PHYS-401 Astrophysics III : stellar and galactic dynamics ...
Galactic Dynamics I - old.phys.huji.ac.il
Transcript of Galactic Dynamics I - old.phys.huji.ac.il
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Galactic Dynamics IRe’em Sari (Hebrew University, Jerusalem)
Dynamical Friction
Modes in disks
Stability
Lindblad & Corotation resonances
Migration
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Dynamical Friction
GM/v2
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Estimating Dynamical FrictionFrom conservation of momentum
deflected orbits
From gravity of the wake:contracting spheres
M vb
δb ∼ GM
b2
b
v
2
∼ GM
v2
δM ∼ ρb2δb
dv
dt∼ GδM
b2∼ G2ρM
v2
Independent of b!All scales contributeCoulomb logarithm: counting scales
v⊥ ∼ GM
b2b
v∼ GM
bv
Mdv
dt∼
b2ρv
v||
dv
dt∼ G2Mρ
v2
δv|| =
v2 − v2⊥ − v ∼= −v2⊥v
∼ G2M2
b2v32
(first order)
(second order)
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Dynamical Friction - More Accurately Momentum
Vector form:
Where is the velocity relative to the stationary background.
vperp ∼= +∞
−∞
GM
x2 + b2dx
v=
2GM
bv
dv
dt= −
2πbdb ρδv||
M= −4πG2ρ
v2
db
b= −4πG2ρ
v2ln(bmax/bmin)
dv
dt= −4πG2ρ ln
bmax
bmin
v
v3
v
δv|| ∼= −2G2M2
b2v3
M M
M
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Dynamical Friction - SummaryA moving object in initially stationary background creates a wake.
Natural scale
Density on that scale higher by order unity, then as b-1.
Vector form:dv
dt= −4πG2ρ ln
bmax
bmin
v
v3
b0
b0 ∼ GM
v2
4b02b0
vM
δρ/ρ ∼ 1/4
δρ/ρ ∼ 1/2δρ/ρ ∼ 1
M
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Dynamical Friction - Finite velocity dispersionBody with velocity v, background with velocity distribution
- Like Newton
For isotropic velocity distribution - analogy to Gauss
For small velocity, there is a deceleration timescale independent of v
For large velocity, deceleration timescale = static background
dv
dt= −4πG2ρ ln
bmax
bmin
v − vb|v − vb|3
f(vb)d3vb
f(vb)
f(vb)
Chandrasekhar’sDF formula
1
v
dv
dt= −16π2
3G2ρ ln
bmax
bminf(0)
1
v
dv
dt= −4πG2ρ ln
bmax
bminv−3
dv
dt= −16π2G2ρ ln
bmax
bmin
v
0f(vb)v
2bdvb
v
v3
τ ∝ v3
τ−1 =
τ−1 =
M
M
M
M
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Modes in Disks
Axis symmetric modes Dispersion relationStability
Physical argumentsDispersion relation
Spiral wavesDispersion relationExcitations
h/r~10-6
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Physical effects
Self Gravity
Shear and rotation
Pressure/velocity dispersion
h/r~10-6
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Pressure WavesOrder of magnitude
Sphere of size r, with sound speed cExpansion in time r/c.ω~c/r or ω~kc
Rigorous:
∂ρ
∂t= −∂(ρv)
∂x
∂v
∂t+ v
∂v
∂x= −1
ρ
∂p
∂x
p ∝ ργ
∂δv
∂t= −1
ρ
∂δp
∂x
∂δρ
∂t= −ρ
∂δv
∂x
δp =γp
ρδρ ≡ c2δρ
∂2δρ
∂2t= c2
∂2δρ
∂x2
ω2 = k2c2
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Self Gravity - in thin disksOrder of magnitude
Sphere of size r, surface density Σ G (Σ r^2)/r^2 t^2 =r ω2~1/t2~G Σ k
Rigorous:
∂δv
∂t= −∂δφ
∂x∇2δφ = 4πGδΣδ(z)
∂δΣ
∂t= −Σ
∂δv
∂x
δφ = δφ exp [i(kx− ωt)− |kz|]δφ = −2πG
|k| δΣ exp [i(kx− ωt)− |kz|] δg = 2πGδΣ
ω2 = −2πGΣ|k|Unstable
like deep ocean waves
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Rotation and Shear - Epicyclic motionGeneral radial acceleration F(r)
Circular orbits at any distancev2/r=F(r)
