Masses and Scaling Relations for Nuclear Star Clusters, and their Coexistence with Central Black Holes

3.2 MB·

by  Iskren Y. Georgiev

Mon.Not.R.Astron.Soc.000,1–22(2014) PrintedJanuary13,2016 (MNLATEXstylefilev2.2)
Masses and Scaling Relations for Nuclear Star Clusters, and
their Coexistence with Central Black Holes
Iskren Y. Georgiev1(cid:63), Torsten Böker2, Nathan Leigh3,4, Nora Lützgendorf2,
6
1 and Nadine Neumayer1
0
1Max-Planck Instiut für Astronomie, Königstuhl 17, 69117 Heidelberg
2
2Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA

n 3Department of Astrophysics, American Museum of Natural History, Central Park West and 79th Street, New York, NY 10024
a 4Department of Astronomy, University of Alberta, 116 St and 85 Ave, Edmonton, AB T6G 2R3, Canada
J

1
1
Accepted2015mmdd.Received2015mmdd
]
A
G ABSTRACT
. GalacticnucleitypicallyhosteitheraNuclearStarCluster(NSC,prevalentingalaxies
h with masses (cid:46) 1010M ) or a Massive Black Hole (MBH, common in galaxies with
p (cid:12)
masses(cid:38)1012M ).Intheintermediatemassrange,somenucleihostbothaNSCand
- (cid:12)
o a MBH. In this paper, we explore scaling relations between NSC mass ( ) and
NSC
r hostgalaxytotalstellarmass( )usingalargesampleofNSCsinlateM-andearly-
t M(cid:63),gal
s type galaxies, including a number of NSCs harboring a MBH. Such scaling relations
a reflecttheunderlyingphysicalmechanismsdrivingtheformationand(co)evolutionof
[
these central massive objects. We find 1.5σ significant differences between NSCs in
∼
1 late- and early-type galaxies in the slopes and offsets of the relations reff,NSC– NSC,
M
v r – and – , in the sense that i) NSCs in late-types are more
eff,NSC (cid:63),gal NSC (cid:63),gal
3 compactMat fixed M andM ; and ii) the – relation is shallower
NSC (cid:63),gal NSC (cid:63),gal
1 for NSCs in late-tyMpes than inMearly-types, similarMto the M – relation. We
BH (cid:63),bulge
6 discusstheseresultsinthecontextofthe(possiblyongoingM)evoluMtionofNSCs,depen-
2
ding on host galaxy type. For NSCs with a MBH, we illustrate the possible influence
0
of a MBH on its host NSC, by considering the ratio between the radius of the MBH
.
1 sphereofinfluenceandr .NSCsharbouringasufficientlymassiveblackholeare
eff,NSC
0 likely to exhibit surface brightness profile deviating from a typical King profile.
6
1 Keywords: galaxies:nuclei–galaxies:starclusters–galaxies:quasars:supermassive
: black holes
v
i
X
r
a
1 INTRODUCTION Object1 (CMO), suggesting that the formation and evolu-
tion of both types of central mass concentration may be
A growing body of observational evidence indicates that
linked by similar physical processes.
the nuclear regions of galaxies are often occupied by a
Indeed,themassrangeofthetwocomponentsofCMOs
nuclear star cluster (NSC) and/or a super massive black
overlap, with SMBHs having M (cid:38)106M (e.g. Gültekin
hole (SMBH), with NSCs being identified in more than BH (cid:12)
et al. 2009; Rusli et al. 2013; McConnell & Ma 2013), and
>60−70% of early- (e.g. Durrell 1997; Carollo et al. 1998;
NSC masses falling in the range 104 (cid:46) M (cid:46) 108M .
Geha et al. 2002; Lotz et al. 2004; Côté et al. 2006; Turner NSC (cid:12)
BothM andM haverepeatedlybeenfoundtocorre-
etal.2012;denBroketal.2014)andlate-typegalaxies(e.g. BH NSC
latewitharangeofhostgalaxypropertiesincludinggalaxy
Böker et al. 2002, 2004; Balcells et al. 2007b,a; Seth et al.
luminosity,mass,stellarvelocitydispersion(σ),AGNactiv-
2006;Georgievetal.2009a;Georgiev&Böker2014;Carson
ity etc. (e.g. Gültekin et al. 2009; Seth et al. 2008a; Kor-
et al. 2015). Driven by apparent similarities in the scaling
mendy & Ho 2013, and references therein). These correla-
relations of SMBHs and NSCs with host galaxy properties,
tions followed the earlier discoveries that the mass of the
Ferrareseetal.(2006a)introducedthetermCentralMassive
SMBHscaleswiththehostgalaxyB-bandbulgeluminosity
1 Toclarifytheterminology,wewillusetheternCMOtodescribe
(cid:63) E-mail:[email protected];[email protected] thesumofNSCandMBH,ifbotharepresent.
c 2014RAS
(cid:13)
2 I. Y. Georgiev, T. Böker, N. Leigh, N. Lützgendorf, N. Neumayer
(Kormendy&Richstone1995),dynamicalmass(e.g.Magor- trueeveninformationscenariosthatinvolvethemergingof
rianetal.1998;Häring&Rix2004),stellarvelocitydisper- systems: regardless of whether a NSC spirals into a nucleus
sion (Ferrarese & Merritt 2000; Gebhardt et al. 2000) and that already contains a MBH, or whether a MBH falls into
central light concentration (e.g. Graham et al. 2001). anucleusoccupiedbyaNSC,thestructureandintegrityof
Similarscalingrelationsarealsofoundtoholdbetween the NSC will be impacted if the MBH mass is a sufficiently
the mass of NSCs and their host galaxy bulge luminosity, high fraction of the bound NSC mass (e.g. Antonini et al.
mass(e.g.Ferrareseetal.2006a;Wehner&Harris2006)as 2012, 2015; Antonini 2013, see also refs. in §3.3)
well as morphological type (e.g. Rossa et al. 2006; Erwin & In massive globular clusters (GCs) and ultra compact
Gadotti2012).Thedetailedshapeoftheserelationspossibly dwarf galaxies (UCDs), the presence of MBHs is hotly de-
dependsonhostgalaxymorphology,assuggestedbyErwin bated. If confirmed, this would add support to the no-
& Gadotti (2012) who report a systematic difference in the tion that some GCs may be the former nuclei of galaxies
NSC mass fraction between early- and late-type hosts. It which lost significant amounts of mass in galaxy interac-
is, however, still hotly debated which is the fundamental tions/merging (e.g. Lützgendorf et al. 2013; Mieske et al.
physical mechanism setting these scaling relations (e.g. gas 2013;Sethetal.2014,andrefstherein).The(non-)presence
accretion, cluster and/or galaxy mergers Silk & Rees 1998; of MBHs is therefore an extremely important factor for the
McLaughlin et al. 2006; Li et al. 2007; Leigh et al. 2012; studyofthevarioustypesofcompactstellarsystems(NSCs,
Antonini 2013), or any combination of these (see reviews UCDs,andmassiveGCs),andpossibleevolutionaryconnec-
by Kormendy & Ho 2013; Cole & Debattista 2015). Proper tionsbetweenthem(e.g.Greggetal.2009;Priceetal.2009;
understanding of these issues is crucial for gaining insight Georgievetal.2009b,a,2012;Tayloretal.2010;Misgeld&
into the formation and growth of CMOs, and, in turn, how Hilker2011;Chiboucasetal.2011;Brünsetal.2011;Norris
a CMO might impact the evolution of the host galaxy. &Kannappan2011;Fosteretal.2011;Pfeffer&Baumgardt
Perhaps one of the most intriguing observations is the 2013;Puziaetal.2014;Georgiev&Böker2014;Frank2014;
coexistence of NSC and SMBH in galaxies with masses Norris et al. 2014; Seth et al. 2014).
around M (cid:39)1010M (Filippenko & Ho 2003; Seth et al. Here,weexplorescalingrelationsbetweenthesize/mass
gal (cid:12)
2008a, 2010; Graham & Spitler 2009; Neumayer & Walcher of NSCs and the stellar mass of their host galaxies, M(cid:63),
2012),withthebest-studiedexamplebeingthecenterofthe sorted by host morphology. In this context, it is reasonable
Milky Way (Schödel et al. 2007; Ghez et al. 2008; Gillessen to consider the total mass of the host galaxy (rather than
etal.2009;Genzeletal.2010;Feldmeieretal.2014;Schödel justbulgemass).Thisisbecausethebulgemassofanearly-
etal.2014).Throughoutthispaper,wewillusetheterm“co- type, elliptical galaxy is effectively equal to its total stellar
existing” wheneverdescribinganNSCthatcontainsaMBH. mass,whileinlate-typegalaxies,thebulge-ifitexistsatall
FindingcoexistingNSCsandMBHshastriggerednumerous - is negligible compared to the disk component. Therefore,
studiestounderstandthenatureofthisco-existenceandthe the main mass reservoir for NSC and/or SMBH formation
processes involved in their formation, growth, mutual influ- inlate-typediskswouldbeignoredinstudiesthatonlycon-
ence,andco-evolution(e.g.McLaughlinetal.2006;Lietal. siderthebulgemassofthehost.Whileafewpreviousstud-
2007; Nayakshin et al. 2009; Bekki & Graham 2010). ies (Carollo et al. 1998; Erwin & Gadotti 2012) have taken
For example, Neumayer & Walcher (2012) discuss the theapproachofconsideringthetotalhostgalaxymass,our
possibilitythatNSCsaresusceptibletodestructionbyBHs worksignificantlyimprovesonthenumberofobjectsandthe
when M /M >> 1, or when the MBH sphere of in- galaxymassrange,takingadvantageofourrecentcatalogue
BH NSC
fluencebecomescomparabletothesizeoftheNSC(Merritt of NSCs in disk galaxies (Georgiev & Böker 2014).
2006).RecentN−bodysimulationshavedemonstratedthat In §2, we describe the galaxy sample and the calcula-
the capture and accretion of stars migrating within the BH tionofphotometricmassesforNSCandhostgalaxy.In§3,
sphere of influence can significantly contribute to the mass we present the analysis and comparison between late- and
growth of black holes as well as the central core density of early-type galaxies using relations between NSC mass and
the host galaxy (Brockamp et al. 2011, 2014). From the- size,MNSC-reff,NSC aswellasbetweenNSCpropertiesand
oretical arguments, the growth rate of MBHs is expected host galaxy stellar mass, reff,NSC-M(cid:63) (§3.1), and MNSC-
to increase with MBH mass and likely requires a seed BH M(cid:63) (§3.2).Fornucleiwithco-existingNSCandSMBH,we
withM>100M ,unlesstheBHhostclusterisverydense show in §3.3 the corresponding relations for the combined
(cid:12)
(Baumgardt et al. 2004a,b, 2005, 2006). These simulations CMO mass, MBH+NSC-M(cid:63), and the ratio between the ra-
alsoshowthatasignificantfractionofstarscanescapefrom dius of the BH sphere of influence and the NSC effective
the cluster due to close encounters with the MBH (Baum- radius,rinfl,BH/reff,NSC.Theresultsarediscussedin§4and
gardt et al. 2004a, 2006). our conclusions are summarized in §5.
The presence of a MBH can inhibit the onset of core-
collapsein theNSC,and cause theNSCto expand,andul-
timatelytobedisrupted(e.g.Merritt2006,2009;Tremaine
1995).DependingonM andtheclustercoredensity(con- 2 DATA SAMPLES AND DERIVING NSC AND
BH
centrationandcorevelocitydispersion),theimpactoftidal GALAXY PHOTOMETRIC MASS
stressforcesfromtheMBHontheNSCwillbecomesignifi-
2.1 Morphological Sample Definitions
cantataradiuscomparabletothatoftheMBHsphereofin-
fluence, r which scales linearly with M . Therefore, Oneofourmaingoalsforthisstudyistocheckfordifferences
infl,BH BH
theeffectofaMBHonthestellarorbitsintheNSCislikely in the CMO properties between early- and late-type galax-
to be more pronounced in massive host galaxies (because ies,i.e.innucleiofdynamically“hot” (bulge-dominated)and
more massive galaxies host more massive MBHs). This is “cold”, (disk-dominated) host galaxies. Such a morphology-
c 2014RAS,MNRAS000,1–22
(cid:13)
NSCs in late- and early-type hosts 3
based separation could indicate two different modes of evo- Morphological type
E E-S0S0 S0a Sa Sab Sb Sbc Sc Scd SdSdmSm Irr
lution, e.g. active and inactive. Any observed differences 45
between late- and early-type NSCs could therefore reflect Early-types
40 Late-types
the underlying environmental conditions for CMO forma-
In Fornax (3) and Virgo (9)
tion (e.g. Leigh et al. 2015). 35
To define our NSC sample, We use the t−type galaxy 30
morphological parameter defined by de Vaucouleurs et al.
25
(1991).TheoverallsampleiscomprisedofNSCsinspheroid- N
dominatedgalaxiesfromCôtéetal.(2006)andTurneretal. 20
(2012),aswellasindisk-dominatedgalaxiesfromGeorgiev 15
& Böker (2014) and Georgiev et al. (2009a). We divide this
10
master sample into sub-samples of NSCs in early- and late-
type hosts using the following criteria: the early-type sub- 5
sampleiscomprisedofallgalaxieswitht<0(i.e.bulgedom-
0
inated Es-S0s), while the late-type sub-samples contains all -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 11
galaxies with t>3 (i.e. disk dominated, Sb and later). To Morphological type code [t-value]
demonstratetheclearseparationofthetwosub-samples,we
showinFigure1histogramsofthet-typedistributionwithin Figure 1.Morphologicaldistributionoftheearly-andlate-type
sub-samples. The t-type values are from HyperLEDA (based on
each subsample. This approach enables us to identify gen-
deVaucouleursetal.1991).
eral trends and differences between the properties of NSCs
inbulgeanddisk-dominatedhostgalaxies,suchasthosere-
portedbyErwin&Gadotti(2012)whofindachangeinthe
massbyafactoroftwo(atfixedageandmetallicity),which
the mass ratio M /M occuring around t (cid:39) 3 (their
NSC (cid:63),gal needs to be considered when estimating the systematic un-
Fig.4), i.e. close to the morphological separation between
certainties(seealso§2.2anddiscussionin§3.2).Thevalues
our late- and early-type sub-samples.
forM andM derivedasdescribedinthenextsections
NSC (cid:63)
We also note that our two sub-samples are dominated are tabulated in TableA13.
by galaxies in different environments. While virtually all
The sample of NSCs in late-type galaxies in this work
NSC hosts in the early-type sample are located in a clus-
comesfromtherecentlypublishedcatalogueof228NSCsin
ter environment (Virgo or Fornax), the late-type galaxies nearby((cid:46)40Mpc),moderatelyinclinedspiralgalaxieswith
are found mainly in a lower density (group) environment, t(cid:62)3 (Georgiev & Böker 2014). These selection criteria en-
exceptfor12galaxies(<10%)thataremembersofVirgoor
surethattheeffectsofanylightcontaminationfromthehost
FornaxaccordingtocataloguesofBinggelietal.(1985)and
galaxy disk and (pseudo-)bulge on the derived NSC prop-
Ferguson & Sandage (1990). We highlight these 12 objects
erties are minimized. The catalogue contains luminosities
with a solid histogram in Figure1.
calculated from the flux within the best fitting King model
Relevant only for the NSC-MBH discussion in §3.3.1
of a given concentration index. This provides the most ac-
and 3.3.2, we also use data for galaxies hosting both a
curate photometry in a nuclear environment because is less
NSCandaMBHfromNeumayer&Walcher(2012)andfor
affected by nearby contaminating sources.
SMBH host galaxies from McConnell & Ma (2013). Both
TheBelletal.(2003)mass-to-lightratio(M/L)-colour
studiescontaingalaxieswithawiderangeofmorphological
relations are available for the SDSS, 2MASS, and John-
types,andinordertoseparatetheseobjectsintoearly-and
son/Cousins magnitude systems. However, as discussed in
late-types, we adopt a galaxy morphology dividing line at
Georgiev & Böker (2014), we prefer to work in the na-
t=3.
tive WFPC2 magnitudes to avoid propagating uncertain-
ties from transformations between the various photomet-
ric systems. For each NSC, we obtain the M/L-ratio using
2.2 NSC photometry and mass
the NSCs magnitudes in Georgiev & Böker (2014) and the
Derivingaccuratephotometricmassesreliesonproperlyac- Bruzual & Charlot (2003) SSP models for solar metallicity
countingforforegroundGalacticextinctiontoagivengalaxy andaKroupa(2001)IMF.Asshownbyspectroscopicstud-
and precise knowledge of its distance. For this purpose, we ies of NSCs in late-type galaxies, the assumption of solar
retrieved the foreground Galactic extinction E(B−V) and metallicity is a reasonable one for these objects (e.g. Rossa
the (median value of the) distance modulus for all sample etal.2006;Walcheretal.2006;Sethetal.2006).Tocalcu-
galaxies from NED2. The NED extinction values are based latetheluminosityweightedphotometricmassoftheNSCs,
on the Schlafly & Finkbeiner (2011) recalibration of the M ,weusedtheavailablecolourinformationinthevari-
NSC
Schlegeletal.(1998)extinctionmap,forwhichwecalculate ouscombinationsofthemostreliablycalibratedWFPC2fil-
filter-specificvaluesassumingtheFitzpatrick(1999)redden- ters (F300W,F450W,F555W,F606W,F814W). We obtain
inglawwithR =3.1.ThevaluesforE(B−V)andm−M the SSP model M/L by matching the NSC colours to the
V
used for the computation of photometric stellar masses are model colours. If more than one colour is available, we cal-
listedinTableA1.WeemphasizethatpossibleNSCredden- culate the error weighted mean of the different M/L-values
ing due to host galaxy self-absorption is not accounted for. toobtainM ,whichhelpstominimizesystematicuncer-
NSC
A correction for an A (cid:39) 0.4mag would increase the NSC
V
3 Fullversionofthetableisavailableintheelectronicversionof
2 http://ned.ipac.caltech.edu thejournal
c 2014RAS,MNRAS000,1–22
(cid:13)
4 I. Y. Georgiev, T. Böker, N. Leigh, N. Lützgendorf, N. Neumayer
SCSCSC 111000 analysis (from stellar population fitting, Rossa et al.2006,
NNN MM // MM or line widths, Walcher et al.2006). For those objects with
MMM NNSSCC,,ssppeecc NNSSCC,,ccooll
c c c both types of measurements, Figure2 plots their ratio as a
metrimetrimetri 111 +++ 555000%%% NGC5585 NGC1N38G5C0428NGC3455NGNCG13C245775 function of NSC mass. The shaded area is the rms scatter
otootooto --- 555000%%% NGC0300 NGC1493 NGC7421 NGC4030 ofthedataaroundthebestfit(σ=0.42dex).Forreference,
PhPhPh NGC2552 dashedhorizontallinesshowthe±50%rangearoundamass
pic/pic/pic/ 000...111 NGC4701 ratioof1.Theplotshowsthatwithintheuncertainties,both
ooo
scscsc estimates are generally in good agreement. The photomet-
ooo rms= 0.42
pctrpctrpctr 000...000111 Fit: 0.80 ± 0.13 NGC2139 ric estimates appear to be higher by about 20%, but the
eee significance of this difference is only <1.5σ. Nevertheless,
SSS 111000555 111000666 111000777 111000888 111000999
a slight overestimation of the photometric mass could be
MMM [[[MMM ]]]
NNNSSSCCC ⊙⊙⊙ expectediftheassumedNSCmetallicityistoohigh.Forex-
Figure 2. Ratio between measurements from spec-
MNSC ample, the M/Ls would differ by about 20% between solar
troscopy (from Rossa et al. 2006) to those from photometric
and sub-solar metallicity in the Bruzual & Charlot (2003)
colours (this study). The error-weighted straight line fit (solid
SSPmodels.Thus,thesystematicuncertaintyinthephoto-
horizontal line) is plotted, together with the rms scatter of the
metric and spectroscopic mass estimates due to our choice
data (shaded area). The dashed horizontal lines show the 50%
rangearoundamassratioof1.Theplottederrorsareonly±from of metallicity is much smaller than that caused by the de-
our study, as there are no error bars provided by Rossa et al. generacy with stellar age, as discussed below.
(2006). The NSC photometry in early-type galaxies is col-
lectedfromtwogalaxyclustersurveysconductedwithwith
HST/ACS-theVirgoClusterSurvey(ACSVCS,Côtéetal.
tainties (cf McGaugh & Schombert 2014). For NSCs with
2004) with photometry for 56 NSCs (Côté et al. 2006) and
photometry in only one band (i.e. without colour informa-
the Fornax Cluster Survey (ACSFCS, Jordán et al. 2007)
tion), we used the sample median colours containing that
with measurements of 31 NSCs (Turner et al. 2012). The
filtertocalculatetheerrorweightedmeanoftheM/Lfrom
photometric mass of the NSCs of these samples are calcu-
the possible colour combinations containing that filter, e.g.
lated in a similar way as for the late-types by using the
foraNSCwithonlyF814W magnitude,weusedthemedian
g −z colour and z-band magnitude in the ta-
F475W F850LP
F300W−F814W, F450W−F814W and F606W−F814W
bles of Côté et al. (2006) and Turner et al. (2012). We note
colours of the NSC sample to calculate the error weighted
that our approach in deriving M (from colours at fixed
NSC
SSPmodelM/L .Wechecked,andexpectedly,thecal-
F814W solar metallicity) differs from that adopted in those studies
culated M using the sample median colours showed no
NSC (M at fixed age of 5Gyr). According to the Bruzual &
NSC
systematic difference between those NSCs with mass calcu-
Charlot(2003)SSPmodels,atafixedageof5Gyr,theM/L
lated from measured colour(s).
canvarybyafactorof2withmetallicity.Forsolarmetallic-
Although these colours are representative for the en-
ity, the M/L increases again by a factor of 2 between ages
tire sample of NSCs, we caution that there may still be a
of 5 and 14Gyr. Therefore, either method carries an equal
small bias in the NSC masses derived from different filters.
amountofuncertainty,roughlyafactoroftwo.Ourmethod
We checked for this using NSCs observed in multiple fil-
thereforeshouldyieldM valuesthatareconsistentwith
NSC
ters, but did not find any systematic differences. We also
other studies to within a factor of two.
note that NSCs with uncertainties larger than > 100% in
M (shownwithgraysymbolsinsubsequentfigures)are
NSC
excluded from the various fits4. 2.3 Properties of NSCs with massive BHs
The formal errors of the measured colours are small
ForthesubsampleofNSCswithMBHs,weusethemeasure-
due to the generally very high S/N of the NSC, and thus
ments of Neumayer & Walcher (2012) who provide upper
introduceonlynegligibleuncertaintiesintheresultingM/L-
limits for M based on velocity dispersions and dynami-
ratios.However,theymaystillbeaffectedbythepossibility BH
cal mass modelling from VLT/UVES spectra.
that a small mass fraction of the NSC (δ ≈10%) is com-
M Twelve NSCs (one late- and 11 early-type galaxies) in
posed by a younger stellar population (∆t>5Gyr) which
their sample are not in Georgiev & Böker (2014). For those
willoutshinethemoremassiveolderstellarcomponent.This
NSCs, we use the luminosities obtained by Neumayer &
will cause a bias towards bluer integrated colours, younger
WalcherfromaMulti-GaussianExpansionfittingtechnique
SSP ages, and a lower M/L values, i.e. towards lower to-
(MGE, Emsellemetal.1994;Cappellari2002).Theremain-
tal M mass (by up to a factor of 5, see also discussion
NSC ing seven NSCs in the Neumayer & Walcher (2012) sample
in §3.1 and 3.2). Our approach to derive M in late-
NSC arepresentinourHSTsample,andwethereforeuseourpho-
type hosts is similar to that in Seth et al. (2008a) who find
tometrytocalculatetheNSCsmasses,asexplainedin§2.2.
good agreement between photometric and dynamical mass
To check for systematic differences between the two stud-
estimates to within a factor of two, which is comparable to
ies, we calculate the ratio between our and the Neumayer
themassuncertaintiescalculatedhere.Nevertheless,inFig-
&WalcherNSCsizesandmasses.Wefindgoodagreement,
ure2 we illustrate how well our colour-based photometric
with a mean ratios of 0.81±0.18 for the NSC sizes, and
NSC masses compare to those obtained from spectroscopic
0.98±0.16 for the NSC masses. The small apparent differ-
enceinderivedsizescanlikelybeattributedtothedifferent
4 We found no significant differences in the best-fit parameters fitting techniques used, elliptical King profiles in Georgiev
when including these NSCs in the fits, using weights that are & Böker vs. MGE technique in Neumayer & Walcher.
inverselyproportionaltotheiruncertainty. We also include in our sample the NSC and MBH
c 2014RAS,MNRAS000,1–22
(cid:13)
NSCs in late- and early-type hosts 5
masses of the Milky Way (MW) and Andromeda (M31). We have therefore calculate M using the empirically
(cid:63),gal
Theirproximitymakesthesetwonucleithebest-studiedex- calibratedM/L-galaxycolourrelationsofBelletal.(2003).
amplesofsystemswithreliablemassmeasurementsofboth TheserelationswereobtainedbycomparinggalaxySEDsat
NSC and MBH. For the MW NSC, we use mass and size optical and NIR wavelengths, and by using composite stel-
estimates from Schödel et al. (2014), M =(2.5±0.4)× larevolutionarymodelsforarangeofmetallicitiesandstar
NSC
107M , r = 4.2±0.4pc. The mass of the MBH in formation histories.
