Black Hole Astrophysics

Black Hole Astrophysics
Cole Miller, University of Maryland
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Outline
•  Why do we think BHs exist?
•  The feeding of BH
•  The dynamics of BH
Ask questions any time!
Would you be comfortable with group discussion?
Approaches in theoretical astrophysics
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Range of BH Masses
•  We are confident that 3-20 Msun BH exist
•  Similarly for 106-1010 Msun BH
•  Not as clear: 102-105 Msun BH
(“intermediate-mass black holes”)
•  More generally, what observational
evidence suggests that black holes exist?
3
Accretion Disks
•  Matter spirals onto
BH from companion
or surrounding gas
•  Produces X-rays,
UV, opt, ...
•  Need to ensure that
object is too massive
to be NS (spectra,
timing very similar)
http://upload.wikimedia.org/wikipedia/commons/2/2a/Accretion_disk.jpg
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Stellar Orbits
•  Easiest in our
Galaxy (see right)
•  Otherwise, see
blended lines (look
for Keplerian)
•  Must argue that
nothing else could
explain data (such
as dense cluster)
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Masers
•  Natural masers provide high intensity,
angular localization is excellent
•  Can see Keplerian motion
NGC 4258:
Swinburne
astronomy
group,
Australia
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Gravitational Lensing
•  Achromatic
brightening of
background star
•  Duration depends
on lens mass but
also on angular
speed
•  BH in galaxy
minimal compared
to galaxy mass
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http://www.spacetelescope.org/static/archives/images/screen/opo0003e.jpg
Brightness and Variability
•  Very bright things
could be accreting
BHs, but to
establish size need
to have fast
variability
•  D<~cΔt, with
possible beaming
modifications
http://www.astr.ua.edu/keel/agn/vary.gif
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Feeding Black Holes
•  As in, how do you get gas to a BH to light
it up?
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Wind Accretion
•  From massive or
giant stars
•  Little net angular
momentum
•  Short-lived phase
(little mass
transferred)
http://cdn.phys.org/newman/gfx/news/hires/2013/fastfuriousr.jpg
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Roche Lobe Overflow
•  Donor overflows L1
•  Easier to get high
accretion rates
•  Can have from
low-mass donor
•  Can thus last
longer
•  Still doesn’t add
much mass or spin
to BH (does to NS)
http://www4.nau.edu/meteorite/meteorite/Images/X-ray_Binary.jpg
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Jets
•  Small opening
angle outflows
•  Seen in protostars,
white dwarfs, NS,
BH
•  Seems to require
spin, probably mag
field
•  But much is not
known
Image credit: NASA
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Bondi-Hoyle Accretion
•  Accretion from ISM/IGM
•  Easiest in center of galaxy; cold flows
http://quasar.cc.osaka-kyoiku.ac.jp/~fukue/Hoyle/hoyle-1b.jpg
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More About Bondi-Hoyle
Mass accretion rate:
2
GM
2 )1/2
Ṁ = ⇡ ⇢1 B@ c2+v 2 CA (c2s + v1
s 1
0
1
ρ=density at infinity, cs=sound speed, v=relative
speed at infinity, M=mass of BH, λ~1=eigenvalue.
But note: if luminous, feedback increases cs,
decreases ρ. Extremely difficult to grow stellar
mass BH (straightforward to grow SMBH because
T~M/(dM/dt)~1/M).
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Eddington Luminosity
•  In sph symm, outward rad force balances
inward gravitational force
•  LEdd=1.3x1038 erg/s (M/Msun) typically
•  Surprisingly well-obeyed
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http://www.ppl.phys.chiba-u.jp/lecture/radiation/Timg55.png
Efficiency of Accretion
•  At luminosities
~0.01-1 LEdd,
L~(0.05-0.3)c2dM/dt
•  Much below LEdd,
get radiatively
inefficient flow
•  Much above,
trapped radiation?
http://ned.ipac.caltech.edu/level5/March08/Ho/Figures/figure13.jpeg
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Characteristic Growth Time
•  If L=ηc2(dM/dt), then
M/(dM/dt)=45 Myr (η/0.1)-1 at L=LEdd
•  Interesting puzzle: 109 Msun BH seen at
z=7, but not enough time to grow from 10
Msun seed
Supermassive star seed? Runaway
cluster collapse? Slightly super-Edd
accretion? Currently under debate
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Transport of Angular Momentum
•  To accrete: mass in, ang mom out
•  But how? Molecular viscosity too small
•  Shakura+Sunyaev (1973): anom. viscosity
Trφ=αP (α-model), α~0.1; physical origin?
