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