Non circular orbits
Conservation of angular momentum: Small perturbations around circular orbits:
r = −F (r) + θ2r
r = −F (r) + L2r−3
δr = −F (r)δr − 3L2r−4δr
δr = −(F (r) + 3Ω2)δr
κ2 = Ω2(3 +d lnF
d ln r)
κ2 = Ω2(4 + 2d lnΩ
d ln r)
Keplerian disksPrecessionSolid body rotationUniform velocityConstant LDecreasing L - Instability
κ = Ωω = Ω− κ
κ = 2Ωκ =
√2Ω
κ = 0
Ω
κ2Ω/κ
Aspect Ratio
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Axisymmetric Modes - Stability
ω2 = k2c2 − 2πGΣk + κ2
ω2min = κ2 − π2G2Σ2
c2
ω2min = κ2(1−Q−2)
Q < 1Instability:
Most unstable:
Q =κc
πGΣToomre’s Q parameter
k =πGΣ
c2
Q < 1
ω2
k
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Stability - Order of magnitudePressure stabilizes short wavelengths:
Thermal energy per unit mass c2
Grav. energy per unit mass GΣrInstability r> c2/GΣ
Rotation and shear:Rotation energy per unit mass (κr)2
Instability r<GΣ/κ2
Together, instability for c2/GΣ <r<GΣ/κ2
Stable disk for c2/GΣ > GΣ/κ2
or Q~ cκ/GΣ >1.
ω2 = k2c2 − 2πGΣk + κ2
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Spiral Density Waves
Leading (k>0) & trailing (k<0) wavesm=number of armsTight winding kr>>1Pattern speed
Corotation resonance
Lindblad resonance
At Lindblad k=0Pressure: away from corotationGravity: towards corotation
(ω −mΩ)2 = κ2 − 2πGΣ|k|+ k2c2
exp(imφ+ i
kdr)
Ωp = ω/m
Ωp = Ω
m|Ωp − Ω| = κ
Ω = Ωp ± κ/m
(Find relation between first wavelength and scaleheight H)
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Resonances• Out of resonance:
• disk particles are drifting relative to the potential
• torque averages to 0!• Corotation resonance:
• particles move together with the potential• feel a constant force.
• Lindblad resonance:• each disk particle experiences both signs of
force but at different phases of its epicycle• each epicyclic period it drifts one potential
peak.• Inner Lindblad resonance (ILR)• Outer Lindblad resonance (OLR)
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Migration in disks• Proper calculations
– involves excitation of density waves.– Inner & outer lindblad resonances.– modes carry all energy and angular
momentum deposited in the disk away.• Dynamical friction??
– which velocity should we take?– what is the Coulomb logarithm?– what is the direction at which it acts
• “One sided” torque:
• Total torque
• Migration time
Σr2(Ωr)2(r/h)3(m/M)2
Σr2(Ωr)2(r/h)2(m/M)2
tdyn(M/m)(h/r)2(M/mdisk)
H
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Marginally Stable Galaxies
ω2 = k2c2 − 2πGΣk + κ2
λc =2πGΣ
κ2
Mc =M3
d
M2tot
= δ2Md
δ ≡ Md
Mtot
Q =κc
πGΣ< 1
c
v= δ
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Marginally Stable Galaxies
Q < 1ω2
Q > 1Q = 1
ω2 = k2c2 − 2πGΣk + κ2
k
κ2
Slope -2πGΣ
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Migration & Bulge Formation
tmig =Mtot
Mc
Mtot
Mdisk
σ
V
2tdyn
δ−3 δ−1 δ2
tmig = δ−2tdynMbulge = Mdisk/tmig
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Steady State GalaxiesDekel, Sari & Ceverino
Mbulge = Mdisk/tmig
tmig = δ−2tdyn
δ ≈
tdyntHubble
1/3
Mbulge = Mbulge/tHubble
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Steady State Galaxy - Summary
δ ≈
tdyntHubble
1/3
Nc ≈
tdyntHubble
2/3
z=2tdyn = 50Myr
tHubble = 3Gyrδ = 1/4 Nc ≈ 15
Mc ≈
tdyntHubble
Mtot
Mdisk ≈
tdyntHubble
1/3
Mtot
-