(cid:12) eff,NSC
theMW,M =4.26×106M ,isfromChatzopoulosetal. Forthemajorityofthegalaxiesinoursamples,wecol-
BH (cid:12)
(2014)basedonstellarkinematics.Forthetotalstellarmass lectphotometry(B,B−V,I magnitudes)fromHyperLEDA,
oftheMW,weusethevaluederivedbyLicquia&Newman i.e.forthegalaxiesinGeorgiev&Böker(2014),Turneretal.
(2014), M = (6.08±1.14)×1010M , which is based on (2012), Neumayer & Walcher (2012) and McConnell & Ma
(cid:63) (cid:12)
animprovedBayesianstatisticalanalysisaccountingforun- (2013).TheonlyexceptionistheACSVCSsample,forwhich
certaintiesinliteraturemeasurements.TheMWgasmassis we use the photometry derived from a dedicated isophotal
M =1.25×1010M (about 17% of its stellar mass), analysis (Ferrarese et al. 2006b).
MWgas (cid:12)
of which atomic Hydrogen constitutes M = 8×109M , The McConnell & Ma (2013) catalogue provides host
HI (cid:12)
warm ionized medium M = 2×109M , and molecular galaxybulgestellarmass(eitherfromsphericalJeansmod-
H+ (cid:12)
gas M =2×109M (Kalberla & Kerp 2009). elling of the bulge stellar dynamics, or from M/L-modeling
H2 (cid:12)
The M31 nucleus is a rather complex system (Lauer basedongalaxycolours,thelatterapproachbeingidentical
et al. 1993). It is composed of a cluster that clearly stands to ours). However, McConnell & Ma (2013) do not provide
out above the surrounding bulge within the central 10pc stellarmassestimatesforthosegalaxiesintheirsamplethat
and isdominated by light fromold stellar populations (Ko- containasignificantdiskcomponent,i.e.S0andlatertypes.
rmendy & Bender 1999). Its inner 1.8pc core features a bi- ForaconsistentcomparisontotheNSCsample,wecalculate
modal component (Lauer et al. 1993, 1998, 2012), which the total (bulge+disk) stellar masses of their entire sample
is interpreted as a projection of the Keplerian orbits of using magnitudes and colours obtained from HyperLEDA
starsinacentraleccentricdiscaroundtheMBH(Tremaine andtheBelletal.(2003)M/L-colorrelations.Tocheckthe
1995; Peiris & Tremaine 2003). The mass of the MBH is consistency of our results, we compared our galaxy masses
M = 1.4×108M (Bender et al. 2005) and that of the tothoseofMcConnell&Ma(2013)fortheearly-typegalax-
BH (cid:12)
NSC M = 3.5±0.8×107M (Lauer et al. 1998; Ko- ies in their sample, i.e. for cases where M (cid:39) M , and
NSC (cid:12) bulge (cid:63)
rmendy & Ho 2013). We calculate the total stellar mass find a very good agreement (to within 10%). Uncertainties
of M31 to be M = 7.88±4.23×1010M , using B,V,I ofthephotometricmasseshavebeencalculatedbypropaga-
(cid:63) (cid:12)
photometry from HyperLEDA5 and using Bell et al. (2003) tion of the photometric uncertainties and the uncertainties
M/L-colourrelations(seealso§2.5).OurvaluefortheM31 associated to the coefficients of the M/L-color relation. To
mass is consistent with other stellar population based esti- avoid over-crowding in the figures, galaxies with uncertain-
mates in the literature, e.g. 10-15×1010M (Tamm et al. ties larger than 100% are shown with grey symbols.
(cid:12)
2012).
2.6 HI and X-ray gas masses
2.4 Sample of massive black holes
A significant baryonic mass component in late-type galax-
Massesof72SMBHsandtheirhostgalaxiesaretakenfrom ies is in the form of atomic HI gas. We therefore calcu-
McConnell & Ma (2013). They collect literature data from late the HI mass from the HyperLEDA 21-cm line mag-
varioussourcesofthemostuptodateMBH measurements. nitudes, m21, converted to flux (FHI = 10−0.4×(17.40−m21))
This sample is used in §3.3.2 for calculating the SMBH using the relation between the MHI and FHI, i.e. MHI =
sphere of influence radius, rinfl,BH. 2.36×105×D2×FHI,whereD isthedistanceinMpc,cal-
culated from the same distance modulus in NED used for
calculating galaxy mass from its luminosity. We note that
2.5 Photometric stellar mass of NSC host galaxies wedidnotcorrecttheHImassforHefractionormolecular
gas.
Wealsocalculatethetotalgalaxystellarmass(i.e.thesum
Early-typegalaxiesareknowntocontainahotgascom-
oftheirbulgeanddiskcomponents)forallNSChostgalaxies
ponent, detected as an X-ray halo resulting from thermal
inoursample.Thetotalgalaxymassisanimportantquan-
Bremsstrahlung emission, which is known to trace well the
tityforthediscussionofformationscenariosforbothNSCs
total gravitating mass (e.g. Forman et al. 1985; Fukazawa
and SMBHs. This is especially true for late-type galaxies
et al. 2006). Typically, the hot gas mass is no more than a
withoutprominentbulges,wherematerialforthegrowthof
few times 109M for a range of galaxy morphologies, envi-
either CMO must come predominantly from the disk. (cid:12)
ronments, and luminosities (L (cid:39) 3−15×1010L ) (e.g.
Calculating galaxy photometric mass, M , from in- K (cid:12)
(cid:63),gal
Bogdán et al. 2013b,a; Anderson et al. 2013). This is only
tegrated colours in the optical is a challenging task, mainly
6-7% of the galaxy stellar mass (e.g. O’Sullivan et al. 2003;
due to the age-metallicity degeneracy and assumptions of
Su & Irwin 2013) and is therefore not a significant compo-
galaxy star formation history used by synthetic models.
nentofthebaryonmassbudget.Nevertheless,fortheearly-
However, it has been demonstrated that the B−V colour
typemassivegalaxiesintheMcConnell&Ma(2013)SMBH
(including for disk galaxies) offers a good representation of
samplewecollectthehotgasmassmeasuredbySu&Irwin
their stellar population (e.g. McGaugh & Schombert 2014).
(2013), based on Chandra and XMM data. Unfortunately,
noX-raymeasurementsexistformostofourlate-typesam-
5 http://leda.univ-lyon1.fr(Patureletal.2003) ple, and we therefore do not list the hot gas mass fraction
c 2014RAS,MNRAS000,1–22
(cid:13)
6 I. Y. Georgiev, T. Böker, N. Leigh, N. Lützgendorf, N. Neumayer
111111000000000000
inTableA1.Giventhatthehotgasmassfractionissmaller
in late-type galaxies than in ellipticals Li et al. (2011), and
in any case accounts for only a small fraction of the total
galaxy mass, this does not significantly affect our analysis
or conclusions.
c]c]c]c]c]c] 111111000000
pppppp
[[[[[[
CCCCCC
SSSSSS
3 RESULTS AND ANALYSIS NNNNNN
Quantifying the relations between NSCs and their host rrrrrreff, eff, eff, eff, eff, eff,
galaxy, and possible dependence on galaxy morphology, 111111
bears important constraints for models of NSC formation NSCs in late-types
and evolution, as well as for possible evolutionary connec- err > 100%
tionstothevariousincarnationsofcompactstellarsystems Early-types
(e.g.massiveGCs,UCDs).Inaddition,theyprovideinsights 000000......222222
into mechanisms that may transform galaxies from late- to 111111000000444444 111111000000555555 111111000000666666 111111000000777777 111111000000888888 111111000000999999 111111000000111111000000
early-type morphologies. With our large sample of NSCs MMMMMM [[[[[[MMMMMM ]]]]]]
in late-type hosts, we significantly increase the number of NNNNNNSSSSSSCCCCCC
110000 ⊙⊙⊙⊙⊙⊙
wellstudiednucleiindisk-dominatedgalaxies.Thisenables
a more statistically meaningful comparison of r and
eff,NSC
M between early- and late-type galaxies, and extends
NSC
the range of host galaxy masses to less massive systems.
c]c] 1100
pp
3.1 Relations between NSC size and NSC and [[
CC
host-galaxy masses SS
NN
1.34
TherelationsbetweentheNSCeffectiveradiusanditsmass eff, eff, 3.21
(r -M )aswellasthestellarmassofitshostgalaxy rr
eff,NSC NSC
(r -M )areshowninFigures3and4,respectively. 11
eff,NSC (cid:63),gal
Inbothfigures,late-andearly-typehostgalaxiesareshown
with different symbol and line types, as indicated in the EEaarrllyy--ttyyppeess
figure legend. We do not plot unresolved NSCs, i.e. those LLaattee--ttyyppeess
00..22
with only an upper limit to r (see Fig.4 in Georgiev
eff,NSC 44 55 66 77 88 99 1100
& Böker 2014). In both figures, the bottom panel shows 1100 1100 1100 1100 1100 1100 1100
the two-dimensional probability density distribution func- MM [[MM ]]
NNSSCC
tion (2D-PDF). The uncertainty weighted 2D-PDFs are es- Figure3.Top:Nuclearstarclustersiz⊙⊙e-massrelation,reff,NSC
timatedwithinarunningboxofsize0.3dexwithinR6.The vs. - for late- and early-type host galaxies. Small grey
MNSC
2D-PDFsprovideafirstorderquantificationoftheobserved (light) symbols are for NSCs with uncertainties >100%. Bot-
r -M distribution.Thethickdashedandsolidcon- tom: a contour plot of the two-dimensional probability density
eff,NSC NSC
tourlinesinFigures3and4indicatethe1σdispersionofthe distribution(2DPDF)forthetwosubsamples.Thedifferentsym-
bols and line types are for the different samples, as indicated in
data.
thelegend.Thickcontourlinesmarkthe1σofthe2DPDFs.The
To more robustly quantify any differences in the
fit to the data is shown with lines, where the narrower darker
r -M and r –M relations between the
eff,NSC NSC eff,NSC (cid:63),gal (colour)shadedregionindicatestheuncertaintiesrangeofthefit
two subsamples, we perform a maximum likelihood, linear
slopeandintercept.Thewiderandlighter(colour)shadedregion
(in log-log space) regression analysis by bootstrapping the isthe1σ dispersionofthedata.
data to account for the finite data sample and construct
the posterior PDFs. Our fitting also accounts for the non-
symmetricmeasurementuncertainties,whicharetreatedas
acombinationoftwoGaussians,i.e.asplitnormaldistribu- Thevaluesofthenormalizationconstantsarethehigh-
tion. The fitted linear regression is of the form: estprobabilitydensityvalueofthecenterpeaksinthecon-
tour plot in Fig.3. We find that these normalization con-
log10(reff,NSC/c1)=α×log10(MNSC/c2)+β, (1) stantsareimportantforminimizingthecorrelationbetween
the slope and intercept, which provides a more realistic un-
where the normalization constants (c1, c2) and the best fit
certaintyestimateofthefits.Theposteriorprobabilityden-
values for the slope (α) and intercept (β) for the different
sitydistributionsoftheslopeandinterceptforeachfittedre-
subsamples are tabulated in Table1. Description of the fit-
lationareshowninFigureA1in§A.Wefindthatlogr
ting technique and results for each relation are provided in eff,NSC
scales with logM with a slope of α = 0.321+0.047 for
the Appendix §A. NSC −0.038
late-typesand0.347+0.024 forearly-types.Thelogr is
−0.024 eff,NSC
also observed to scale with host galaxy stellar mass with a
6 Risafreesoftwareenvironmentforstatisticalcomputing.The slope of α = 0.356+−00..005567 for late-types and 0.326+−00..005551 for
R-project is an official part of the Free Software Foundation’s early-types.
GNUproject(http://www.r-project.org/). The comparison between the 2D-PDFs of NSCs in the
c 2014RAS,MNRAS000,1–22
(cid:13)
NSCs in late- and early-type hosts 7
111111000000000000
Table 1. Parameters of the fitted relations for late- and
early-type NSC host galaxies.
Host c1 c2 α β σ
c]c]c]c]c]c] 111111000000 type
pppppp (1) (2) (3) (4) (5) (6)
[[[[[[

CCCCCC
SSSSSS
NNNNNN
f, f, f, f, f, f,
rrrrrrefefefefefef log(reff,NSC/c1)=α×log(MNSC/c2)+β
111111 Late 3.31 3.60e6 0.321+0.047 0.011+0.014 0.133
−0.038 − −0.031
NSCs in late-types Early 6.27 1.95e6 0.347+−00..002244 −0.024+−00..002221 0.131
err > 100%
Early-types
000000......222222 777777 888888 999999 111111000000 111111111111 111111222222 log(reff,NSC/c1)=α×log(M(cid:63),gal/c2)+β
111111000000 111111000000 111111000000 111111000000 111111000000 111111000000 Late 3.44 5.61e9 0.356+0.056 0.012+0.026 0.139
−0.057 − −0.024
MMMMMM★★★★★★ hhhhhhoooooosssssstttttt ggggggaaaaaallllllaaaaaaxxxxxxyyyyyy [[[[[[MMMMMM ]]]]]] Early 6.11 2.09e9 0.326+−00..005551 −0.011+−00..001450 0.143
110000 ⊙⊙⊙⊙⊙⊙
log(MNSC/c1)=α×log(M(cid:63),gal/c2)+β
Late 2.78e6 3.94e9 1.001+0.054 0.016+0.023 0.127
−0.067 −0.061
Early 2.24e6 1.75e9 1.363+0.129 0.010+0.047 0.157
−0.071 −0.060
]]
cc 1100
pp
[ [CC log(MNSC/c1)=α×log(M(cid:63)+HI,gal/c2)+β
SS Late+HI 2.87e6 1.17e10 1.867+0.158 0.041+0.041 0.126
NN −0.133 −0.042
f, f, 1.21
ff
ee
rr 2.01
11 log(MNSC+MBH/c1)=α×log(M(cid:63),gal/c2)+β
MBH+NSC 5.03e7 2.76e10 1.491+0.149 0.019+0.111 0.233
−0.097 − −0.054
EEaarrllyy--ttyyppeess
LLaattee--ttyyppeess
00..22 Note. — The fitted scaling relations are of the form log (y/c1) = α
110077 110088 110099 11001100 11001111 11001122 log10(x/c2)+β, where in column (1) is the NSC host morp1h0ological type∗,
columns (2) and (3) are the normalization constants obtained from the 2D
MM hhoosstt ggaallaaxxyy [[MM ]]
★★ PDFs (see §3.1), in columns (4) and (5) are the slope and intercept and in
Figure4.Top:Nuclearstarclustersizeversu⊙⊙stotalstellarmass (6)isthefitrmsdispersionofthedata,σ.
ofthehostgalaxyforNSCsinlate-andearly-typegalaxies.Bot-
tom: A contour plot of the two-dimensional probability density
distribution of the two subsamples. The symbol, line types and
3.2 NSC mass – host galaxy stellar mass relation
shadedareasarethesameasinFig.3.
In Figure5, we explore the relation between the NSC mass
and host galaxy stellar mass, again separately for late-
late- and early-type subsamples suggests that the r – (Fig.5a) and early-type galaxies (Fig.5b). We fit the two
eff,NSC
M distributionsareconsistentwitheachothertowithin subsamples with the same technique as described in §3.1.
