Populations & Components of the Milky Way

Astronomy 626: Spring 1995

Recognition of Stellar Populations

Resolution of M31 enabled Baade (1944) to discern two distinct populations of stars. The disk of M31 yielded HR diagrams like those of galactic open clusters, while the bulge yielded HR diagrams like those of globular clusters. These two categories were adopted as the archetypical examples of Populations I and II, respectively.

The idea of stellar populations, each characterized by a distinct spatial distribution, kinematics, and metal content, proved to be a key concept in interpreting observations of our own galaxy.

Characteristics of Standard Populations

Increasingly detailed studies of the Milky Way, along with a growing understanding of `chemical' evolution, culminated in 1957 Vatican Symposium, which legitimized an extended version of Baade's system including several intermediate populations. These populations were viewed as a continuous sequence, in accord with the idea that the Milky Way formed from the coherent collapse of a slowly-rotating gas cloud (Eggen et al. 1962). Thus the oldest Pop. II stars were taken to define a nearly-spherical, slowly-rotating halo, while younger populations defined flatter and more rapidly rotating distributions, blending smoothly from spheroid to disk.

The idea of continuous transitions between populations, although appealingly simple, has come under considerable revision in the light of new evidence. In particular, the realization that the central bulge is metal-rich challenges the one-dimensional taxonomy of traditional stellar populations. At present the galaxy is thought to contain two or three luminous populations (e.g. Wyse 1992). The thin disk and stellar halo correspond to Baade's Pop. I and II, respectively. Still under debate is the existence of a thick disk population which might correspond to the thick disks seen in some other disk galaxies.

Thin disk (Population I)

The thin disk of the Milky Way has sustained ongoing star formation for ~10^10 years. Consequently it contains stars with a wide range of ages, and the thin disk may be divided into a series of sub-populations of increasing age.

The spiral-arm population is the youngest in the disk; it appears to trace the spiral pattern of the Milky Way. This population is concentrated very close to the disk plane, with a scale height of ~100 pc. Representative objects include H I and molecular clouds, H II regions, protostars, stars of types O & B, supergiants and classical cepheids. The metallicity of this population is somewhat higher than that of the Sun (MB81).

Attempts have been made to reconstruct the large-scale distribution of the H I from 21-cm observations. The radial distribution is less centrally concentrated than the disk stars, and the inner ~3 kpc are almost free of neutral hydrogen (MB81); thus the Milky Way is one of those galaxies with a central hole in H I. It is now realized that non-circular motions seriously confuse `galactic velocity tomography'.

The disk population proper is more smoothly distributed and does not seem to trace the spiral structure. This population may be further subdivided into young, intermediate, and older categories, with ages of ~1, ~5, and ~10*10^9 years, respectively (MB81). The characteristic scale-height of this population increases with age, ranging from ~200 to ~700 pc, while the metallicity declines to perhaps ~20% of the solar value. Representative objects include stars of type A and later, planetary nebulae, and white dwarfs.

Stellar halo (Population II)

The stellar halo of the Milky Way includes the system of globular clusters, metal-poor high-velocity stars in the solar neighborhood, and metal-rich dwarf stars seen toward the galactic center. While star formation in the outer halo largely ceased more than 10^10 years ago, the situation in the inner kpc of the galaxy is not so clear.

Globular clusters are the classic tracers of the galactic halo; their spatial distribution provided the first real clues of the true size and shape of the galaxy. The metal-poor clusters have a nearly-spherical distribution extending to many times the Sun's distance from the galactic center, while the metal-rich clusters are concentrated towards the center of the galaxy and may have a more flattened distribution (Harris 1976).

Metal-poor subdwarfs in the solar neighborhood have large velocities with respect to the Sun and other disk stars. These stars are on highly eccentric orbits about the galactic center; the net rotation of this population is amounts to less than ~40 km/sec, while their random motions are quite large. The metallicity of these stars ranges from ~0.1 to ~10% of solar (MB81). RR Lyrae variables are useful in tracing the large-scale distribution of this population because they can be identified by their characteristic light variation throughout the stellar halo.

The shape of the metal-poor halo population represents something of a puzzle. Direct star-counts indicate that the halo is only mildly flattened, with an axial ratio c/a > ~0.6; the degree of flattening seems to decrease with distance from the center of the galaxy. On the other hand, the nearby metal-poor subdwarfs have an anisotropic velocity distribution, with sigma_r/sigma_z = ~2, so this population is expected to have c/a < 0.4 or less (Gilmore et al. 1989).

Metal-rich subdwarfs in the central bulge of the galaxy are observed through Baade's window and other regions of low absorption. These stars span a wide range of metallicity, from ~0.1 to > 100% of solar (Searle & Zinn 1978). The inner kpc of the bulge also appears to contain stars of type A, implying that some star formation has occurred rather recently (Gilmore et al. 1989).

The density profile of the inner stellar halo has been measured from counts of RR Lyrae variables (Oort & Plaut 1975). A power-law with rho(R) ~ R^-3 matches the observations quite well. A similar power law, with a slightly steeper slope, also matches the density profile of the outer halo as traced by the globular clusters (Harris 1976).

Thick disk (Intermediate Population II)

The existence of an intermediate population was explicitly recognized at the Vatican Symposium. Representative objects of this population include Mira variables with periods of 150 to 200 days and RR Lyrae variables with metallicities > 10% of solar (Gilmore et al. 1989).

Star-counts suggest that this intermediate population is distributed in a thickened disk with a scale height of ~1 to 1.5 kpc. This population accounts for only ~1% of the stars in the vicinity of the sun but dominates the high-altitude tail of the thin disk population at z > 1 kpc.

The true nature of this stellar population is not completely clear; although it was originally classified as Pop. II (halo), it is much flatter than any halo population observed at the solar radius. Kinematic studies imply that the thick disk rotates with a velocity of ~180 km/s (Gilmore et al. 1989), compared to the less than 40 km/s rotation of the metal-poor subdwarfs. This would indicate that the thick disk is more closely associated with the thin disk population, which rotates at ~220 km/s. Metallicity measurements also support the idea that the thick disk is distinct from the stellar halo; the characteristic metal abundance of thick disk stars is ~25% of solar, while the stellar halo is significantly more metal-poor in the solar neighborhood.

It is less obvious if the thick disk is distinct from the thin disk; in many respects it appears to represent a continuation of the trends of metallicity, velocity dispersion, and scale height with age that we see in the thin disk. On the other hand, the velocity dispersion and scale height of the thick disk are significantly greater than even the oldest thin disk sub-population, suggesting that some discontinuity might occur between these groups.