http://inspirehep.net/record/849684/files/002-figure-angular-momentum.png
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Magnetorotational Instability
•  Velikhov 1959; astro
appl. Balbus+Hawley
1991
•  Spring analogy
•  In disk: weak magnetic
field lines tangle, amplify,
transport angular
momentum
•  Turbulence! Don’t find in
α-model
Credit: John Hawley
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MRI Movie (John Hawley)
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Open Questions
•  Is a net vertical magnetic field needed for
observed angular momentum transport?
What about for jets?
•  Quasiperiodic brightness oscillations are
seen from many stellar-mass BH. What
causes them? MRI destroys some modes
•  Quasar microlensing suggests that the
standard model isn’t quite right; annular
fluctuations (Dexter+Agol)? Cause?
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Spin Alignment of SMBH Binaries
•  When spinning BHs of comparable mass
merge, kick could be thousands of km/s
•  If the spins are aligned, kick is <200 km/s;
implications for retention of BH postmerger
•  Are there processes that might tend to
align SMBH spin with each other and
orbit?
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Bardeen-Petterson Effect
CXC
“Back-reaction” of frame-dragging of
disk by black hole causes hole to realign… large lever-arm leads to
effective realignment
9/18/14
Bardeen-Petterson (1975)
Natarajan & Armitage (1999)
Sorathia et al. 2013a,b
Tremaine & Davis 2014
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Circumbinary Disk
Miller+Krolik 2013
Initial spins generally misaligned with each other, orbit,
and the normal to the circumbinary plane at large distance.
Spins dictate gas plane close to holes; binary dictates gas
plane far from holes but close enough to binary.
Net result: fast alignment of spins with each other and orbit.
9/18/14
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What Could Prevent Alignment?
•  If stellar dynamics is important, will tilt orbit
but (usually) not individual spins
Thus mergers of gas-poor galaxies might
lead to large kicks
•  If gas is prevented from getting to
individual black holes, spins aren’t aligned
(Gerosa & Lodato)
•  If gas all flows to one hole, the other is not
aligned
•  TBD whether these are realistic
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Coevolution of SMBH, Galaxies
•  Properties of SMBH
are pretty well
correlated with
galaxies (e.g., M-σ)
•  Many explanations!
•  Deviations at high
and low mass
•  Important, but
causes, scatter are
not well understood
Gultekin et al. 2009
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Black Hole Feedback: Galaxies
•  Galaxy formation is
inefficient at low,
high masses
•  Low mass: star
formation feedback?
•  High mass: AGN
feedback?
•  Seems necessary to
explain colors, too
Dark matter halo
Galaxy mass function
http://ned.ipac.caltech.edu/level5/March05/Read/Figures/figure4.jpg
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Black Hole Feedback: Clusters
•  Centers of many
galaxy clusters
should have cool
gas flow, SFR~103
Msun/yr
•  But they don’t; best
guess is feedback
from central AGN
Perseus Cluster
http://newswatch.nationalgeographic.com/files/2014/01/perseushalloween_cxc_big.jpg
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Questions About Accretion?
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The Dynamics of Black Holes
•  How do black holes interact gravitationally
with stars and gas?
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Dynamical Friction
•  Related to BondiHoyle accretion
•  Heavy objects sink
in grav. potential
T~1/M
•  SMBH in galaxies
have bulge around
them, ~500x more
massive; speedup!
Until bulge stripped
http://abyss.uoregon.edu/~js/lectures/cannibalism/dyn_friction.gif
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Final Parsec Problem
•  In galaxy merger, SMBHs with bulges drift
rapidly to center
•  But at ~0.1-1 parsecs, SMBHs have
kicked out most stars
Requires relaxation time to restock; can be
>1010 years for many galaxies
•  Will stall unless BHs can get to ~10-3 pc
•  Solutions? Triaxiality, massive perturbers,
gas interactions
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Extreme Mass Ratio Inspirals
•  EMRIs
•  Stellar-mass object (star, WD, NS, BH)
spiraling into supermassive black hole
•  One of main gravitational wave targets of
eLISA; very precise mapping of spacetime
•  Also important for tidal disruption events
•  But what are the important dynamical
processes?
