GALACTIC STRUCTURE, STELLAR POPULATIONS ROSEMARY F.G. WYSE A stellar population will here mean a group of stars that is characterized by several properties. These properties are (i) its structure-the spatial distribution; (ii) the motions of its member stars (the kinematics)-both the mean streaming motions, which will essentially be rotation, because we assume the Galaxy is neither expanding nor contracting, and the random motions about these means, (iii) the chemical enrichment of its member stars-both the overall enrichment (metallicity) and the abundances of specific elements, such as oxygen and iron; (iv) the ages of the stars-both the age of the oldest stars and the spread in ages are important; and last (v) the formation mechanism-different physical processes may give rise to similar stellar properties. The Milky Way galaxy offers a unique opportunity to identify stellar populations and determine their characteristic properties, due to our ability to measure three-dimensional quantities in a model-independent way-we can analyze the light from individual stars, unlike studies of external galaxies where the properties of many stars, all those along the line-of-sight, must be analyzed. Most models of a protogalaxy envisage a mixture of gas and dark matter, with the gas being transformed into stars as the galaxy evolves. The dark matter is assumed present as a means of explaining the flat rotation curves of disk galaxies (constancy of rotational velocity with distance from the center). However, there is no generally accepted theory of galaxy formation. The more of the defining properties of a stellar population listed above that are known, the more one can infer about disk galaxy formation and evolution. The kinematic properties of stars in a galaxy are related to their spatial distribution through the properties of the gravitational potential of the galaxy. Most of the mass of galaxies is believed to be composed of dark matter, that is, objects that are not ordinary stars, and the gravitational potential is in general unknown, to be determined by analysis of the motions of test particles such as stars. The study of galactic structure is strongly motivated by this fact, given the uniqueness of our location within the Milky Way. The defining properties of stellar populations are dependent on the ratios of several timescales-the time for gas to be consumed through star formation; the time for gas to radiate binding energy and dissipate, sinking deeper into the galactic potential well; the collapse time of the initial protogalaxy under its self-gravity. These of course are not the only parameters that determine the nature of a stellar population and they are probably themselves controlled by initial conditions such as the density and angular momentum content of the protogalaxy, but they are the parameters that one can hope to constrain by study of stellar populations and galactic structure. CONCEPT The concept of stellar populations was essentially introduced by Walter Baade in 1944, when he succeeded in resolving into individual stars the central regions of our companion spiral galaxy the Andromeda nebula (M31), as well as its elliptical galaxy satellites M32 and NGC 205. The only information that Baade had available for these stars was their location in the color-magnitude (HR) diagram, since the available technology could not obtain kinematic data for stars in external galaxies and the theory of stellar nucleosynthesis had not yet been established for chemical abundance data to be meaningful. Thus his sole criterion for the assignment of systems of stars to different populations was their HR diagrams. This limited information led to the simplest scheme-Population I stars were those whose HR diagram resembled that of the open clusters in the disk of our galaxy, whereas the red giant branch of Population II stars occupied the same locus in the HR diagram as the galactic globular cluster giants. Baade assigned those stars that lie outside the thin disks of spiral galaxies-in globular clusters, elliptical galaxies, and the stellar halo components of disk galaxies, to Population II. The kinematics of disk stars in our galaxy had earlier led Bertil Lindblad to model the disk as consisting of a superposition of many subsystems with different mean rotational velocities and random velocities, the subsystems becoming rounder in shape as the mean rotation velocity decreased. These ideas were combined with the newly understood concept of chemical evolution at the 1957 Vatican Symposium, where a scheme of stellar populations was agreed upon that included as discrete classes, between those of Baade, Intermediate Population II and Disk Population. The term Population II became commonly understood to refer to old, metal-poor stars on low-angular-momentum orbits. This followed from the seminal work of Olin Eggen, Donald Lynden-Bell, and Allan Sandage in 1962, whose detailed observations of the galactic subdwarf stars, observed passing through the solar neighborhood at high velocity, revealed them to be metal poor and members of a system with little or no net rotation, but with large amplitude random motions. This work provided evidence for the existence of a smooth relationship between kinematics and chemistry from thin disk stars through to the extreme subdwarfs. Assuming that the chemical enrichment of a star is a monotonic and universal function of time alone, as thought in the 1960s, implies that the properties of stellar populations are fully determined by the epoch at which those stars formed. The interpretation-and indeed reality-of this correlation has been debated over the years. THREE POPULATIONS* Modern data have revealed, perhaps not surprisingly, that the Galaxy is a complicated system and much current research aims to understand it. We now know that the different stellar populations are not a one-parameter sequence, as had been suggested by the smooth correlations that were evident between kinematics and chemistry, and between chemistry and time. Scatter and discontinuities in such relationships may be indicative of such exotic phenomena as mergers between our galaxy and another. There are most probably three major stellar populations in the Galaxy. This is a controversial statement. In order of decreasing total mass, these stellar populations are the thin disk, the thick disk, and the stellar halo. The characteristic properties of these components are as follows. THIN DISK The radial light distribution follows an exponential law, so that the intensity bas fallen from its central value by a factor of * by *4 kpc from the galactic