NSC
1σofthedispersionofthedata(cf.thesolid1σcontourlines The best-fit relations are shown with solid lines, while the
in Fig.3, bottom). Within the uncertainties, the relations shaded regions represent the uncertainties of the fit coeffi-
for late- and early-type hosts also have very similar slopes, cients(thenarrower,darkerregion)andthe1σdispersionof
however, the zeropoint of the fitted relations differ beyond the data (the broader, lighter region). The direct compari-
their1σdispersion(cf.thebroadershadedregionaroundthe sonbetweentherelationsforlate-andearly-typeNSChost
highest density peaks in Fig.3). This suggests that at fixed galaxies in Figure5c shows that within 1σ, their 2D-PDFs
cluster mass, NSCs in late-type hosts are smaller by about (thick contour lines) are indistinguishable from each other.
a factor of 2 than their counterparts in early-type hosts. On the other hand, the comparison also shows that the fit-
Asimilardifferenceinr betweenlate-andearly- ted slopes (solid and dashed lines in Fig.5c) are different
eff,NSC
typegalaxiesisalsoobservedasafunctionofgalaxystellar between the two sub-samples beyond the 1σ level (i.e. the
mass, (r -M ), shown in Figure4. The r in- darkershadedregioninFig.5cdonotoverlap).Thisimplies
eff,NSC (cid:63),gal eff,NSC
creaseswithM withanidenticalslopeforbothsamples, that at higher galaxy mass, early-types have more massive
(cid:63),gal
however, at fixed M , NSCs in late-type hosts are more NSCs than late-types. A similar difference as a function of
(cid:63),gal
compact by about a factor of 2. In this case, however, the galaxy morphology has been also reported earlier (Rossa
statistical significance that both distributions differ is less et al. 2006; Seth et al. 2008a; Erwin & Gadotti 2012).
than 1σ, for the offset, slope and the 2D density distribu- Thevaluesofthebest-fitcoefficientsaresummarizedin
tions of the data (cf fits’ shaded regions and solid density Table1.OurresultthatthemassoftheNSCscaleswithhost
contours in Fig.4). These differences are discussed in §4.2. galaxy stellar mass with a slope near unity for late-types,
c 2014RAS,MNRAS000,1–22
(cid:13)
8 I. Y. Georgiev, T. Böker, N. Leigh, N. Lützgendorf, N. Neumayer
NSCs: late-types NSCs: early-types
111111000000111111000000 err > 100% 11111000001111100000 111000111000
aaaaaa)))))) bbbbb))))) ccc)))
111111000000999999 111110000099999 111000999
]]]]]]⊙⊙⊙⊙⊙⊙111111000000888888 ]]]]]⊙⊙⊙⊙⊙111110000088888 ]]]⊙⊙⊙111000888
MMMMMM MMMMM MMM
[ [ [ [ [ [CCCCCC111111000000777777 [ [ [ [ [CCCCC111110000077777 [ [ [CCC111000777
SSSSSS SSSSS SSS
NNNNNN NNNNN NNN
MMMMMM MMMMM MMM
111111000000666666 111110000066666 1.08 111000666 0.93 2.79
111111000000555555 111110000055555 111000555
EE
111111000000444444 111110000044444 111000444 SS
111111000000777777 111111000000888888 111111000000999999 111111000000111111000000 111111000000111111111111 111111000000111111222222 111111000000111111333333 10 30 50 111110000077777 111110000088888 111110000099999 11111000001111100000 11111000001111111111 11111000001111122222 11111000001111133333 4 14 24 111000777 111000888 111000999 111000111000 111000111111 111000111222 111000111333
MMMMMM★★★★★★ hhhhhhoooooosssssstttttt ggggggaaaaaallllllaaaaaaxxxxxxyyyyyy [[[[[[MMMMMM ]]]]]] N MMMMM★★★★★ hhhhhooooosssssttttt gggggaaaaalllllaaaaaxxxxxyyyyy [[[[[MMMMM ]]]]] N MMM★★★ hhhooosssttt gggaaalllaaaxxxyyy [[[MMM ]]]
⊙⊙⊙⊙⊙⊙ ⊙⊙⊙⊙⊙ ⊙⊙⊙
Figure5.Relationbetweennuclearstarclustermass, ,andhostgalaxystellarmass, .Panelsa)andb)showseparately
MNSC M(cid:63),gal
therelationsforlate-andearly-typegalaxies.Thehistogramsonthey2-axesshowtheNSCsmassdistributionsforthedifferentsamples.
In panel c) we compare the fitted relations from panels a) and b) and their 2D PDF distribution (E for early- and S for late-types).
Thickcontourlinesindicatethe1σ ofthedataPDF.Symbols,linetypesandshadedareasarethesameasinFig.3.
α = 1.001+0.054, is in good agreement with the literature, with the values (0.1-0.2%) reported by earlier studies (e.g.
−0.067
e.g. Erwin & Gadotti (2012), who derive a slope of α = Rossaetal.2006;Sethetal.2008a;Graham&Spitler2009;
0.90±0.21betweenM andtotal(bulgeplusdisk)stellar Erwin&Gadotti2012;Kormendy&Ho2013).Wenotethat
NSC
galaxymassinasmallersampleofmassivelate-typespirals. Erwin & Gadotti (2012) reported M /M ∼0.2% for
NSC (cid:63)gal
We note at this point that the slope of the relation HubbletypesearlierthanSbc(consistentwithourresults),
for the late-types is similar to the slope defined by the but∼0.03%forlaterHubbletypes,whichislowerthanthe
MBH mass and host spheroid mass, M –M , which peak of the distribution in this study. However, partly due
BH (cid:63),sph
is α = 1.05 (McConnell & Ma 2013). However, the MBH– totheslopeoftheMNSC-M(cid:63)gal relationtheMNSC/M(cid:63)gal
M hasazeropointof8.46,whichis0.6dexhighercom- distribution shows a large dispersion in Fig.6c with a 1σ
(cid:63),sph
pared to the 7.86±0.1 for our late-types relation. In §4.2 range between 6×10−6 (0.006%) and 3×10−3 (0.1%).
wediscussfirstwhetherthedifferencesbetweentherelations
for late- and early-types are due to measurement biases or
3.3 Relations for coexisting NSCs and MBHs
evolutionary differences, and then discuss the implications
for the M –M relation. The identification and study of systems in which NSC and
CMO (cid:63),gal
ItisperhapsequallyinterestingtoseehowtheM – MBH coexist is important in order to make progress on
NSC
M relation changes when including the HI mass to the a number of open questions. For example, it is not clear
(cid:63),gal
total host galaxy mass. Naturally, the effect on the M – whetherthiscoexistenceispossibleonlyin thenuclei ofin-
NSC
M(cid:63),gal relationwillbelargerforgas-richlate-typegalaxies. termediatemassgalaxies(few×1010M(cid:12)),orwhatthephys-
Togaugethemagnitudeofthiseffect,weshowinFigure6a ical reason is for the dominance of one or the other at low
the NSC mass distribution for both early- and late-types and high galaxy mass. Understanding whether there is a
against host galaxy stellar mass, M , and in Figure6b common scaling relation for NSC and MBHs with host M
(cid:63),gal (cid:63)
against the total galaxy mass, M . To guide the eye, promises to shed light on the processes that govern their
(cid:63)+HI
we overplot again the best-fit relations from Fig.5 in Fig- growth, i.e. the processes funnelling matter (gas, stars, star
ure6a. It is evident from Figure6b that when the HI mass clusters)towardsthedeepestpointofthehostgalaxypoten-
isincluded,therelationforlate-typessteepenssignificantly. tial (e.g. Li et al. 2007; Mayer et al. 2010; Hartmann et al.
Thisismostlybecausethelowmass,late-type,galaxieshave 2011;Antoninietal.2012,2015),andthefeedbackprocesses
the highest HI mass fraction, and thus move noticeably to affecting the growth by either NSC or MBH.
the right (i.e. toward higher total mass) in Figure6b, caus- Observationally,however,itisextremelychallengingto
ing the relation to steepens. Due to the purely illustrative populate the respective scaling relations, mostly because in
purposes of this comparison, we did not attempt to include the absence of accretion activity, the dynamical effects of a
He or molecular mass corrections. Those will only further low-massMBHonthesurroundingNSCarebelowthedetec-
strengthen the differences. As mentioned in §2.6, ignoring tionthresholdofcurrentinstruments.Onthehigh-massend,
thesmallfraction(<5%)ofthetotalgalaxymasscontained one could ask why NSC are not observed around SMBHs
inX-rayemittinghotgasmassshouldnotsignificantlyaffect with MBH> 108M(cid:12)? To address these questions, we first
our results. look at the combined mass of the NSC and MBH in coex-
In Figure6c we plot M /M , i.e. the fraction of isting systems (Neumayer & Walcher 2012), with an eye on
NSC (cid:63)gal
galaxy mass contained in the NSC. Overall, both for late- theimpactofaMBHthatismassiveenoughtoaffectmore
and early-type hosts, the mass of the NSC is about 0.1% of than 50% of the NSC.
the galaxy stellar mass (M /M (cid:39) 10−3, cf the his-
NSC (cid:63)gal
tograminFig.6cwithadispersionofaboutafactorofthree.
