Black Holes in Globular Clusters

Transcription

1 Black Holes in Globular Clusters Douglas Heggie University of Edinburgh

2 Papers on black holes and globular clusters

3 Spot the odd man out Cen M15

4 The pioneer phase:

5 Discovery of variable X-ray sources in globular clusters Clark et al, 1975, ApJ, 199, L93-L96 NASA/CXC/MIT/D.Pooley et al

6 An early theoretical model of X-ray sources in globular clusters 1. Bahcall J.N., Ostriker J.P., 1975, Massive Black Holes in Globular Clusters, Nature, 256, Silk, J., Arons, J., 1975, On the nature of the globular cluster X-ray sources, ApJ, 200, L131

7 What effect would a black hole have on the cluster? 1. Peebles P.J.E., 1972, Star Distribution near a Collapsed Object, ApJ, 178, Bahcall, J.N., Wolf, R.A., 1976, Star Distribution around a Massive Black Hole in a Globular Cluster, ApJ, 209, Bahcall, J.N., Wolf, R.A., 1977, Star Distribution around a Massive Black Hole in a Globular Cluster. II. Unequal Star Masses, ApJ, 209, Shapiro, S.L., Lightman, A.P., 1976, The Distribution of Stars around a massive black hole, Nature, 262, 743

8 The distribution of stars around a black hole Black Hole M stars, mass m, density, velocity v Inward flux of mass requires outward flux of energy F This is mediated by two-body relaxation, and so F ~ r3v2/tr, where TR ~ v3/(g2m ) and v2 ~ GM/r. Hence F ~ G3/2m 2r7/2/M1/2 In steady state F = const, and so r-7/4

9 An N-body model of the Bahcall-Wolf cusp Preto et al, 2004, N-Body Growth of a Bahcall-Wolf Cusp around a Black Hole, ApJ, 613L, 109

10 The surface brightness profiles of globular clusters: M15 Newell et al, 1976, Evidence for a Central Massive Object in the X-Ray Cluster M15, ApJ, 208L, 55 King 1966 Note core radius

11 Surface brightness profiles of globular clusters Djorgovski, S.; King, I. R., 1984, Surface photometry in cores of globular clusters, ApJ, 277L, 49 ~25% of all galactic globular clusters have a collapsed core surface brightness profile.

12 How the case for black holes in globulars was undermined. I 1. The X-ray sources are accreting neutron stars/white dwarfs Grindlay et al, 1984, Determination of the mass of globular cluster X-ray Guhathakurta et al, 1996, Globular sources, ApJ, 282L, 13 Cluster Photometry With the Hubble Space Telescope. V. WFPC Study of AC211 M15's Central density Cusp, AJ, 111, cusp 267

13 How the case for black holes in globulars was undermined. II Alternative explanations of the cusp 1. A core of neutron stars Illingworth, G.; King, I. R., 1977, Dynamical models for M15 without a black hole, ApJ, 218L, 109

14 How the case for black holes in globulars was undermined. II Alternative explanations of the cusp 2. The phenomenon of core collapse Hénon, M., 1961, Sur l'évolution dynamique des amas globulaires, AnAp, 24, 369 Lynden-Bell, D., Wood, R., 1968, The gravo-thermal catastrophe in isothermal spheres and the onset of red-giant structure for stellar systems, MNRAS, 138, 495 Lynden-Bell, D.; Eggleton, P. P., 1980, On the consequences of the gravothermal catastrophe, MNRAS, 191, 483

15 Core collapse: Two simulations Entire system 13 seconds to t = 3.3 Central area 13 seconds to t = 330 The initial conditions

16 The quiet phase: to 2000 Growth of black holes during core collapse Time Black Hole Mass Duncan, M. J., Shapiro, S. L., 1982, Star clusters containing massive, central black holes. IV - Galactic tidal fields, ApJ, 253, but how was the process seeded?

17 The Modern Phase: 2000 to the present day Creation of seed black hole by runaway merger in young, dense star clusters Example: R136 in the LMC Portegies Zwart et al, 1999, Star cluster ecology. III. Runaway collisions in young compact star clusters, A&A, 348, 117

18 Established the possibility of formation of massive star in compact star forming regions

19 Conditions for runaway coallescence Concentration Portegies Zwart et al, 2004, Formation of massive black holes through runaway collisions in dense young star clusters Nature, 428, 724 Dynamical friction time scale Another movie...

