A Galactic halo road map

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A Galactic halo road map
The halo stars where, whither, whence?
Chris Thom, Jyrki Hänninen, Johan Holmberg, Chri
s Flynn Tuorla Observatory Swinburne Universit
y of Technology
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The Milky Way I
Disk, bulge and halo
Disk, bulge and halo
Disk and bulge dominate emission at all wavele
ngths
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The Milky Way II
Visible mass Diskbulge 99 Stellar Halo
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Stellar halo about 1 globular clusters and 99
stars.
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The Milky Way's halo
Stellar halo about 1 globular clusters and 99
stars. Mainly metal poor stars less than 10 of
Solar metallicity. Large random motions and
little net rotation around the Galactic center.
Old amongst the first stars born in the Galaxy.

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The halo's kinematics I
Disk star orbits
Halo star orbits
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Kinematics balances gravity
Disk star orbits velocity translates d
irectly to rotation support Vrot 200
km/s
Halo star orbits random velocities pro
vide pressure support
Vrandom 200 km/s
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Structure in Andromeda's halo
Spur of stars in M31's halo (Irwin et al 2001)
Stellar halos are probably built up from
fragments.
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There be fragments yet...
The Milky Way has fragments breaking up even now
Sagittarius Dwarf
Canis Major Dwarf
The Magellanic stream
Globular cluster tidal tails
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The inner halo
On phase mixing After dozens of orbits aro
und the Galaxy, structure in coordinate and velo
city space becomes smoothed out. Stars are sa
id to be well mixed in phase space.
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The Galactic Mixmaster
The stellar halo Inner regions probably ni
cely mixed. Outer regions probably st
ill a little lumpy.

Dissolving satellites and stellar streams
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On halo stars nearby I
Components of the velocity dispersion
???????? (?R,????????? ??????????????????????
??????????????????????km/s Morrison, Flynn and
Freeman 1990 Chiba and Beers 2000 Gould 2003
Determined from 1000 halo stars found
over many decades in searches out to about 1 kpc.
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On halo stars nearby II
Nearby halo stars appear well mixed.
CDM models predict a few hundred streams in th
e solar neighbourhood. Gould (2003) analysed 4
500 motions of nearby halo stars. No strong e
vidence for any streams in the inner halo.
50 dwarf galaxies accreted over 10 Gyr sim by
Paul Harding
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Distant halo stars tell about dark matter and
the formation of the galaxy

Probing the distant halo I
Useful probes G and K giants RR Lyrae sta
rs
BHB stars
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Distant halo stars tell about dark matter and
the formation of the galaxy

Probing the distant halo II
Flynn, Sommer-Larsen, Christensen 1996 used BHB
stars to probe halo. Found 100 (!) stars up to
50 kpc distant. Measured radial velocities o
ne-by-one. Developed weird model of halo to f
it.
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Distant halo stars tell about dark matter and
the formation of the galaxy

Weird science
Very peculiar kinematics found in halo
Near sun ????? ??????????????????km/s Outer
halo??? ??????????????????km/s
Orbits of stars very different in outer halo (ro
und) compared to near Sun (elliptical).
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Distant halo stars tell about dark matter and
the formation of the galaxy

Weird science II
The model...
Not the world's greatest fits!
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Distant halo stars tell about dark matter and
the formation of the galaxy

6dF
1.2 m UK Schmidt telescope 6dF fibre fed spectrog
raph 150 objects observable simultaneously 6 deg
ree field
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Distant halo stars tell about dark matter and
the formation of the galaxy

6dFgalaxies
The main 6dF survey is aimed at cosmic struct
ure. Program aim 180,000 2MASS selected (K
band)
galaxies.
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Distant halo stars tell about dark matter and
the formation of the galaxy

They do stars too...
Hamburg/ESO survey (faint blue objects). 600
new BHB stars. 6dF spectra from UK Schmidt f
or velocities. Mainly toward inner halo. 1
0 kpc distant.
Chris Thom's thesis work
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Distant halo stars tell about dark matter and
the formation of the galaxy

Sloan Digital Sky Survey
Dedicated 2.5 m wide FOV telescope
Fibre fed spectrograph 640 objects simultaneously
Quarter sky to be surveyed Aiming for 1,000,000
galaxian redshifts
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Distant halo stars tell about dark matter and
the formation of the galaxy

Stars too...
Selection by Sloan colours 1170 new BHB star
s. Sloan velocities Fields mainly toward ou
ter halo.
20-100 kpc distant.
Stellar program as well ... 1170 BHB stars
now released by Sirko et al 2003
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Distant halo stars tell about dark matter and
the formation of the galaxy

The 3D view
1770 stars in many directions (now) compared to
100 stars in 4 directions (1996)

(X,Y,Z) coordinates 600 BHB stars (Thom) 1
170 BHB stars (Sirko) /home/cflynn/heli
o/3dview
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Distant halo stars tell about dark matter and
the formation of the galaxy

Weird science III
As we'll see, with the new data, the weird model

predictions weren't too bad!
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Distant halo stars tell about dark matter and
the formation of the galaxy

Weird science IV
But... a constant velocity dispersion 'model
' fits too! Actually, both models are equall
y mediocre
fits to the data...
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Distant halo stars tell about dark matter and
the formation of the galaxy

LOS velocity dispersion
Model predictions verus observations in 6
different fields
?LOS km/s
Thom et al 2004 data
Distance along line of sight from Sun (kpc)
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Distant halo stars tell about dark matter and
the formation of the galaxy

LOS velocity dispersion
Model predictions verus observations in 12
different fields
?LOS km/s
Sirko et al 2003 data
Distance along line of sight from Sun (kpc)
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Model summary
Two models of the kinematics of halo stars in th
e literature. Flynn et al 1996, Sirko et al 200
3. Circa 2000 distant halo stars, and 2000 ve
ry nearby halo stars as model constraints.
Our model is physical, the Sirko 'model'
phenomological. The physical model fits every
where, the other only far from the Sun.
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Distant halo stars tell about dark matter and
the formation of the galaxy

Halo road maps I
We can set up test particles following the model,
and integrate their orbits for a Hubble time in
the Galactic gravitational potential
N 50,000 massless stars in potential due to di
skbulgedark halo Power law density distributi
on, velocities as ? (?R,?????????
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Distant halo stars tell about dark matter and
the formation of the galaxy

Halo road maps II
Time Gyr
Initially weird
0
5
10
15
Stays nicely weird!
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Distant halo stars tell about dark matter and
the formation of the galaxy

Halo road maps III
Time Gyr
Initially isothermal
0
5
10
15
Stays quite isothermal
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Distant halo stars tell about dark matter and
the formation of the galaxy

Conclusions
Whence and wither the halo stars?
Inner halo well mixed, stars on elliptical
orbits
Outer halo stars on roundish orbits?
State of play
Finally, there is plenty of data the models are
really naïve Inner halo orbits by far most prom
ising test