3.3.1 M - host galaxy stellar mass relation
The slope of the M -M relation adds to the broaden- NSC+BH
NSC gal
ing of the histogram projections. The observed NSC mass In Figure7, we show the combined mass of the CMO (i.e.
fractionsinthislate-typehostgalaxysampleareconsistent M + M ) against host galaxy stellar mass for late-
NSC BH
c 2014RAS,MNRAS000,1–22
(cid:13)
NSCs in late- and early-type hosts 9
1111111 00
NSCs: late-types NSCs: late-types Late-types
11111111000000001111111100000000 early-types 11111111000000001111111100000000 early-types ccccccc))))))) Early-types
aaaaaaaa)))))))) bbbbbbbb)))))))) 11111110000000-------1111111 1100--11
111111110000000099999999 111111110000000099999999
11111110000000-------2222222 1100--22
M]M]M]M]M]M]M]M]⊙⊙⊙⊙⊙⊙⊙⊙111111110000000088888888 M]M]M]M]M]M]M]M]⊙⊙⊙⊙⊙⊙⊙⊙111111110000000088888888 MMMMMMM★★★★★★★
[ [ [ [ [ [ [ [CCCCCCCC111111110000000077777777 [ [ [ [ [ [ [ [CCCCCCCC111111110000000077777777 / / / / / / / CCCCCCC11111110000000-------3333333 1100--33
SSSSSSSS SSSSSSSS SSSSSSS
NNNNNNNN NNNNNNNN NNNNNNN
MMMMMMMM 111111110000000066666666 MMMMMMMM 111111110000000066666666 MMMMMMM 11111110000000-------4444444 1100--44
111111110000000055555555 Late-types 111111110000000055555555 11111110000000-------5555555 1100--55
Early-types
111111110000000044444444 111111110000000044444444 11111110000000-------6666666 1100--66
111111110000000077777777 111111110000000088888888 111111110000000099999999 11111111000000001111111100000000 11111111000000001111111111111111 11111111000000001111111122222222 11111111000000001111111133333333 111111110000000077777777 111111110000000088888888 111111110000000099999999 11111111000000001111111100000000 11111111000000001111111111111111 11111111000000001111111122222222 11111111000000001111111133333333 111111100000007777777 111111100000008888888 111111100000009999999 1111111000000011111110000000 1111111000000011111111111111 1111111000000011111112222222 1111111000000011111113333333 2200 4400 6600 8800
MMMMMMMM★★★★★★★★ hhhhhhhhoooooooosssssssstttttttt ggggggggaaaaaaaallllllllaaaaaaaaxxxxxxxxyyyyyyyy [[[[[[[[MMMMMMMM ]]]]]]]] MMMMMMMM★★★★★★★★ ++++++++ HHHHHHHHIIIIIIII hhhhhhhhoooooooosssssssstttttttt ggggggggaaaaaaaallllllllaaaaaaaaxxxxxxxxyyyyyyyy [[[[[[[[MMMMMMMM ]]]]]]]] MMMMMMM★★★★★★★ hhhhhhhooooooosssssssttttttt gggggggaaaaaaalllllllaaaaaaaxxxxxxxyyyyyyy [[[[[[[MMMMMMM ]]]]]]] NN
⊙⊙⊙⊙⊙⊙⊙⊙ ⊙⊙⊙⊙⊙⊙⊙⊙ ⊙⊙⊙⊙⊙⊙⊙
Figure 6. Panel a) – relation and in panel b) with added HI mass, . NSCs in late- and early-type galaxies
MNSC M(cid:63),gal M(cid:63)+HI,gal
are shown with different symbol types and colours as indicated in the figure legend. For reference, with lines in panel a) we show the
samefitsasinFig.5,whereasinpanelb)arethefitsincludingtheHImass.ThefitvaluesforthedifferentsamplesaregiveninTable1.
Panel c)showsthemassofNSCcomparedtohostgalaxystellarmass.Symbols,linetypesandshadedareasarethesameasinFig.3.
and early-type galaxies, plotted with light and dark sym- 111100005555
bols,respectively.ThevaluesforMNSC arecalculatedfrom 1111111000000011111110000000 aaaaaaa))))))) bbbb)))) 111100004444
luminosities in Georgiev & Böker (2014) and Neumayer &
Walcher (2012) as described in §2.2 and 2.3. 111111100000009999999 111100003333
The maximum likelihood, bootstrapped, non- ]]]]]]]⊙⊙⊙⊙⊙⊙⊙ 111100002222
symmetric error weighted fit is shown with a solid MMMMMMM 111111100000008888888 M31 SCSCSCSC 11110000
line in Figure7a. As before, the shaded regions indicate [ [ [ [ [ [ [CCCCCCC MW MMMMNNNN M31
tTohvheeerupnfilocttervttahaleiunetMsyaorfet-hMleistfietdanirndelatThtaieobn1leσf1rd.oimFspoerMrscciooCmnonponafertilhsloe&nd,aMtwaae. MMMMMMMBH + NSBH + NSBH + NSBH + NSBH + NSBH + NSBH + NS111111111111110000000000000067676767676767 MMMM / / / / BHBHBHBH1111111100000000--------111121212121 MW
BH bulge
(2013) with a dashed line, and with a dash-dotted line the 111111100000005555555 11110000----3333
MNSC-M(cid:63)gal relation for late-type galaxies obtained in Early-types 11110000----4444
§3.2(cfFig.5).WefindthatthesumoftheNSCandMBH 111111100000004444444 Late-types 11110000----5555
massesalsodefinesarelationwithhostgalaxystellarmass, 111111100000007777777 111111100000008888888 111111100000009999999 1111111000000011111110000000 1111111000000011111111111111 1111111000000011111112222222 1111111000000011111113333333 111100009999 1111000011110000 1111000011111111 1111000011112222
withaslopeofα=1.491+−00..104997.Thisslopeissimilartothat MMMMMMM★★★★★★★ hhhhhhhooooooosssssssttttttt gggggggaaaaaaalllllllaaaaaaaxxxxxxxyyyyyyy [[[[[[[MMMMMMM ]]]]]]] MMMM★★★★ hhhhoooosssstttt ggggaaaallllaaaaxxxxyyyy [[[[MMMM ]]]]
of the early-type MNSC–M(cid:63)gal relation (α = 1.363+−00..102791), Figure 7. Sum of the mass⊙⊙⊙⊙⊙⊙⊙of NSC with MBH (panel a) and⊙⊙⊙⊙
but significantly steeper than the one for late-type hosts.
theirmassratio(panelb)againsthostgalaxystellarmass(data
We note that the M −M is steeper than the
NSC+MBH (cid:63),gal from Neumayer & Walcher (2012)). The different symbol types
M -M relation (e.g. α = 1.05 ± 0.11,1.12 ± 0.06,
BH bulge indicatelate-andearly-typegalaxies,asindicatedinthelegend.
respectivelyMcConnell&Ma2013;Häring&Rix2004).In Labeled also are the Milky Way and M31. The fit through the
their sample of massive late-type spirals, Erwin & Gadotti data in panela) is shown with solid line and the shaded region
(2012) find a slope of α=1.27±0.26 for the MBH-M(cid:63)bulge indicatestheuncertaintyofthefitvaluesandthermsofthedata.
relation,butunfortunately,theydonotprovideafitagainst Forreference,dashed line istheMcConnell&Ma(2013) -
MBH
total galaxy mass. Mbulge relation and dash-dotted line is the MNSC-M(cid:63) relation
forlate-typegalaxiesobtainedin§3.2(discussionin§3.3and4.3).