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Two-Body Relaxation
•  (largely) distant grav. 2-bod
interactions
•  Time when dE/E~1 is energy
relaxation time
•  For MBH~few x 106Msun,
tr~few Gyr (10Msun/m).
Less for smaller SMBH
•  For equal-mass, core
contracts, halo expands
(increases entropy)
•  Binaries, MBH have effects
on distribution
9/18/14
Evol. of low mass star dist.
Decressin et al. 2008
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Mass Segregation
•  For distributed masses,
heavies sink
•  Seen in many globulars
•  Boosts BH EMRI rate
dramatically
•  Combined with greater
visibility, BH-SMBH
dominate rate
•  Pre-segregation or topheavy IMF?
M22 (GC) radial annuli
Albrow, De Marchi, Sahu 2002
9/18/14
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Net Rotation?
•  2-bod relaxation and mass segregation
are enhanced when relative speeds of
stars are decreased
•  If there is net rotation in the inner ~1 pc,
relaxation times are therefore decreased
•  Some simulations suggest this could make
huge difference to rates, properties
E.g., work by Spurzem and colleagues
9/18/14
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Resonant Relaxation
•  Near SMBH, orbits are
nearly Kepler ellipses
•  If orbit orientation is
~fixed, torques can
change angular
momentum faster than
2-bod relaxation
•  Issue: GR precession
“Schwarzschild barrier”
Rauch & Tremaine 1996
9/18/14
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Triaxiality
•  Galaxy collisions can cause
cores to be triaxial
•  Then no symmetry
preserves angular
momentum of individual
orbits
•  Not as true close to SMBH
•  Increased feeding rates to
center, boosting EMRIs?
9/18/14
M87
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Expected Eccentricities
•  Gravitational radiation circularizes orbits
except very close to ISCO
For highly eccentric, circularize at roughly
constant pericenter distance
•  Interactions with stars can increase or
decrease eccentricities
•  How do these play out in different
circumstances?
9/18/14
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Circularization
•  Under pure GW evolution, time is
Here µ is the reduced mass, M is total, f is
GW frequency. Time is ~same for inspiral,
circularization.
f=10-4 Hz, µ=10 Msun, M=106 Msun, e=0.5:
τGW~8x105 yr. Shorter than relaxation time.
9/18/14
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EMRI 1: High Eccentricity Inspiral
• 
• 
• 
High apocenter orbit
2-body rel -> plunge
Small pericenter means loss of
energy
• 
• 
Inspiral over 104-5 orb
Eccentric in LISA band
Arbitrary inclination
• 
• 
Energy dissipation
by gravitational
radiation
Triaxiality unlikely to boost
Related to tidal disruptions
9/18/14
Eccentric in LISA band
Courtesy V. Lauburg
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EMRI 2: Binary Tidal Separation
•  What if BH in binary?
Miller et al 2005
•  Binary separates
No energy loss needed
•  High pericenter, low
apocenter
Low e, arb i in LISA band
•  Triaxiality might boost
9/18/14
No energy
dissipation
required for capture
Circular in LISA band.
Arbitrary inclination.
Courtesy of V. Lauburg
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EMRI Scenario 3:
Settling in Accretion Disk
•  Miralda-Escude &
Kollmeier 2005
(also Yuri Levin)
•  Star plunging through
disk settles in disk
•  Zero eccentricity
•  Zero inclination
http://apod.nasa.gov/apod/image/0503/accretion_mpowen_c1.jpg
Yunes et al. 2009: e=0, i=0, a/M=0 EOB EMRI
9/18/14
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Rates?
•  Very uncertain!
•  Estimates of LISA detection rates of
EMRIs range from 1-1000 per year
•  Lots of theoretical and observational work
needed to reduce the uncertainties
9/18/14
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Questions about BH Dynamics?
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Summary
•  Far from being loners, black holes affect
the evolution of objects as large as galaxy
clusters
•  Observations are coming at an increasing
rate, as are computational models
•  Rich range of astrophysics!
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