Roberto Soria CfA

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Intermediate-Mass BHs and star clusters
Roberto Soria (CfA / University College London)
Thanks to C Copperwheat, M Cropper, C Motch, R
Mushotzky, M Pakull, K Wu
The Antennae, courtesy of G Fabbiano (CfA)
Roberto Soria (CfA)
The Antennae, from A Zezas
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Outline
ULXs contain young BHs
How to form IMBHs in young environments?
Stellar coalescence in a star cluster
Protostellar coalescence in a protocluster
IMBHs as seeds for SMBH growth at high z Early
nuclear starburst can provide seeds for SMBH, not
Pop-III BHs
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Basic terminology
Intermediate-mass BHs here means M gt 30 Msun
(gt mass of known Galactic BHs)
(Luminosity arguments suggest M 30 -- 200 Msun
Only very few are brighter than 3 x 1040 erg/s)
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Where and when were IMBHs formed?
In star-forming regions?
Remnants of Pop-III stars?
In globular clusters?
old BHs, recycled systems?
Young BHs?
Observational evidence favours association with
starburst, colliding, merging systems
Found near OB stars (age 5--50 Myr) Candidate
donor stars may have M 10--15 Msun
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Standard model
IMBH formation in a young super-star-cluster
Dynamical friction
106 Msun cluster
Mass segregation Runaway core-collapse
1000 Msun BH
Stellar collisions/mergers in the core
Short-lived, very massive star (1000 Msun)
Hypernova or direct collapse into IMBH
Numerical simulations by Portegies Zwart et
al and by Gurkan, Rasio et al.
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Observational evidence for ULXs in SSCs?
ULX in a young super-star-cluster in M82
Lx varying from 1039 to 1041 erg/s
Mbh 1000 Msun Mcl 4 105 Msun
Portegies Zwart et al, Nature, 2004
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Problem most ULXs are not in super-star-clusters
Near OB stars but not inside a bound cluster
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Not in clusters
4 ULXs in the colliding galaxies NGC 7714 / 7715
with Lx 2 8 1040 erg/s
2 are in clusters, 2 are not
Smith et al 2005, AJ, 129, 1350
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Near clusters but not in one
ULX in the starburst dwarf NGC 5408
with Lx 1040 erg/s
Near B stars but not in a cluster
Kaaret et al 2003 Soria et al 2004
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Near OB stars but not in a super-star-cluster
ULX in the dwarf galaxy NGC 5204
Liu et al 2004
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Not in super-star-clusters
ULX in the starburst dwarf NGC5408
with Lx 1040 erg/s
Two ULXs in NGC4559 with Lx 1 4 1040 erg/s
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NGC4559 X-10 near OB stars, no super cluster
A few B stars but no big clusters
Cropper et al 2005
Soria et al 2005
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NGC4559 X-7 near OB stars, no super cluster
A few B stars but no SSCs
Soria et al 2005
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Antennae lots of ULXs, displaced from clusters
ULXs are displaced from SSCs by 100 300 pc
Zezas, Fabbiano et al 2002
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Why are ULXs / IMBHs not inside super clusters?
Were they ejected?
Inconsistent with IMBH, would require low BH
mass (eg, Zezas et al 2002 Belczynski et al 2005)
Have their parent clusters dispersed?
Tidal disruption always too slow (gt 50 Myr) SN
disruption perhaps.but there are no signs
of the dispersed super
clusters
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106 Msun super star clusters with 1000
Msun BHs
NOT FOUND
NOT NEEDED
Probably we only need M 30 -- 200 Msun
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Suggestion IMBHs formed in smaller
proto-clusters, not super clusters
Ionized gas
protostars
Neutral gas
cluster
t 0.5 Myr
OB assoc
protocluster
(eg, Kroupa Boily, 2002-2004 Geyer Burkert
2001)
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sh lt 10 km/s
M 103.5 -- 105 Msun
Ideal conditions for
forming BHs with M 30 -- 200 Msun
dispersing the protocluster
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Dense proto-clusters ideal for coalescence
Stellar captures and mergers are favoured by
proto-stellar disks / envelopes
Collision cross section enhanced at low velocity
dispersion (gravitational focussing)
Collision rates maximum BH mass enhanced at
high density
(Collision rates enhanced in strongly magnetized
clouds?) (Elmegreen Shadmehri 2003)
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Mid-size proto-clusters are very fragile
M 103.5 -- 105 Msun
sh lt 10 km/s
They may evaporate explosively
when proto-stars stars, on a timescale
t 0.5 - 1 Myr
when a few massive stars coalesce in their
core ( merger-induced outflows)
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Massive proto-stellar mergers
proto-cluster disruption
Explosive expulsion of gas
Merger of 100 100 Msun stars
releases 1051 erg
(Bally Zinnecker 2005)
Binding energy of gas in a 105 Msun cluster a
few 1050 -- 1051 erg
Single SN releases 1051 erg
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Two regimes for coalescence IMBH formation?
