Many, if not all, galaxies host massive compact objects at their centers.
They are present as singularities (super massive black holes) or high density
star clusters (nuclear tar clusters). In some cases they coexist, and interact
more or less strongly. In this short paper I will talk of the 'merger' globular
cluster scenario, which has been shown in the past to be an explanation of the
substantial mass accumulation in galactic centers. In particular, I will
present the many astrophysical implications of such scenario pointing the
attention on the mutual feedback of orbitally decaying globular clusters with
massive and super massive black holes.Comment: 4 pages, 1 fiugre. Presented at the MODEST 16/Cosmic Lab conference
  in Bologna, Italy, April 18-22 2016. To be pusblshed in Mem. S.A.It.
  Conference Serie

Capuzzo-Dolcetta, R.

English

arXiv

Many, if not all, galaxies host massive compact objects at their centers. They are present as singularities (super massive black holes) or high density star clusters (nuclear tar clusters). In some cases they coexist, and interact more or less strongly. In this short paper I will talk of the merger globular cluster scenario, which has been shown in the past to be an explanation of the substantial mass accumulation in galactic centers. In particular, I will present the many astrophysical implications of such scenario pointing the attention on the mutual feedback of orbitally decaying globular clusters with massive and super massive black holes

Capuzzo Dolcetta R.

