1. Theoretical ideas for the formation and feeding of IMBHs
Cole Miller
University of Maryland and Joint Space-Science Institute
11/27/2018

2. Outline
Formation and challenges Early stars Dynamics: young massive clusters Dynamics: black hole mergers
Feeding Stars: massive or giants Temporary, from molecular cloud Other?
I will provide far more questions than answers!
11/27/2018

3. Formation of IMBHs Problem: ~103 Msun too much for normal star!
Population III stars Low Z; weak winds
Collisions or mergers Needs dense clusters Young: collisions Old: three-body

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)?
11/27/2018

5. Cluster Thermodynamics
Close analogy with thermo!
Equipartition of energy => heavy things sink Mass seg seen in GC (e.g., Sosin 1997)
Increase of entropy => with time core gets denser, outside expands
11/27/2018

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
11/27/2018

7. Formation of IMBHs Problem: ~103 Msun too much for normal star!
Population III stars Low Z; weak winds
Collisions or mergers Needs dense clusters Young: collisions Old: 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
11/27/2018

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
11/27/2018

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
11/27/2018

11. Formation of IMBHs Problem: ~103 Msun too much for normal star!
Population III stars Low Z; weak winds
Collisions or mergers Needs 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
11/27/2018

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
11/27/2018

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 occur Thus 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
11/27/2018

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
11/27/2018

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
11/27/2018

17. Tidal Disruptions by IMBHs?
Haas et al. 2012
11/27/2018

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
11/27/2018

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
11/27/2018

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!
11/27/2018