Theoretical ideas for the formation and feeding of IMBHs

1. Theoretical ideas for the formation and feeding of IMBHs Cole Miller University of Maryland and Joint Space-Science Institute

2. Outline • Formation and challengesEarly starsDynamics: young massive clustersDynamics: black hole mergers • FeedingStars: massive or giantsTemporary, from molecular cloudOther? I will provide far more questions than answers!

3. Formation of IMBHs • Problem: ~103Msuntoo much for normal star! • Population III starsLow Z; weak winds • Collisions or mergers Needs dense clusters Young: collisions Old: three-body http://www.npaci.edu/enVision/v17.4/images/star2.gif

4. Are Early Stars Actually Large? Stacy & Bromm 2013 More recent high-resolution work has shown that primordial gas might fragment into 10s of Msun stars, not 100s. On the other hand, stars with MZAMS>200 Msun are seen in the LMC (Crowther et al. 2010); might they produce MBH>100 Msun (Belczynski et al. 2014)?

5. Cluster Thermodynamics • Close analogy with thermo! • Equipartition of energy => heavy things sinkMass seg seen in GC (e.g., Sosin 1997) • Increase of entropy => with time core gets denser, outside expands

6. 3-body interactions • Binary-single interactions harden hard binaries, soften soft binaries • Soft binaries are eventually “ionized”: they separate • But recoil from 3-body interactions with hard binaries supplies energy to cluster, holds off core collapse if there is enough energy in the binaries

7. Formation of IMBHs • Problem: ~103Msuntoo much for normal star! • Population III stars Low Z; weak winds • Collisions or mergersNeeds dense clusters Young: collisionsOld: three-body Gurkan et al. 2006

8. Problems With Winds? • Suggested that winds might blow away more mass than is added in runaway collisions • Does this prevent the formation of ultramassive stars? Glebbeek et al. 2009

9. Not Necessarily • As Melvyn Davies has pointed out, collisions happen faster than Kelvin-Helmholtz time • Thus stars don’t relax; they are a bag of cores • Far from clear that winds will resemble those of a star of the same mass

10. Not necessarily, pt 2 • Multiple M>150 Msun stars seen in R136 • LMC metallicity is ~40% of solar, so don’t need Z<<1 • Wind losses apparently not catastrophic Crowther et al. 2010

11. Formation of IMBHs • Problem: ~103Msuntoo much for normal star! • Population III stars Low Z; weak winds • Collisions or mergersNeeds dense clusters Young: collisions Old: three-body From Steinn Sigurdsson

12. Problems With Ejection? Maybe... • Early calculations assumed that in GC size clusters, stellar-mass BHs would quickly form a dense subcluster • Then, they would kick each other out via three-body interactions • Few mergers; if they did merge, GW recoil would certainly kick them out • Few options for growth

13. ...But Maybe Not • More recent sims show that BHs do not form dense subcluster (also Trenti+van der Marel) • There is thus time to accumulate • GW kicks still bad, but not for BH-star Morscher et al. 2013

14. Long-Term Core Collapse • In GC, binaries prevent deep core collapse • But in Miller & Davies 2012, ApJ, 755, 81:If vdisp>~40 km/s, binaries will be destroyed and core collapse will occurThus MBH production seems inevitable in relaxed clusters with at least this vdisp • Consistent with galaxies known without SMBH: NGC 205, 39 km/s; M33, 24 km/s • Might be a way to form IMBHs in sufficiently compact clusters

15. Feeding IMBH: RLOF • Roche lobe overflow • Star needs to be massive, or a giant, for needed dM/dt • Implies Porb~days to months/years • Implications for line variation studies Kalogera et al. 2004

16. Feeding HLX-1 With Wind? • Could a wind power a ULX? • We proposed that a wind from a recently-stripped RG core could power HLX-1 • Maybe easier to explain burst trends Miller, Farrell, Maccarone 2014

17. Tidal Disruptions by IMBHs? r T Haas et al. 2012

18. Tidal Disruptions by IMBHs? • WD disruption suggested as cause of Swift beamed tidal disruption event (Krolik & Piran), some other events • Possible source of gravitational waves, if very close • Not good for persistent fueling of ULXs

19. Accretion from Molecular Clouds • Krolik 2004, ApJ, 615, 383 • Steady accretion is minimal; luminosity heats up cloud • But an IMBH passing through a MC can acquire a disk before it lights up • Active time few x 105 years, but could repeat many times • Would associate with SF regions

20. Conclusions • Without a dynamical mass for ULX primaries, we cannot say for sure whether some or most are powered by IMBHs • Within substantial uncertainties, there do appear to be ways form and power IMBHs • More study is necessary!