Archivio della ricerca- Università di Roma La Sapienza

Mem. S.A.It. Vol. 87, 634c© SAIt 2016 Memorie dellaGlobular cluster-massive black hole interactionsin galactic centersR. Capuzzo-DolcettaDipartimento di Fisica, Università di Roma ”La Sapienza”, piazzale A. Moro 5, I-00185 Roma,Italy, e-mail: roberto.capuzzodolcetta@uniroma1.itAbstract. Many, if not all, galaxies host massive compact objects at their centers. They arepresent as singularities (super massive black holes) or high density star clusters (nuclear tarclusters). In some cases they coexist, and interact more or less strongly. In this short paper Iwill talk of the merger globular cluster scenario, which has been shown in the past to be an ex-planation of the substantial mass accumulation in galactic centers. In particular, I will presentthe many astrophysical implications of such scenario pointing the attention on the mutual feed-back of orbitally decaying globular clusters with massive and super massive black holes.Key words. galaxies: globular clusters – galaxies: nuclei – galaxies: massive black holes1. IntroductionGalaxies of all the Hubble types have pecu-liarities in their inner region. It is clear nowa-days that brighter galaxies host massive andeven super massive black holes (SMBHs) os-cillating around their gravity centers whilefainter galaxies often host very dense stellaraggregates, the so called nuclear star clusters(NSCs). The coexistence of an SMBH and asurrounding NSC is not uncommon, anyway.In a recent work Georgiev et al. (2016) showedthat for stellar host masses above ∼ 5 × 1010M the BH mass begins to dominate over theNSC mass, while for lower galaxy masses, theNSC outweighs the MBH.A collection of recent data indicates thatNSCs are present in 75% of late-type spirals(Scd-Sm), 50% of earlier type sp. (Sa-Sc), and70% of spheroidal (E and S0) galaxies. A rel-evant issue is that NSCs contain both a old(≥ 1 Gyr) and a young (≤ 100 Myr) stellarpopulation. This gives an important constraintto NSC formation hypotheses. The presencein different galaxies of compact massive ob-jects (CMOs), although different in structureand type, suggests the existence of some cor-relation between the galactic environment andthe CMO, which requires a convincing theoret-ical interpretation.So far, two main frameworks have beensuggested. One, called in situ model, is stillmore a hypothesis than a detailed model. Itclaims a local, central, star formation givingrise to a dense star cluster. Neither the originof the gas (funnelled toward the centre by someangular momentum loss?) nor the modes of thenecessarily efficient central star formation inpresence of a massive black hole therein withits strong tidal disturbance effect, have been ex-plained in a convincing way, so far. The otherproposed explanation for galactic nucleus for-mation is via orbital decay and subsequentmerger in the galactic center of massive glob-ular clusters This scheme was first suggestedCapuzzo-Dolcetta: GCs and MBHs 635by Tremaine et al. (1975) and later developedand made more straightforward by Capuzzo-Dolcetta (1993) and a series of following pa-pers of his research group.It is, indeed, relevant to acknowledge thatthis infall and merging scheme, upon whichmany people have been working so far, has hadthe Tremaine et al. (1975) and the Capuzzo-Dolcetta (1993) as seminal papers. Actually,these two papers were able to quantify the pos-sibility of carrying many stars in a compactstructure around a galactic center forming a re-solved stellar nucleus therein even before thatwhat have been later called nuclear star clus-ters were discovered.2. Compact massive objectsAs said above, galaxies use to host massive ob-jects in their central region. The compactnessof these CMOs is an increasing function of theparent galaxy luminosity (mass). In Table 1 wegive values of characteristic physical scales forsome CMOs.The values reported in the table essentiallysay that in order to make the infall and mergerhypothesis viable, a shrink of the GC distribu-tion length scale (i.e. a shrink of the spatial dis-tribution of a sub sample of GCs) for a factor1000, reducing the kpc GC spatial distributionscale to the inner pc scale, is needed.Is this possible? And how?A straightforward positive answer to these,and other, questions related to the suitability ofthe merger hypothesis has already been done inmany papers since Capuzzo-Dolcetta (1993).Here I limit to give a qualitative physicalinsight to this topic.GCs move as internally structured test ‘par-ticles’in the external galactic potential. Alongthe motion, different degrees of freedom are di-versely excited: the interaction with the galac-tic field induces some reduction of the GC or-bital energy (quenching of what I call external,orbital degrees of freedom, that of the GC asa whole) and a contemporary excitation of in-ternal degrees of freedom, i.e. a ‘heating’ ofthe GC. The first phenomenon corresponds todynamical friction, tending to shrink the GCdistribution length scale in a rate directly pro-portional to the GC mass, and is strongly de-pendent upon the GC orbital distribution. Theother phenomenon acts oppositely, in the sensethat the larger the heating, the higher the evap-oration of stars from the cluster with conse-quent reduction of its mass and thus of the ef-ficiency of dynamical friction breaking with afollowing fate of GC dissolution in the fieldbefore it reaches an orbital equilibrium aroundthe galactic center. The likelihood of a signifi-cant excitation of the internal degrees of free-dom is, roughly, evaluated by this simple backof the envelope calculation.Let E be the GC binding energy per unitmass and Eo its orbital energy per unit mass.Assuming that the GC, considered as a homo-geneous sphere of mass MGC and size RGC ,moves in a homogeneous galactic bulge whosemass and size are Mb and Rb, we haveEEo=35MGCMbRGCRb32−(rRb)2 , (1)which, assuming for both MGC/Mb = 0.01and MGC/Mb = 0.01 as typical values forthe GC moving on a circular orbit of radius rwithin the bulge (r/Rb < 1), leads toEEo≤ 9 × 