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A Galactic halo road map
The halo stars :
where, whither, whence?
Chris Thom, Jyrki Hänninen,
Johan Holmberg, Chris Flynn
Tuorla Observatory
Swinburne University of Technology
The Milky Way I
Disk, bulge and halo
Disk and bulge
dominate emission
at all wavelengths
The Milky Way II
Visible mass
Disk+bulge 99%
Stellar Halo 1%
Stellar halo : about 1% globular clusters and 99% stars.
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.
The halo's kinematics I
Disk star orbits
Halo star orbits
Kinematics balances gravity
Disk star orbits
velocity translates directly to rotation support
Vrot ~ 200 km/s
Halo star orbits
random velocities provide pressure support
Vrandom ~ 200 km/s
Structure in Andromeda's halo
Spur of stars in M31's halo (Irwin et al 2001)
Stellar halos are probably built up from fragments.
There be fragments yet...
The Milky Way has fragments breaking up even now
Sagittarius Dwarf
Globular cluster tidal tails
Canis Major Dwarf
The
Magellanic
stream
The inner halo
On phase mixing
After dozens of orbits
around the Galaxy, structure
in coordinate and velocity space
becomes smoothed out.
Stars are said to be well mixed in phase space.
The Galactic Mixmaster
The stellar halo
Inner regions :
probably nicely mixed.
Outer regions :
probably still a little lumpy.
Dissolving satellites and stellar streams
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.
On halo stars nearby II
Nearby halo stars appear well mixed.
CDM models predict a few hundred
streams in the solar neighbourhood.
Gould (2003) analysed 4500 motions
of nearby halo stars.
No strong evidence for any
streams in the inner halo.
50 dwarf galaxies accreted over 10 Gyr : sim by Paul Harding
Probing the distant halo I
Useful probes :
G and K giants
RR Lyrae stars
BHB stars
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
one-by-one.
Developed weird model
of halo to fit.
Weird science
Very peculiar kinematics found
in halo:
Near sun:  km/s
Outer halo: km/s
Orbits of stars very different in outer halo (round)
compared to near Sun (elliptical).
Weird science II
The model...
Not the world's greatest fits!
6dF
1.2 m UK Schmidt telescope
6dF fibre fed spectrograph
150 objects observable simultaneously
6 degree field
6dFgalaxies
The main 6dF survey
is aimed at cosmic
structure.
Program aim:
180,000 2MASS
selected (K band)
galaxies.
They do stars too...
Hamburg/ESO survey
(faint blue objects).
600 new BHB stars.
6dF spectra from
UK Schmidt for
velocities.
Mainly toward
inner halo.
10 kpc distant.
Chris Thom's thesis work
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
Stars too...
Selection by Sloan colours
1170 new BHB stars.
Sloan velocities
Fields mainly toward
outer halo.
20-100 kpc distant.
Stellar program as well ... 1170 BHB stars
now released by Sirko et al 2003
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)
1170 BHB stars (Sirko)
/home/cflynn/helio/3dview
Weird science III
As we'll see,
with the new data,
the weird model
predictions
weren't too bad!
Weird science IV
But...
a constant velocity
dispersion 'model'
fits too!
Actually, both models
are equally mediocre
fits to the data...
LOS velocity dispersion
Model predictions verus observations in 6 different fields
LOS
Thom et al 2004 data
km/s
Distance along line of sight from Sun (kpc)
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)
Model summary
Two models of the kinematics
of halo stars in the literature.
Flynn et al 1996, Sirko et al 2003.
Circa 2000 distant halo stars, and
2000 very nearby halo stars as
model constraints.
Our model is physical, the Sirko 'model' phenomological.
The physical model fits everywhere, the other only far from the Sun.
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 disk+bulge+dark halo
Power law density distribution, velocities as  = (R,
Halo road maps II
Time Gyr
Initially
weird
0
5
10
Stays nicely
weird!
15
Halo road maps III
Time Gyr
Initially
isothermal
0
5
10
Stays quite
isothermal
15
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 promising test