CHAOS
Stellar Streams
Adrian Price-Whelan
+
K. V. Johnston, M. Valluri, S. Pearson,
A. H. W. Küpper, D. W. Hogg, B. Sesar, H-W Rix
(Columbia University)
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Stellar streams and tidal debris
Milky Way: ~20 known, large range in distance and mass
Bonaca, Giguere, Geha
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Stellar streams and tidal debris
Milky Way: ~20 known, large range in distance and mass
Bonaca, Giguere, Geha
(globular?)
(globular)
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Stellar streams and tidal debris
Milky Way: ~20 known, large range in distance and mass
Bonaca, Giguere, Geha
(globular?)
(globular)
(dwarf Gal)
(dwarf Gal?)
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1. Dark matter halo properties
2. Accretion histories
3. Formation of stellar halos
Stellar streams and tidal debris
Thesis work: Connecting galaxies to cosmology
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1. Dark matter halo properties
2. Accretion histories
3. Formation of stellar halos
Stellar streams and tidal debris
Thesis work: Connecting galaxies to cosmology
- Shape / triaxiality? Mass profile?
(Price-Whelan et al. 2013, 2014, 2016)
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1. Dark matter halo properties
2. Accretion histories
3. Formation of stellar halos
Stellar streams and tidal debris
Thesis work: Connecting galaxies to cosmology
- How many accreted? When? Orbits? Masses, etc.?
(Price-Whelan et al. 2016)
- Shape / triaxiality? Mass profile?
(Price-Whelan et al. 2013, 2014, 2016)
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1. Dark matter halo properties
2. Accretion histories
3. Formation of stellar halos
Stellar streams and tidal debris
Thesis work: Connecting galaxies to cosmology
- All accreted? Kicked out of disk? Formed in situ?
(Price-Whelan et al. 2015)
- How many accreted? When? Orbits? Masses, etc.?
(Price-Whelan et al. 2016)
- Shape / triaxiality? Mass profile?
(Price-Whelan et al. 2013, 2014, 2016)
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• Predictions for dark matter halo shapes
• Implications for stream evolution
• Modeling a stream to study the Galactic bar
Outline
DM halo properties affect stream morphology
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(Caterpillar simulations; Griffen et al. 2015)
Simulated dark matter halos
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(Caterpillar simulations; Griffen et al. 2015)
Simulated dark matter halos
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Simulated dark matter halos
Milky Way-mass halos are strongly triaxial
(Millennium-II; Schneider et al. 2012)
pdf
0.2 0.4 0.6 0.8 1.0
0.0
(c/a)
(b/a)
Axis ratio in density
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Orbits in triaxial systems
Regular motion
3 integrals of motion
3 fundamental
frequencies, O
(⌦1, ⌦2, ⌦3)
x
(
t
) =
1
X
k
ak e
i !k t
motion in any
component:
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Orbits in triaxial systems
Regular motion
3 integrals of motion
3 fundamental
frequencies, O
(⌦1, ⌦2, ⌦3)
x
(
t
) =
1
X
k
ak e
i !k t
!k = n1⌦1 + n2⌦2 + n3⌦3
(for integers n)
motion in any
component:
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Orbits in triaxial systems
Regular motion
3 integrals of motion
3 fundamental
frequencies, O
Chaotic motion
only conserve energy
frequencies evolve
with time
(⌦1, ⌦2, ⌦3)
x
(
t
) =
1
X
k
ak e
i !k t
!k = n1⌦1 + n2⌦2 + n3⌦3
(for integers n)
motion in any
component:
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x(t)
time time
Orbits in triaxial systems
Regular motion
3 fundamental
frequencies, O
Chaotic motion
frequencies evolve
with time
(⌦1, ⌦2, ⌦3)
x(t)
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FFT
frequency frequency
FFT
Orbits in triaxial systems
Regular motion Chaotic motion
3 fundamental
frequencies, O
frequencies evolve
with time
(⌦1, ⌦2, ⌦3)
wk
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window 1 window 2
e.g., Valluri & Merritt (1998)
Frequency diffusion time
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window 1 window 2
e.g., Valluri & Merritt (1998)
⌦w1 ⌦w2
Frequency diffusion time
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window 1 window 2
e.g., Valluri & Merritt (1998)
⌦w1 ⌦w2
T
Frequency diffusion time
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Triaxial NFW
b/a = 0.77
c/a = 0.55
}in density
Static triaxial potential model
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Frequency diffusion time
