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

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

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 http://www.npaci.edu/enVision/v17.4/images/star2.gif

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

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

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

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

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

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

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

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

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

...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

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

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

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

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

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

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

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