M gt105.5 Msun
M lt 105 Msun
sh lt 10 km/s
sh gt 10 km/s
tcc lt 0.5 Myr
tcc lt 3 Myr
IMBH formation in unbound proto-cluster
IMBH formation in bound cluster
ULX in a sparse OB assoc (size gt 100
pc) with expanding gas nebula
ULX in a cluster (size lt 3 pc)
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Additional advantage of the proto-cluster
scenario
Same physical process that creates massive O
O binaries, progenitors of BH HMXBs
ULXs high-luminosity end of HMXBs (recall Lum
Functions in Grimms and Zezass talks)
IN SUMMARY protocluster scenario may explain
formation of BHs with M
100 Msun why they are no
longer in a cluster after 10 Myr
(why some are surrounded by gas nebula)?
why ULX population looks
like tail end of HMXBs
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IMBHs formed in young (proto)clusters in the
local Universe
Natural outcome of clustered star
formation dont need old Pop-III remnants
and in the early Universe?
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IMBHs as seeds for SMBHs at z gt 6
Hierarchical mergers accretion (models by
Volonteri et al)
Galaxy merger / satellite accretion
Star formation and/or starburst
Infall of seed Pop-III IMBHs from satellite
galaxies
Some IMBHs sink to center
Some IMBHs left wandering across galaxy
Merge into SMBH
Gas accretion
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IMBHs as seeds for SMBHs at z gt 6
Galaxy merger / satellite accretion
Nuclear starburst
IMBHs formed in nuclear starburst (reach
galactic center on shorter timescale)
Merge into SMBH
Gas accretion
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NGC 7714/5
Early phase of assembly of an SMBH from IMBHs
in a nuclear starburst?
(Smith et al 2004)
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Summary we speculate that
Most ULXs could be BHs with M 30--200 Msun
formed in medium-size, dense proto-clusters,
via merger of a few proto-O stars
Essentially same process that forms HMXBs,
normal outcome of clustered star formation
Massive nuclear starbursts at 3 lt z lt 10 create
their own IMBH seeds, to assemble early SMBHs
(no need for Pop-III star remnants)
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END OF TALK
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Main differences
Seed IMBHs from clustered star-formation in
galactic nuclei, not from Pop-III halo stars
Shorter dynamical timescale for seed IMBHs to
sink and merge
Dont need actual satellite mergings, just tidal
interactions and gas inflow
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Ionized ULX nebulae signature of
dispersed protocluster?
(Pakull Mirioni 2002)
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Parameter space for IMBH formation from core
collapse
Parameter space for IMBH formation from core
collapse
Parameter space for IMBH formation from core
collapse
Parameter space for IMBH formation from core
collapse
Parameter space for IMBH formation from core
collapse
Parameter space for IMBH formation from core
collapse
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Formation of an IMBH in a young star cluster
Two necessary conditions
Core collapse timescale lt lifetime of the O stars
tcc lt 3 Myr tcc 0.1 0.2
trh (relaxation timescale)
trh lt 30 Myr
Mass of the cluster Mcl gt 105 Msun
Mbh 0.001 0.002 Mcl allows Mbh gt 100 Msun
Numerical simulations by Portegies Zwart et
al and by Gurkan, Rasio et al.
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Observational evidence for ULXs in clusters?
4 ULXs in the colliding galaxies NGC7714 / 7715
with Lx 2 8 1040 erg/s
2 are in clusters, 2 are not
Smith et al 2005, AJ, 129, 1350