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 Distant halo stars tell about dark matter and the formation of the galaxy Useful probes : G and K giants RR Lyrae stars BHB stars

Probing the distant halo II Distant halo stars tell about dark matter and the formation of the galaxy 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 Distant halo stars tell about dark matter and the formation of the galaxy Near sun:   km/s Outer halo:   km/s Orbits of stars very different in outer halo (round) compared to near Sun (elliptical). Very peculiar kinematics found in halo:

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

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

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

They do stars too... Distant halo stars tell about dark matter and the formation of the galaxy Chris Thom's thesis work Hamburg/ESO survey (faint blue objects). 600 new BHB stars. 6dF spectra from UK Schmidt for velocities. Mainly toward inner halo. 10 kpc distant.

Sloan Digital Sky Survey Distant halo stars tell about dark matter and the formation of the galaxy 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... Distant halo stars tell about dark matter and the formation of the galaxy Selection by Sloan colours 1170 new BHB stars. Sloan velocities Fields mainly toward outer halo kpc distant. Stellar program as well BHB stars now released by Sirko et al 2003

The 3D view Distant halo stars tell about dark matter and the formation of the galaxy (X,Y,Z) coordinates 600 BHB stars (Thom) 1170 BHB stars (Sirko) /home/cflynn/helio/3dview 1770 stars in many directions (now) compared to 100 stars in 4 directions (1996)

Weird science III Distant halo stars tell about dark matter and the formation of the galaxy As we'll see, with the new data, the weird model predictions weren't too bad!

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

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

LOS velocity dispersion Distant halo stars tell about dark matter and the formation of the galaxy  LOS km/s Distance along line of sight from Sun (kpc) Model predictions verus observations in 12 different fields Sirko et al 2003 data

Model summary Two models of the kinematics of halo stars in the literature. Flynn et al 1996, Sirko et al 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 Distant halo stars tell about dark matter and the formation of the galaxy 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 Distant halo stars tell about dark matter and the formation of the galaxy Time Gyr Initially weird Stays nicely weird!

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

Conclusions Distant halo stars tell about dark matter and the formation of the galaxy Inner halo : well mixed, stars on elliptical orbits Outer halo: stars on roundish orbits? Finally, there is plenty of data; the models are really naïve Inner halo orbits by far most promising test Whence and wither the halo stars? State of play