Copyright © 1996 by Annual Reviews. All rights reserved
|
| Annu. Rev. Astron. Astrophys. 1996. 34:
511-550 Copyright © 1996 by Annual Reviews. All rights reserved |
2.1. Definition and Nomenclature
The population that we are interested
in describing here should more rightly be called the oldest observable
population
or the first substantial population in the Large and Small Magellanic Clouds
(we use MC when referring to both Clouds). If there were a small tail of
stars more metal poor than [Fe/H] = -3.0
in the MCs, therefore presumably older than the first major population, it
would be impossible to detect it given currently available sample sizes,
just as stars in the Milky Way halo with [Fe/H] < -3.0 have only been
found by heroic efforts to search large numbers of Galactic halo stars
(e.g.
Beers et al 1992).
We do not call the oldest major population in the MC "Population II" or the
"halo population," as these words have connotations of abundance, age, and
kinematics that may be inappropriate for the oldest MC populations. The word
"halo" is used when these connotations are intended.
In this section we concentrate on describing the properties
of the oldest population, including the mean age and kinematics. It is
important to determine with certainty whether this "ancient" population
exists in either
Cloud or if the first dominant population is demonstrably younger in those
galaxies than in the Milky Way. By an ancient population we mean a
population coeval with the oldest globular clusters in the Milky Way,
currently believed to be ~
15 Gyr old. We acknowledge that the age scale may change in the future, but
we believe that the age ranking of populations within the two Magellanic
Clouds and relative to the Milky Way globulars will remain secure.