20 Observational evidence of black holes in clusters in young compact star forming regions NOAO M82 Galactic Centre cluster Chandra IR

21 Another example of an intermediate-mass black hole in a young star cluster? Maillard et al, 2004, The nature of the Galactic Center source IRS 13 revealed by high spatial resolution in the infrared, A&A, 423, 155 IR

22 Did globular clusters start very compact? Phinney, E.S., 1993, Pulsars as Probes of Globular Cluster Dynamics, ASPC, 50, 141 Best initial model of M15 has M = 1.4x106, R = 1.5pc Tdf ~ 50Myr, c ~ 1.8 well away from the domain where collision runaway occurs Anyway......does a massive star give rise to an intermediate-mass black hole?

23 Globular clusters and the M - relation Gebhardt et al, 2002, A 20,000 M Black Hole in the Stellar Cluster G1, ApJ, 578L, 41

24 Observations of M15, models without mass segregation Gerssen et al, 2002, Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15. II. Kinematic Analysis and Dynamical Modeling, AJ, 124, 3270

25 Models with mass segregation

26 Some responses to Gerssen et al 1. Mass segregation was estimated from published models of M15 by Dull et al, 1997, The Dynamics of M15: Observations of the Velocity Dispersion Profile and Fokker-Planck Models, ApJ, 481, 267. Old New

27 Some responses to Gerssen et al (1 cont) In their Addendum (2003, ApJ, 585, 598) Dull et al note On the basis of the original, incorrect version of Figure 12, Gerssen et al. (2002) concluded that the D97 models can fit the new data only with the addition of an intermediate-mass black hole. However, this is counter to our previous finding, shown in Figure 6 of D97, that the Fokker-Planck models predict the sort of moderately rising velocity dispersion profile...

28 Response 2: An N-body model of M15 Baumgardt et al, 2003, On the Central Structure of M15, ApJ, 582L, 21 Conclusions independent of retention fraction of neutron stars Note: scaling required

29 Evolutionary N-body models with a black hole Baumgardt et al, 2005, Which Globular Clusters Contain Intermediate-Mass Black Holes?, ApJ, 620, 238 density profile Cf M15: Guhathakurta et al, 1996, AJ, 111, 267

30 What determines the core radius? Recall 1. estimate for flux of energy in the cusp F ~ G3/2m 2r7/2/M1/2 2. velocity dispersion in the cusp v2 ~ GM/r Hence F ~ G5mM3 2/v7 At the edge of the cusp,,v are values in the core, where G rc2 ~ v2. Hence F ~ G5mM3 2/v7 ~ G3mM3/(v3rc4) Just as in a main sequence star, F is determined by conditions well outside the core (Hénon). Hence rc M3/4, i.e. a massive black hole requires a large core. (H, Mineshige, Makino, Hut, Baumgardt, in preparation)

31 Check of the relation between black hole mass and core radius. I

32 Check of the relation between black hole mass and core radius. I

33 The case of Omega Cen Surface brightness profile Noyola et al, 2006, Evidence for an Intermediate Mass Black Hole in Centauri, ASPC, 352, 269

34 The case of Omega Cen. II Inferred black hole mass 5x104M Velocity dispersion profile M/L Variation of stellar M/L is ignored, because mass segregation cannot be an important effect.

35 A dynamic evolutionary model of Cen Mean mass Giersz, M., H, 2003, MNRAS, 339, 486 Time (Myr) There is mass segregation...

36 A dynamic evolutionary model of Cen... though this model does not produce a sufficient velocity dispersion at the centre

37 Summary The early years: X-ray sources in globular clusters as accreting black holes. Do collapsed-core clusters harbour black holes? The intermediate years: Growing black holes from seeds, during core collapse The modern era: Creating the seeds in compact star forming reegions Return to the globular clusters: do clusters with extended cores harbour black holes? As of Prague (Aug 2006) K. Gebhardt pins his case on Cen and G1.