. These R = Re = 80′′, γ = 1.89. The associated negative loga-
spheroidluminositieswerethenconvertedintomassesusing rithmic slope of the nuclear cluster’s internal 3D (i.e. non-
an appropriate stellar mass-to-light (M/L) ratio: for ellip- projected) density profile is 2.0 and 2.7, respectively (Gra-
tical galaxies (and lenticular bulges) we assumed an age of ham et al. 2006, theirEq.23). This is steeper than previous
13 Gyr and [Fe/H] = 0.5 dex (and 7 Gyr and [Fe/H] = estimatesof∼1.4to1.8(Becklin&Neugebauer1968;Catch-
0.3 dex), which gave a K-band stellar mass-to-light ratio pole et al. 1990; Sch¨odel et al. 2009a, 2009b and references
of log[M/LK] = 0.0 dex (and log[M/LK] = −0.22 dex). therein) which havebeen biased by bulge stars.
Given that theK-band stellar mass-to-light ratio varieslit- M32 Ferrarese et al. (2006a) suggest that M32 may
tle with metallicities ranging from [Fe/H] = 0.0 to 0.6 dex, contain both a nuclear star cluster (Smith 1935; Burbidge
andwithagesrangingfrom5to13Gyr,theaboveselection 1970;Worthey2004)andaBH(Verolmeetal.2002,MBH =
of age and metallicity has little effects on the results. For 2.5+−00..55×106M⊙). InFigure 7.5 we present an I-bandlight
the bulges in spiral galaxies, and when no reliable K-band profile which we have fitted with an inner nuclear compo-
magnitudeswereavailable,thederivationofthestellarM/L nent, plus a S´ersic bulge and an outer exponential enve-
ratio is given below. The adopted ratios for each passband lope/disc(seeGraham2002).Theinnercomponentandthe
areshownincolumn5ofTable2,whilethespheroidmasses mainspheroidcomponenthaveanapparentF814W magni-
are given in column 6. Given the various sources of uncer- tudeof10.0and7.5mag,respectively.TheS´ersicindexnof
taintyin each step,thesevaluesare likely tobeaccurateto theinner component is 2.3 (theeffective half-light radius is
within a factor of two. 1.65 arcseconds, equal to ∼6 pc). Together with the Milky
BecausetheNCsmayhaveaconsiderablerangeofages Way’snuclearstarcluster,thispaperpresentsthefirstclear
(and metallicities), individual stellar M/L ratios were de- evidence/statement that nuclear excesses do not all have a
termined for each NC. To estimate these we have used the Gaussian-like structure (i.e. n=0.5);
available NC colours (see below) together with the models Although we initially excluded M32 due to the some-
by Bruzual & Charlot (2003) and a Chabrier (2003) ini- whatunknownnatureofitscentralexcess,spectroscopyhas
tial mass function (see Figure 4). The approach adopted is revealed that the inner region of M32 does possess a differ-
discussed below for each NC, and the results are shown in entmeanchemistryandage(e.g.Worthey2004;Roseetal.
columns 7 to 9 of Table 2. In some instances we bypassed 2005)tothemainspheroid,althoughnoobvioustransitional
this process and used literature established masses. radius is apparent. This difference has most recently been
quantified by Coelho et al. (2009), using an inner 1.5 arc-
second slit to sample the nuclear region. Using the spread
7.1 Notes on individual galaxies of ages and metallicties from theirTable 3 givesan I-band,
stellar M/L ratio of 0.75±0.17 and 0.92±0.18 for thenu-
Milky Way: Due to its proximity, the centre of our own
cleus and main spheroid of M32.
galaxy,theMilkyWay,haslongbeensuspectedofharbour-
ingbothanuclearstarclusterandasupermassiveblackhole Exclusion of M32 from the analysis has no significant
(Rubin1974).Referencestothemassesoftheseentities(not effect on any of theresults.
the mass within the inner parsec: Sch¨odel et al. 2009b) are NGC 1023:Boweretal.(2001)showthattheaverage
provided in Table 2, along with an estimate of the mass of V−Icolourwithin1.0(0.1)arcsecondis1.4(1.25).Fromthe
theMilky Way’sbulge. innermostcolourouradoptedstellar populationmodels tell
The nuclear star cluster has most recently been exam- usthatthemetallicityissuper-solarandthatthepopulation
ined by Sch¨odel et al. (2009a) and Oh et al. (2009). In Fig- must be of an intermediate to old age, in agreement with
ure 7.5 we haveperformed a separation of the Milky Way’s the 7 Gyr old age from Sil’chenko (1999). From Figure 4
NClightfromthebackgroundbulge(andatsomelevelbar) the associated V-band, stellar M/L ratio is in the range
0.2 < log[M/L ] < 0.6 and we adopt M/L = 2.5 for the
V V
NCmagnitude from Bower et al. (2001, their Eq.2).
6 http://www.ucolick.org/∼cnaw/sun.html Faberetal.(1997)reportB−V =0.93forthisgalaxy,a
(cid:13)c 0000RAS,MNRAS000,000–000