10−5. (2)This means that a transfer of just 0.01%of the orbital energy into GC internal energy(i.e. into internal degrees of freedom) sufficesto disperse a loose GC during its infall to thegalactic center. GCs can be fragile systems. Ofcourse, the effectiveness of energy swap fromthe orbital ‘reservoir’ to the external and in-ternal GC degrees of freedom is a strongly nonlinear process that, to be thoroughly quantified,needs sophisticated N-body simulations of theGC motion in a particle-sampled galaxy envi-ronment. We refer to Arca-Sedda et al. (2016)for such work.3. Consequences of GC infallThe GC infall and merger scenario is a frame-work which has many intriguing fallouts. Wecan list:636 Capuzzo-Dolcetta: GCs and MBHsTable 1. CMO characteristic parametersCMO Mass (M) Length (pc) density (M pc−3) locationSMBH 106 − 1010 10−7 − 10−2 1027 gal. centerNSC 106 − 108 4 106 inner pcsGC 104 − 106 2 − 5 103 − 106 kpci) NSC formation;ii) NSC vs host galaxy properties scaling re-lations;iii) mutual feedback with a local MBH (seeFig. 1);iv) high- and hyper-velocity star generation;v) solution of the inner pc problem?I limit here to give some, non exhaustive,example references for the various points andto make, in the following subsection, an easytreatment of one specific point in topic ii) (scal-ing relations).Some references:– Point i): Tremaine et al. (1975); Capuzzo-Dolcetta (1993); Antonini et al. (2012);Antonini (2013).– Point ii): Erwin & Gadotti (2012),Georgiev et al. (2016), Tosta e Melo& Capuzzo–Dolcetta (2016), Capuzzo–Dolcetta & Tosta e Melo (2016).– Point iii): Capuzzo-Dolcetta (1993), Arca-Sedda et al. (2016).– Point iv): Capuzzo-Dolcetta & Fragione(2015), Fragione & Capuzzo-Dolcetta(2016).– Point v): no papers published, yet.3.1. A straightforward correlationOne fundamental correlation in the context ofCMO studies is that linking the CMO mass andthe velocity dispersion of the parent galaxy.While the SMBH mass vs galaxy velocity dis-persion is a steep increasing function (MS ∝σ5, Graham et al. 2011), the NSC mass seemsto correlate to σ with a shallower profile(MN ∝ σ1.6, Graham et al. 2012). Intriguingly,-40-30-20-10 0 10 20 30-20 -10  0  10  20  30  40  50  60y/pcx/pcMg = 3.2·1011 M⊙Fig. 1. Various snapshots of the GC moving in acounter clockwise motion on an eccentric orbit ina 3.2 × 1011 M galaxy. Escaping stars are in green,while red dots identify the stars that remain bound tothe cluster. The black filled circle labels the 5 × 108M SMBH, while the blue asterisks represent thelagrangian points L1 and L2 (from Arca-Sedda etal. (2016).)this shallower profile has a straightforward in-terpretation in the infall and merger scenariofor NSC formation.This result can be derived, again, froma simple formal development. Following thederivation in Arca-Sedda & Capuzzo-Dolcetta(2014), based on the assumption of GCs ofequal mass M, spatially distributed accordingto a mass density power law ρ(r) ∝ rα in a sin-gular isothermal spherical galaxy (ρg(r) ∝ r−2)with mass Mg, (constant) velocity dispersion σand spatially cut at R, the nucleus mass result-ing from GC merger is, at every time tMn = f2G(0.6047G ln ΛM)α+3tα+32σ1−α2Rα+2, (3)for t ≤ σR20/(0.6047G ln ΛM), while Mn(t) sat-urates to MGCS at t = σR20/(0.6047G ln ΛM).Capuzzo-Dolcetta: GCs and MBHs 637Equation 3 (in which f is the fraction ofthe total GC mass to the galactic mass) is ob-tained by a straightforward analytical integra-tion of the 1st order differential equation gov-erning the orbital angular momentum evolutionof the GC in the host galaxy. Note that eq. 3 re-duces to the Mn−σ scaling relation, Mn ∝ σ3/2,obtained by Tremaine et al. (1975) in the caseof α = −2, i.e. for GC distributed the sameway as the galactic isothermal background andis independent of the galactic radius R. Thisis the only case where the dependence on thegalactic radius R cancels out. For other valuesof α in the allowed range, the dependence ofMn on σ, in the assumption of a virial relationbetween galactic R and Mg (R ∝ Mg/σ2), be-comesMn(t) ∝ σ9+3α2Mg, (4)which corresponds to a slope in the range from0 of the steeper (α = −3) GCS radial distribu-tion to 9/2 of the flat (α = 0) distribution.The relevant result here is that the slope ofthe Mn −σ relation in the regime of dynamicalfriction dominated infall process is expected tohave an upper bound in any case smaller thanthat of the MBH − σ relation.4. ConclusionsThe globular cluster infall and merging sce-nario is an extensively studied frame whichrepresents an attractive self-consistent expla-nation of various astrophysical phenomena.This approach was originally proposed byTremaine et al. (1975) and later developed byCapuzzo-Dolcetta (1993). Many other authorshave later followed the lines shown in thosetwo seminal paper. As a not exhaustive list ofthe topics connected to the infall and mergerscenario, we indicate:i) NSC formation;ii) NSC vs host galaxy properties scaling re-lations;iii) mutual feedback with a local MBH;iv) high- and hyper-velocity star generation;v) solution of the inner pc problem.ReferencesAntonini, F. 2013, ApJ, 763, 62Antonini, F., et al. 2012, ApJ, 750, 111Arca-Sedda, M., Capuzzo-Dolcetta, R. &Spera, M. 2016, MNRAS, 456, 2457Arca-Sedda, M., Capuzzo-Dolcetta, R. 2014,MNRAS, 444, 3738Capuzzo-Dolcetta, R. 1993, ApJ, 415, 616Capuzzo-Dolcetta, R. & Fragione, G. 2015,MNRAS, 454, 2677Capuzzo–Dolcetta, R. & Tosta e Melo 2016, inprep.Erwin, P. & Gadotti, D. A. 2012, Advances inAstronomy, 946368Fragione, G. & Capuzzo-Dolcetta, R. 2016,MNRAS, 458, 2596Georgiev, I. Y. et al. 2016, MNRAS, 457, 2122Graham, A. W., et al. 2011, MNRAS, 412,2211Graham, A. W., et al. 2011, MNRAS, 422,1586Tosta e Melo, I. & Capuzzo-Dolcetta, R. 2016,Journal of Physics: Conf. Ser., 689, 012008Tremaine, S. D., Ostriker, J. P. & Spitzer, L.Jr. 1975, ApJ, 196, 407

Globular cluster-massive black hole interactions in galactic centers

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Globular cluster-massive black hole interactions in galactic centers

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