Price-Whelan et al. (2016)
more chaotic less chaotic
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Frequency diffusion time
Long-axis
tube
Short-
axis tube
Box
ZVC (Box)
Price-Whelan et al. (2016)
more chaotic less chaotic
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104 M
Progenitor mass =
Star particle positions after 10 Gyr
Price-Whelan et al. (2016)
x
y
y
z
20 kpc
Streams in triaxial systems
Regular orbits
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Stream star frequencies
relative to progenitor orbit
Price-Whelan et al. (2016)
2%
⌦1
⌦2 ⌦3
⌦2
Streams in triaxial systems
Regular orbits
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Stream star frequencies
relative to progenitor orbit
Price-Whelan et al. (2016)
2%
⌦1
⌦2 ⌦3
⌦2
Streams in triaxial systems
Regular orbits
1 > 2 > 3
1
2 3
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2%
⌦1
⌦2 ⌦3
⌦2
Frequencies relative to progenitor
Price-Whelan et al. (2016)
Streams in triaxial systems
Chaotic orbits
⌦2 ⌦3
Weak chaos:
Strong chaos:
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Star particle positions after 10 Gyr
Price-Whelan et al. (2016)
x
y
y
z
Streams in triaxial systems
Chaotic orbits
y z
20 kpc
Weak chaos:
Strong chaos:
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Price-Whelan et al. (2016)
Dark matter halos are expected to be triaxial
Chaotic orbits are prevalent in triaxial systems
Streams affected by chaos disperse faster:
Form low density “fans” with large velocity dispersion
Conclusions
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In the Milky Way
radius-dependent axis ratios
time-dependence (e.g., mass growth)
subhalos
disk, bulge, bar
Triaxiality PLUS:
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Thin streams exist around the Milky Way!
Bonaca, Giguere, Geha
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Implications
Connecting galaxies to cosmology
1. Dark matter halo properties
Do the known thin streams map the regular orbits?
Is the MW potential more regular?
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Implications
Connecting galaxies to cosmology
1. Dark matter halo properties
Do the known thin streams map the regular orbits?
Is the MW potential more regular?
2. Accretion histories
Measuring dynamical age / accretion time difficult
if stream is chaotic
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Implications
Connecting galaxies to cosmology
1. Dark matter halo properties
Do the known thin streams map the regular orbits?
Is the MW potential more regular?
2. Accretion histories
Measuring dynamical age / accretion time difficult
if stream is chaotic
3. Formation of stellar halos
Substructure smoothed out by chaos — less halo
substructure in Gaia?
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Blue Horizontal
Branch stars
The Ophiuchus Stream
Sesar et al. (2015)
Bernard et al. (2014)
Short: 2º, 1.5 kpc
Age: ~10–12 Gyr
(R,z) ~ (1,4) kpc
Orbital period: ~400 Myr
but…
l [deg]
v
los
[km/s]
b
[deg]
4
5
6
30
31
32
294
290
286
282
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Blue Horizontal
Branch stars
The Ophiuchus Stream
Sesar et al. (2015)
Bernard et al. (2014)
Short: 2º, 1.5 kpc
Age: ~10–12 Gyr
(R,z) ~ (1,4) kpc
Orbital period: ~400 Myr
but…
l [deg]
v
los
[km/s]
b
[deg]
4
5
6
30
31
32
294
290
286
282
Kicked on to this orbit,
full disruption ~300 Myr ago?
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l [deg]
v
los
[km/s]
b
[deg]
4
5
6
30
31
32
294
290
286
282
Sesar, Price-Whelan et al. (2016)
l [deg]
Blue Horizontal
Branch stars
The Ophiuchus Stream
Sesar et al. (2015)
Bernard et al. (2014)
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l [deg]
v
los
[km/s]
b
[deg]
4
5
6
30
31
32
294
290
286
282
Sesar, Price-Whelan et al. (2016)
l [deg]
Blue Horizontal
Branch stars
The Ophiuchus Stream
Sesar et al. (2015)
Bernard et al. (2014)
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Sesar, Price-Whelan et al. (2016)
l [deg]
The Ophiuchus Stream
Sesar et al. (2015)
Bernard et al. (2014)
6 of all 40 BHB stars in this field are
consistent in distance, sky position
4 of those 6 w/ large velocity dispersion
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+ Galactic Bar
The Ophiuchus Stream
On a chaotic orbit?
Price-Whelan et al. (in prep.)
Static, Axisymmetric
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The Ophiuchus Stream
Consistent with chaotic stream fanning
(nearby low-density debris with large dispersion?)
Sensitive to the potential of the Milky Way bar
(independent measure of pattern speed, mass)