Originally it was thought that all globular
clusters were part of the halo. Now, however, it is realized
that two distinct populations of globulars exist. Old,
metal-poor clusters ([Fe/H] < -0.8) are part of an extended,
spherical halo, while younger clusters with [Fe/H] > -0.8 are
in a more concentrated and flattened distribution.
The less-metal-poor clusters have a scale height similar to that of the thick disk, and they may be associated. Other ideas have them related to the Galaxy's bulge instead...
Globular clusters are generally old, with ages ranging from 10-14 billion years. However, there's still a lot of controversy about absolute ages here...
How do we find field halo stars? Look at the space velocities of stars with respect to the Sun. If they are low, they are probably disk stars (like the Sun). If they are high, they are usually associated with the halo.If we add up all the mass in the field stars and the metal-poor GCs, we can come up with a rough density distribution for the Galaxy's stellar halo:
Like the GCs, the halo field stars are also metal-poor. This tells us something about the formation of the Galaxy!
The total mass of the halo is about 108
- 109 M sun, about 1% of which is the
globular clusters, and the rest in field stars. So there's not a
lot of stuff in the stellar halo, but what is there holds a lot
of information about the early history of the Galaxy!
|The metallicity distribution of
field stars in the Milky Way's halo shows lots of very
metal poor stars, with an long tail to extremely metal
poor values of [Fe/H] < -3.5. (Figure taken
Spectra of extreme metal-poor stars (solar, -4, -5.3, zero metals), courtesy ESO:
Stars this chemically unenriched must trace the early history of star formation in the Galaxy!
The distribution of stars in the Milky Way's halo is
not smooth, but shows evidence for stellar streams.
Below: Star streams in the galactic halo from the SDSS
survey (Belokurov et al 2006). Upper main sequence/turnoff
stars have been selected via a color cut; these stars should
have similar luminosities, so their apparent magnitude is a
measure of distance. In this plot, color is not the color of
the star, but distance (blue nearer, red further).
On larger scales, these streams can be overlaid on 2MASS measurements of structure in the galactic halo, showing how the big SDSS stream connects with the larger Sagittarius stream detected by 2MASS: