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HALO7D Disentangling the Milky Way’s Accretion History
Emily Cunningham UC Santa Cruz
Alis Deason, Raja Guhathakurta, Connie Rockosi, Zach Jennings, Evan Kirby, Jay Anderson, Tony Sohn, Roeland Van der Marel
Stellar Halos Across the Cosmos Heidelberg July 2, 2018
Gaia is transforming Milky Way science! But…still missing kinematics of distant
main-sequence stars!
Giants are bright, but rare!
Biased populations?
MS stars are faint, but numerous!
All stellar populations have MS stars!
0.0 0.2 0.4 0.6 0.8(gP1 − iP1)0
22
21
20
19
18
17
16
(i P1) 0
31
40
46
(a)
[Fe/
H] =
−2.
23[F
e/H
] = −
1.23
5 0 −5∆RA (arcmin)
−5
0
5
∆D
ec (a
rcm
in)
(b)
not observednon−membermember
not observednon−membermember
Kirby et al. 2017
Triangulum II: UFD
HALO7DKeck/DEIMOS
Spectroscopy + Hubble Proper Motions
of distant (10-100 kpc) MSTO MW halo stars.
HALO7D selection box
This dataset is unique even in the era of Gaia
Requires deep spectroscopy—
8-24 hour integrations!
Faintest sample of stars with
3D kinematics until LSST!
CANDELS Fields
High latitude fields (|b| ~ 42-60 deg)
Characterized by many deep, multi-epoch HST
pointings.
Largest area HST proper motion program
to date!
First HALO7D Results
Two Papers Coming Soon!
HALO7D I. Line of Sight Velocity Distributions
HALO7D II: Velocity Ellipsoid and Anisotropy
First HALO7D Results
Two Papers Coming Soon!
HALO7D I. Line of Sight Velocity Distributions
HALO7D II: Velocity Ellipsoid and Anisotropy
LOS Velocity Distributions
Model the LOS distributions as mixture of disk and halo
Free Parameters: fdisk: fraction of disk
contamination <vLOS>: mean halo LOS
velocity σLOS: halo LOS velocity
dispersion
Cunningham et al. (2018a)
LOS dispersions are consistent across the 4 fields.
LOS Velocity Distributions
Cunningham et al. (2018a)
LOS Velocity Distributions: Bullock & Johnston Simulations
Cunningham et al. (2018a)
LOS distributions are hot and consistent when material from
the same massive accretion event(s) dominate all four
sightlines.
LOS distributions with strong cold peaks are dominated by
lower mass, recent accretion events
We “observed” the BJ05 halos with Galaxia, observing 1 square degree fields centered on the coordinates of the four HALO7D
fields.
First HALO7D Results
HALO7D I. Line of Sight Velocity Distributions • LOS Velocity distributions are consistent across fields; no field
dominated by cold substructure • Comparison w/ simulations indicates this could be evidence that
our sample are dominated by the same, relatively early massive accretion event(s)
HALO7D II: Velocity Ellipsoid and Anisotropy
First HALO7D Results
HALO7D I. Line of Sight Velocity Distributions • LOS Velocity distributions are consistent across fields; no field
dominated by cold substructure • Comparison w/ simulations indicates this could be evidence that
our sample are dominated by the same, relatively early massive accretion event(s)
HALO7D II: Velocity Ellipsoid and Anisotropy
3D Kinematic Sample
Cunningham et al. (2018b)
3D Model
vφ
v⍬
vr
We can use proper motions + LOS velocity to measure the parameters of the velocity ellipsoid:
<vφ>, σφ, σr, σ⍬
Assumptions: • Velocity distributions are
Gaussian and independent • <v⍬>=<vr>=0 • Distance to every star is a free
parameter, with prior based on isochrones
• Population is a mixture of disk and halo, with disk model parameters fixed
3D Kinematics reveal the field-to-field variation!
His
togr
am o
f Po
ster
ior
Sam
ples
Cunningham et al. (2018b)
3D Kinematics
3D kinematics reveal the field-to-field variation! Evidence for recent accretion?
His
togr
am o
f Po
ster
ior
Sam
ples
β =1−σφ2 +σθ
2
2σ r2
RadialIsotropicTangential
Cunningham et al. (2018b)
Spherically Averaged
Cunningham et al. (2018b)
We estimate the parameters of the velocity ellipsoid using the
full HALO7D sample, and split the sample into three apparent
magnitude bins.
3D Kinematics
Spherically averaged estimates of β are consistent!
His
togr
am o
f Po
ster
ior
Sam
ples
β =1−σφ2 +σθ
2
2σ r2
RadialIsotropicTangential
Cunningham et al. (2018b)
3D Kinematics
Cunningham et al. (2018b)
β Variations in Latte
Figures courtesy of Robyn Sanderson
50 kpc < D < 70 kpc
Latte Macchiato
Beta varies across the sky and across halos!
β
M (< r ) = rσ r2
Gd lnρd ln r
−d lnσ r
2
d ln r− 2β
⎛
⎝⎜
⎞
⎠⎟
Does it make sense to assign β(r) a single value? Should we be looking beyond Jeans modeling?
ConclusionsHALO7D I. Line of Sight Velocity Distributions
• LOS Velocity distributions are consistent across fields; no field dominated by cold substructure
• Comparison w/ simulations indicates this could be evidence that our sample are dominated by the same, relatively early massive accretion event(s)
HALO7D II: Velocity Ellipsoid and Anisotropy • Spherically averaged estimates of Beta are consistent (~0.5),
though individual fields show variation in their ellipsoid parameter estimates.
Looking to the future: abundances!! (UCSC Grad Kevin McKinnon)
Modeling the Distances
Deason, van der Marel, Guhathakurta et al. 2013
Deason, van der Marel, Guhathakurta et al. 2013
Modeling the Distances
Modeling the Distances
Deason, van der Marel, Guhathakurta et al. 2013