Angular momentum shapes disk galaxies
J. Herpich , G. S. Stinson , A. A. Dutton , H.-W. Rix , M. Martig
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1
3
4
R. Roškar , A. V. Macciò , T. R. Quinn , J. Wadsley
(2015, MNRASL, 448, 99)
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1
1
1
1
Abstract
We present a simple numerical model of disk pure exponential (quite rare), up-bending and mentum. We find that the outskirts of upgalaxy formation which, for the first time, re- down-bending. The model links a galaxies’ disk bending disks are dynamically very unusual, i. e.
produces all observed stellar radial profile types: profile type to its host halo’s total angular mo- they host stars on very eccentric orbits.
Observational motivation
down-bending (≈ 60%)
up-bending (≈ 30%)
Gravity + SPH simulations
Modified version of ChaNGa
Initial Conditions:
[mag arcsec-2]
pure exponential (≈ 10%)
Simulations
r,g
Isolated halos, (Mvir = 1012M)
Dark matter + spinning gas sphere in hydrostatic
equilibrium
Cosmologically motivated angular momentum
distribution [2]
Explore halo spin parameter range 0.02 < λ < 0.1.
R [arcsec]
R [arcsec]
R [arcsec]
Prototypes of observed disk galaxy surface brightness profiles (Source: [1]). Shown are a pure
exponential (left), a down-bending (middle) and an up-bending (right) profile.
Evolve for 8 Gyr
Results
λ = 0.06
104
102
λ = 0.03
R(t = 8 Gyr)
101
Rform
103
Σ? [M pc−2 ]
λ = 0.03
Where are stars in outskirts from? How did stellar orbits in
up-bending (low λ) disks evolve?
λ = 0.06
One individual star:
100
10−1
1.0
−2
10
0.8
10
M (R > Rb ) [106 M ]
log(Σ? )
0
5
10
15
R [kpc]
20
25
30
Stellar surface density profiles extracted from low
(bottom) to high (top) spin simulations. The profiles are
offset by 1 dex.
low spin
(λ . 0.03), profiles have up-bending break
intermediate spin
(λ ≈ 0.035 − 0.04), pure exponential profile
high spin
(λ & 0.045), profiles have down-bending
break
5
10
15
20
R [kpc]
25
30
What are the kinematics in the
outskirts?
15
0.4
0.2
0.0
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
20
−0.25
−0.30
−0.35
−0.40
−0.45
10
5
0
6.0
6.5
5
0.2
0.4
0.6
jz /jc (e)
0.8
1.0
circular
The circularity parameter distribution of stars in the
galaxy outskirts for different spin parameters.
high spin
Peaks at / 1 → circular orbits; consistent
with radial migration [3]
low spin
Peaks at 1 → eccentric orbits; flung out;
dynamically unusual
References
[1] Pohlen & Trujillo, 2006, A&A, 454, 759
[2] Bullock et al., 2001, ApJ, 555, 240
[3] Roškar et al., 2008, ApJL, 675, 65
Max-Planck-Institut für Astronomie, Königstuhl 17,
69117, Heidelberg, Germany
2
Research Informatics, Scientific IT Services, ETH Zurich,
Weinbergstrasse 11, 8092 Zurich, Switzerland
3
Astronomy Department, University of Washington, Box
351580, Seattle, WA 98195-1580, USA
4
Department of Physics and Astronomy, McMaster
University, Hamilton, Ontario L8S 4M1, Canada
1
0.6
15
λ = 0.02
λ = 0.03
λ = 0.04
λ = 0.05
λ = 0.06
10
0
0.0
radial
0.1
0.06
0.055
0.05
0.045
0.04
0.035
0.03
0.02
30 0
Moderate/significant radial mass
redistribution in high/low spin case
20
=
=
=
=
=
=
=
=
=
25
Final stellar surface density profile (black) vs. profile with
respect to position at birth of the respective stars (red).
Radial redistribution is much stronger in low spin
simulation (left panel).
8 Gyr
λ
λ
λ
λ
λ
λ
λ
λ
λ
15
20
R [kpc]
R [kpc]
How is stellar mass distributed?
5
jz [103 kpc km/s]
Mock observational images of the low (λ = 0.03, left) and
high spin (λ = 0.06, right) simulated galaxies. The low
spin galaxy appears smaller and more centrally
concentrated.
0
jz /jc (e)
10−3
10−4
λ = 0.02, star particle 2853077
e [−min (Φ)]
What do the disks look like?
7.0
Time [Gyr]
7.5
8.0
Gain energy (and angular momentum) at
pericenter
Gradually loose angular momentum while
away form pericenter
Angular momentum loss turns circular orbits
into eccentric orbits
Conclusion
Gravity + SPH simulations of Milky Way mass
galaxy formation
Disk shape correlates with initial halo spin λ
Low/high spin: up-/down-bending disks
high spin
Down-bending breaks previously explained:
Radial migration [3]
low spin
Only eccentric orbits in outskirts of
up-bending disks
Dynamically very unusual
Suggests fundamentally different
formation mechanism