A Galactic halo road map

The halo stars : where, whither, whence?

Chris Thom, Jyrki Hänninen, Johan Holmberg, Chris Flynn

Tuorla ObservatorySwinburne University of Technology

The Milky Way IDisk, bulge and haloDisk, 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 haloStellar 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 aroundthe Galactic center.

Old: amongst the first stars born in the Galaxy.

The halo's kinematics IDisk 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 1990Chiba and Beers 2000Gould 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 giantsRR Lyrae stars

BHB stars

Probing the distant halo II

Flynn, Sommer-Larsen, Christensen 1996used 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

Near sun: km/sOuter 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

Not the world's greatest fits!The model...

6dF

1.2 m UK Schmidt telescope6dF fibre fed spectrograph150 objects observable simultaneously6 degree field

6dFgalaxies

The main 6dF surveyis aimed at cosmic structure.

Program aim:180,000 2MASS selected (K band)galaxies.

They do stars too...

Chris Thom's thesis work

Hamburg/ESO survey(faint blue objects).

600 new BHB stars.

6dF spectra fromUK Schmidt forvelocities.

Mainly towardinner halo.

10 kpc distant.

Sloan Digital Sky Survey

Dedicated 2.5 m wide FOV telescopeFibre fed spectrograph640 objects simultaneouslyQuarter sky to be surveyedAiming for 1,000,000 galaxian redshifts

Stars too...

Selection by Sloan colours

1170 new BHB stars.

Sloan velocities

Fields mainly towardouter halo.

20-100 kpc distant.

Stellar program as well ... 1170 BHB starsnow released by Sirko et al 2003

The 3D view

(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

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 modelsare equally mediocre fits to the data...

LOS velocity dispersion

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

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 summaryTwo 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 IWe 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 IITime Gyr

15

0

5

10

Initially weird

Stays nicely weird!

Halo road maps IIITime Gyr

15

0

5

10

Initially isothermal

Stays quite isothermal

Conclusions

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