A Stellar-Mass Black Hole Population in the Galactic Globular Cluster

AA stellar-massstellar-mass blackblack holehole populationpopulation inin thethe GalacticGalactic GlobularGlobular ClusterCluster NGCNGC 6101?6101? MiklosMiklos Peuten,Peuten, UniversityUniversity ofof SurreySurrey Alice Zocchi, Mark Gieles, Alessia Gualandris, Vincent Hénault-Brunet NGC 6101 (Digitized Sky Survey) (Hubble Space Telescope) NGC 6101 ● GGC in the constellation Apus ● Galactocentric distance: 11.2 kpc (Harris 1996) ● Distance to the sun: 14.6 kpc † ● Half-mass radius: 12.3 pc † ● Age: 13 Gyr (Dotter et al. 2010) ● Metallicity: [Fe/H] = −1.98 (Carretta et al. 2009) ● Large core radius relative to the half- light radius (Rc / Reff ≈ 0.4 †) ● Half-mass relaxation time: ~ 5.8 Gyr † † Dalessandro et al. (2015) NGC 6101 Dalessandro et al. (2015) NGC 6101 Dalessandro et al. (2015) NGC 6101 Dalessandro et al. (2015) Question:Question: Given the age and the current half-mass relaxation time should we expect to see at least some degree of mass segregation, or not? Idea:Idea: Run a N-Body simulation and see if the cluster should be mass segregated. EMACSS Evolve Me A Cluster of StarS (Alexander et al. 2014) A fast star cluster evolution code applying Henon's (1961, 1965) predictions that flow of energy within a cluster is independent of its source. Initial number of stars N 0 EMACSS Cluster Mass Mcl,t Initial half-mass radius rh,0 Half-mass radius rh,t Galactocentric distance RG Galactocentric distance RG,t Initial mean mass <m>0 Mean mass <m>t Age: T NGC 6101: Varied N0 and rh,0 till Mcl and rh are equal to observed values N-Body Simulation ● 105 particles with Plummer (1911) profile ● Kroupa (2001) IMF: 0.01 – 15 M☉ (No BHs) ● No primordial binaries ● A singular isothermal sphere as Milky Way potential ● Circular orbit ● Concurrent stellar and dynamical evolution for 13 Gyr with GPU version of NBODY6 (Aarseth 2003, Nitadori & Aarseth 2012) N-Body Analysis Cumulative radial distribution plots comparing MSTO stars with BSS BSS:BSS: As there are no BSS in the simulation we use White Dwarfs instead NGCNGC 6101:6101: Use two mass ranges: – mBSS = 1.0 – 1.5 M☉ →→ <m>BSS = 1.1M☉ – mBSS = 1.2 – 1.5 M☉ →→ <m>BSS = 1.3M☉ N-Body Results N-Body Model NGC 6101 N-Body Results Judging from the N-Body simulation we should see signs of mass segregation →→ ButBut wewe dodo notnot seesee it!it! Why?Why? IsIs thethe clustercluster simplysimply notnot massmass segregatedsegregated andand ifif soso whatwhat doesdoes thisthis cause?cause? oror DoDo wewe justjust notnot seesee itit andand thenthen whywhy isis thatthat so?so? The Idea ● Large core radius were seen in simulations with stellar mass black hole populations (Merritt et al. 2004; Mackey & Gilmore 2004; Lutzgendorf et al. 2013) ● Recent publications show a higher survival rate for stellar mass black holes in GCs (Breen & Heggie 2013a,b; Sippel & Hurley 2013; Wang et al. 2016) ● The existence of stellar mass BHs in GC was recently confirmed (Strader et al. 2012; Chomiuk et al. 2013) Idea:Idea: Run additional N-Body simulations with different BH retention fractions. Done by varying the initial kick velocity of the BHs. N-Body Results 50% initial BH retention NGC 6101 N-Body Results 100% initial BH retention NGC 6101 N-Body Analysis Note:Note: Using WDs as a proxy for BSSs we overestimate the degree of central concentration (Richer et al. 2013) Cluster Centre Self-similar behaviour Self-similar behaviour N-Body Results Result:Result: ● We found a self-similar behaviour in multi-mass N-Body simulations which needs further research ● A BH population can reproduce the large core radius and the apparent lack of mass segregation, although the cluster is mass segregated! But:But: It has been shown that IMBHs could also reproduce both observed properties (Trenti et al. 2007; Gill et al. 2008; Lützgendorf et al. 2013) N-Body Results Questions:Questions: How can we observationally discriminate between the proposed explanations: – A cluster with stellar mass BHs and mass segregation – A cluster with an IMBH and no mass segregation – A cluster with no mass segregation Problems Problem:Problem: N-Body Model without BHs fails to reproduce lack of mass segregation and large core, while the models with BHs reproduce both! – Do not know whether missing mass segregation is due to BHs or large core – Bianchini et al. (2016) found that the mass below which stars have similar distributions (meq) depends on the cluster concentration Idea:Idea: Use different families of distribution function based dynamical models to take advantage of their predictive power. (LIMEPY software) Dynamical Modelling Create four dynamical models + add a IMBH model Name SM M0 M0.5 M1 IMBH Model Single Multi Multi Multi Single Type Mass Mass Mass Mass Mass Mass Segregation No Yes Yes Yes No Initial BH No No 50% 100% No retention Miocchi (2007) Number density Dalessandro et al. (2015) Equipartition in energy 100% initial BH retention Equipartition in energy No initial BH retention Cumulative Profile Line-of-sight velocity dispersion Conclusion ● We found that a stellar mass BH population can create the apparent effect of a missing mass segregation, although the cluster is mass segregated ● We found that with a measurement of the line- of-sight velocity dispersion, we could differentiate the different proposed models for NGC 6101 ● We found a self-similar behaviour in multi-mass N-Body simulations which needs further research Thank you .

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