The fact that the M -M and M -M rela-
NSC (cid:63)gal BH bulge
tions have similar zeropoint and slope (cf. dash-dotted and
dashedlinesinFig.7a)isperhapsnotsurprising,giventhat
usehere,plotdirectlyM vs.M .Fromthelackofcor-
the bulge mass of early-type galaxies is a good approxima- NSC BH
relationbetweenthetwo,theyconcludethatNSCsandBHs
tion for the total galaxy stellar mass. In §4.3, we further
donotcorrelateasstronglywitheachotherastheydowith
discuss these relations in the context of the coexistence of
their host galaxy.
NSC and MBH and the transition from one to the other.
In Figure7b, we plot the mass ratio between the NSC
and MBH, M /M , against host galaxy stellar mass.
BH NSC 3.3.2 MBH to NSC size ratio
The plot shows that at total stellar host masses around
5 × 1010M , the BH mass begins to dominate over the There has been a large body of analytical and numeri-
(cid:12)
NSC mass, while for lower galaxy masses, the NSC out- cal work to understand the effect on the formation and
weighs the MBH. Mass ratios of (cid:28) 1 in late-type galax- evolution of a NSC due to the presence of a MBH (e.g.
ies were first pointed by Seth et al. (2008a). Subsequently, Tremaine 1995; Milosavljević & Merritt 2001; Peiris &
Graham & Spitler (2009) also included data for coexist- Tremaine 2003; Merritt 2006, 2009; Baumgardt et al. 2005,
ing NSCs and MBHs in early-types, but they considered 2006; Matsubayashi et al. 2007; Bekki & Graham 2010;
thefractionalmassratioM /(M +M )againsthost Brockamp et al. 2011; Antonini 2013; Lupi et al. 2014;
BH NSC BH
spheroidmass.Neumayer&Walcher(2012),whosedatawe Mastrobuono-Battistietal.2014;Antoninietal.2012,2015,
c 2014RAS,MNRAS000,1–22
(cid:13)
10 I. Y. Georgiev, T. Böker, N. Leigh, N. Lützgendorf, N. Neumayer
and many others). In this section, we attempt to explore plottedagainsthostgalaxymass7.Thevastmajorityofthe
this topic from the observational perspective by using the systemshaveasizeratiothatfallssignificantlyaboveunity,
MBH sphere of influence (r ) and the effective (half- whichisinlinewiththeoreticalexpectationsfortheabsence
infl,BH
mass) radius of the NSC (r ). These are observables of a NSC.
eff,NSC
that can be linked to theoretical expectations and provide On the other hand, one could also assume that a pu-
observational information/expectation as to whether a sig- tative NSC in these galaxies has a mass corresponding to
nificant fraction of the NSC stars/mass is influenced by the extrapolation of the r - M relation shown in
eff,NSC (cid:63),gal
the MBH. For an isotropic, virialized stellar cluster, the Figure4. In this case, it implies that the NSC initially out-
size ratio between NSC and MBH is effectively equivalent grewtheMBHbyalargeamount,thetheoreticalsizeratios
to their mass ratio, because in such an idealized system, are significantly smaller (see Figure8), but still fall above
r =GM /σ2 and r =GM /σ2, and hence unity, again favouring the strong impact by the SMBH on
infl,BH BH eff,NSC NSC
r /r ≡M /M . When r /r = 1, the NSC structure and its stellar velocity field. We further
infl,BH eff,NSC BH NSC infl,BH eff,NSC
allstarswithinr arestronglyboundtotheMBHand discuss these observations and their implications in §4.3.
eff,NSC
have mostly Keplerian orbits. Thus, beyond the r ,
eff,NSC
the cluster will have profile represented by a King model.
Therefore,atthislimit,theinner50%oftheNSCpotential
4 DISCUSSION
(i.e. the stellar orbits) are dominated by the MBH, and the
outer50%aredominatedbytheNSCpotential/massdistri- We have found noticeable differences in the fitted relations
bution. It follows that, for r /r >> 1, the “clas- between M / r and host galaxy mass for differ-
infl,BH eff,NSC NSC eff,NSC
sic” NSC SB profile should no longer exist, and the NSC ent morphological types, as shown in Figures3 and 4. We
potential should be entirely dominated and shaped by the now investigate whether these differences could provide in-
MBH. In other words, it is reasonable to expect that when sight into the evolutionary path of NSCs in different hosts.
r /r >> 1, the NSC integrity may be compro- In other words, can the properties of NSCs (size, mass) be
infl,BH eff,NSC
mised, to the point that the very definition of an NSC may tracedtothevariousgrowthmechanism(s)thatresultfrom
change, both theoretically and observationally. theinternalsecularevolutionofthehost,suchasgasaccre-
tionand/ormergingstarclusters?Forexample,inlate-type
Inwhatfollows,wederivethesetwocharacteristicsizes
galaxies that are gas rich and harbor young stars and star
forNSCsandMBHs,firstfornucleiinwhichbothareknown
clusters, NSC growth is more likely to be ongoing, while in
tocoexist.WecalculatetheradiusoftheBHsphereofinflu-
early-type hosts, the only feasible mechanism today is the
enceasr =GM /σ2,whereM isfromMcConnell
infl,BH BH BH infall of old stellar populations.
& Ma (2013) and Neumayer & Walcher (2012) and σ is the
central velocity dispersion taken from HyperLEDA for the
McConnell & Ma sample, or as measured by Neumayer & 4.1 Possible measurement biases in r and
eff,NSC
Walcher for their NSC-MBH sample. We note that the σ M
NSC
values from HyperLEDA may be biased toward higher val-
Wefirstdiscusspossiblebiasesintheestimatesforthesizes
ues due to the often limited spatial resolution in measuring
and photometric masses of NSCs. In late-type galaxies, the
σ. Our calculated r values for the McConnell & Ma
infl,BH
derived values for M can be affected if the light (and
sample may be affected by this bias. NSC
hence the color) of the NSC is significantly influenced by
In Figure8a we plot the ratio r /r against a young stellar population (cf. §2.5). This effect can cause
infl,BH eff,NSC
host galaxy stellar mass. For reference, the data points for the M/L ratio (and thus M ) to be underestimated by
NSC
the Milky Way (MW) and M31 are labelled, and the unity up to a factor of 5, for example if 10% of the stellar mass
ratioisindicatedwithadashedhorizontalline.Asexpected, is ≈5Gyr younger than the rest (cf. §2.2). However, this
the MW has a size ratio below one, while M31 falls above is opposite to what is observed in Figure3, namely that at
the unity line. This is in line with the observed complex fixed r NSCs in late-type galaxies are more massive
eff,NSC
morphologyoftheM31nucleus,whiletheMWNSCstruc- thanthoseinearly-typehosts.Weconcludethattheactual
ture and SB-profile are undisturbed by the presence of the offset between the two populations in Figure3 may well be
MBHinitscenter.Inotherwords,thelargesizeratiorelates more pronounced.
to the larger fraction of the M31 NSC stars (mass) within Another possible bias comes from underestimating
ther thatareaffectedbytheMBH(asalsodiscussed r in late-type hosts if the NSC contains a significant
infl,BH eff,NSC
in e.g. Peiris & Tremaine 2003). We note that the majority fractionofyoungstarsthataremorecentrallyconcentrated.
of the galaxies in the Neumayer & Walcher (2012) sample We detected such an effect in Georgiev & Böker (2014) by
have a size ratio below one. It may be interesting to check measuringtheratioofNSCsizesinblueandredpassbands
whether those galaxies with size ratios similar or greater to (see also Kormendy & McClure 1993, Matthews et al. 1999
M31 have similarly complex central morphologies. and Carson et al. 2015). However, as shown in Figure10 of
Georgiev&Böker(2014),thisbiasis<5%forourNSCsam-
Another question to ask from this observational per- ple,andisthusanegligibleeffectwheninterpretingFigure3.
spective that can be related to theoretical expectations is Wealsodonotexpectasignificantmeasurementbiascaused
to what extent galaxies with a SMBH could also harbor a byanycontaminationofNSClightfromtheunderlyingdisk
“classical“ NSC with a radius in the range 2-5pc? To ad-
dress this question for the McConnell & Ma (2013) sample
of“pure” MBHs,weshowinFigure8btheratiobetweenthe 7 Weassumeavalueof3pcbecauseitisthemostrepresentative
derived rinfl,BH and a “nominal” NSC size of reff,NSC=3pc, (typical)valueforreff,NSC(cf.Fig.11inGeorgiev&Böker2014).
c 2014RAS,MNRAS000,1–22
(cid:13)