The physical origin of stellar envelopes around globular clusters

Transcription

1 The physical origin of stellar envelopes around globular clusters Phil Breen University of Edinburgh in collaboration with A. L. Varri, J. Peñarrubia and D. C. Heggie

2 Current observational evidence Example: NGC 1851 Number density (Olszewski et al. 2009) Velocity dispersion (Marino et al. 2014)

3 Extra-tidal spherical envelopes Recent observations report the existence of diffuse spherical stellar envelopes extending few times r t. But what is r t? Name r t (pc) size (pc) mass % power law ref NGC ,3 NGC M Tuc ) Kuzma et al. 2016, 2) Kuzma et al. 2017, 3) Olszewski et al. 2009, 4) Piatti 2017 r t values from Harris 2010

4 King truncation radius? Some limitations of lowered isothermal models: Phase space truncation: different prescriptions provide different truncation radii. Independent of the Jacobi radius. (e.g., Mclaughlin & van der Marel 2005) Tidal filling: such models can often fit very underfilling (isolated) clusters, but resulting r t does not trace the real tidal limitation (e.g., Baumgardt et al.2010) Potential escapers: physical argument for an energy cut off doesn t account for spatially confined, unbound stars (e.g., Kuepper et al. 2010, Claydon et al. 2016) Kuepper et al 2010

5 Is there even an envelope? 10r t 5r t r t Jacobi radii based on apocentre and pericentre distances, from Balbinot & Gieles 2017 (calculated as in Ernst & Just 2002, assuming logarithmic Galactic potential)

6 More realistic tidal field The Jacobi surface is not spherical, and the flattening of equipotential surfaces is monotonically increasing with radius (e.g., Heggie & Ramamani1995)

7 Possible interpretations Classical interpretation: Tidal shocks/heating & potential escapers Alternative interpretation: Non-baryonic dark matter ( mini halos ) Modified gravity

8 Classical interpretation Stationary field (circular orbit): potentially large population of potential escapers filling the Jacobi surface (see Kuepper et al. 2010, Claydon et al. 2016) Time-variable field (elliptic, or more complex orbit): simple models indicate that disk shocks remove potential escapers, as they are more likely to be heated.

9 Alternative interpretation: external (or modified) potential Cosmological context and motivation: A possible cosmological formation scenario involves globular clusters forming in dark matter mini halos (e.g. Peebles 1984) These mini halos could have then been stripped by tidal forces in the inner regions of galaxies (Bromm & Clarke 2002, Mashchenko & Sills 2005 a,b) Another scenario is that (especially massive) globular clusters may be the remnants of tidally-stripped nucleated galaxies (e.g., Bekki & Yong 2012)

10 Populating stellar envelopes in an external potential Assuming the envelope is populated by ejection from the centre (i.e. radial orbits) (r) = f c 4 r 2 Z p 2(E N(E) )P (E) de Assuming smooth distribution of velocities Therefore we just need to know the potential (which constrains the period) to solve the integral above We can also reverse the problem, given a ρ we can solve for the envelope potential (within the assumptions) Big caveat: tidal effects currently not accounted for!

11 Dark matter - Keplerian potential Peñarrubia, Varri, Breen et al (arxiv: ) r -1 r -2 r -3 r -4 Theory predicts ~r -4 at large radii. Comparison with simple numerical experiments with tracer particles in cluster s + additional potential Equivalent argument in galactic context, linked to violent relaxation (see von Hoerner 1957, Bertin & Stiavelli 1983, Jaffe 1987, Makino 1990, Aguilar 2008)

12 Dark matter Uniform Density Background Theory predicts ~r -2 at large radii (flattening near edge of the sphere) Comparison with simple numerical experiments with tracer particles in cluster s + additional potential

13 MOND potential Models predicts ~r -2 at large radii for deep MOND regime In the Newtonian regime ~r -4 Comparison with simple numerical experiments with tracer particles in cluster s + additional potential

14 Self consistent N-body simulations Breen et al., in preparation (Too) simple theory confronted so far only with numerical experiments with tracer particles in static dark matter halo + cluster potential Next step : full collisional treatment of the cluster within a static dark matter potential. Crucial to explore evaporation vs. ejection behaviour. Modified Nbody6 (Aarseth 2003) to include a static dark matter potential (additional contribution to regular force). Experimented with several dark matter potentials.

15 Collisional dynamics within a dark matter potential Breen et al., in preparation Escapers Envelope stars Green bound to cluster, blue bound to cluster + DM (no tidal effects, for now)

16 Collisional dynamics within a dark matter potential Breen et al., in preparation Without DM Halo With DM Halo Many purely stellar dynamics questions, e.g. what happens to core collapse? It might not happen at all! And other surprises

17 Summary Growing interest for the structural and kinematic properties of the outskirts of globular clusters. High-quality kinematics is needed. Assessment of the tidal limitation should be done with care (a truncation radius is not the Jacobi radius!). Theoretical prediction that envelope stars are distributed with a slope that depends on the choice of the additional potential (dark matter/mond). Understanding of tidal effects in progress The presence of a dark matter potential can significantly change the dynamical evolution of collisional stellar systems (e.g. delayed core collapse). Much to explore and understand.

18 Additional Slides

19 Dark matter in globular clusters Baryonic component: 'dark remnants' White dwarfs, neutron stars, and stellar-mass black holes may make up a significant fraction of a cluster's mass (e.g., Heggie & Hut 1996) Sollima et al 2016

20 Internal kinematics - initial conditions Breen et al., in preparation Plummer model self-consistently embedded in a Hernquist DM halo The velocity dispersion increases as the dark matter halo mass increases, reducing the energy flux

21 Stellar envelopes in dark mini halos Peñarrubia, Varri, Breen et al. 2017, (arxiv: ) Red bound to cluster mass Blue bound to cluster plus dark matter halo Hernquist potential used for dark matter (i.e. -GM/(a+r))