enter. The scale height of the thin disk is <350 pc (for the oldest stars of this component, which contain most of the mass, and less for the youngest stars) resulting in a very flattened-hence, thin-spatial distribution. The total blue-band luminosity is *10*** solar luminosities (L*). This component contains about 10% by mass of gas, with the gas fraction being an increasing function of galactocentric radius, equaling about *** at the Solar distance. The Sun is a typical youngish thin-disk star and moves in an approximately circular orbit, with rotational velocity around the Galaxy of *220 km/sec. The Sun is -******yr old, and there exist stars being born today; the age of the oldest thin disk star is of great interest and (thus!) is very controversial; an age of 12x**** yr has recently been obtained for a cluster of stars in the thin disk. The older thin-disk stars have higher random motions, lower rotational velocities, and lower metallicities than the younger stars, on average. The existence of inherent scatter in the age-metallicity relationship for disk stars has only recently been unequivocally established, although its presence was suspected in earlier samples; scatter is expected in many theories of disk evolution and its amplitude is obviously a crucial constraint. The presence of radial metallicity gradients, with the outer regions of the disk being of lower metallicity than the solar neighborhood at the same age, must all be predicted by successful theories. THICK DISK This stellar component was first detected in star counts towards the south galactic pole, subsequent to the discovery of thick disks in external spiral galaxies. The existence of the thick disk was controversial in the early 1980s, when only star-count data were available. Spectroscopic data have provided indisputable evidence, but the origins and evolutionary status of the thick disk remain subject to debate. The thick disk has a scale height of -**kpc, at least at the solar radius, and of the order of 2% of local stars belong to this component (the other -98% being members of the thin disk, with the stellar halo accounting for merely a fraction of a percent). The total luminosity of the thick disk can at present best be constrained by analogy with external galaxies. The implications are that the thick disk dominates over the stellar halo out to many kiloparsecs away from the plane, and probably has a total luminosity several times that of the metal-poor stellar halo, or a few x*****L*. The detailed values of the descriptive parameters remain poorly determined, however, and await the large statistical spectral surveys currently being completed. The corresponding vertical velocity dispersion for a scale height of *****************, has now been observed in many samples. Typical thick disk stars appear to be on high-angular-momentum orbits lagging behind the Sun by only ********. The thick disk is apparently kinematically distinct from the sub-dwarf system to an adequate approximation. The thick disk is probably kinematically distinct from the galactic old thin disk, though the data remain inadequate for robust conclusions. This is obviously an extremely important point, with major ramifications for the formation mechanism of the thick disk. The mean metallicity of thick disk stars, which dominate 1-2 kpc above the galactic plane, is about one-quarter of the solar metallicity. Provided they are present in sufficient numbers, these stars have a major effect on the chemical evolution of the thin disk. In particular, they can provide a simple, self-consistent solution to the G-dwarf problem, which is the observed paucity of metal-poor G-dwarfs in the solar neighborhood, relative to the predictions of the most naive model of chemical evolution. Since G-dwarfs live for of order of the age of the Galaxy, all G-dwarfs ever born should still be around today, and counts of G-dwarfs in a representative volume will constrain the integrated chemical evolution of that volume. One of the complications of the G-dwarf problem is that it is easy to think of solutions by adopting more sophisticated models. The discovery of the thick disk allows one to retain simplicity by the identification of the thick disk as the reservoir for the missing metal-poor stars. These stars were not found in the earlier small surveys of stars in the solar neighborhood, most probably because of their relatively large scale height and low local space-density normalization. At least the metal-poor tail of the thick disk is comparable in age to the younger members of globular cluster system, ********** yr, leaving little room for a hiatus in star formation between the formation of the stellar halo and the formation of a disk, as envisaged in some models of galaxy formation. The age spread is unknown at present, although some results imply that there exist thick disk stars as young as the oldest thin disk stars. STELLAR HALO We assume that the stellar halo is represented in the solar neighborhood by the high-velocity, metal-poor subdwarfs, the classical (Extreme) Population II of Baade (there are metal-rich stars in the nuclear bulge which may pose a problem). As mentioned above, only **0.1% of local stars belong to the stellar halo, which has total luminosity 10**L*. This component apparently follows a de Vaucouleurs light profile with half of its light being contained within a radius of 3 kpc. The central metal-rich bulge, as detected in the infrared, is more centrally concentrated. The shape of the stellar halo is important for its implications for the early stages of galaxy collapse and star formation, the interpretation of the kinematics of high-velocity stars, and the shape of the underlying dark matter that generates the gravitational potential in which these stars move. The kinematics of the subdwarfs leads to the expectation of a flattened shape, although the classical Population II was round. A recent determination of the shape of the stellar halo from stars counts has reconciled expectation and reality, however, deriving an axis ratio of approximately 2:1. The mean metallicity of the subdwarfs is well established at around ** of the solar value. Several elements are enhanced, when normalized to iron and compared with the solar ratio, with, in particular, oxygen being overabundant by a factor of -3. Analysis of this element ratio, as a function of iron abundance, can lead to constraints on the time scale of stellar halo formation, with the result that a metallicity of * solar was reached in the relatively short time of ***** yr. Indeed, recent analyses of certain unusual products of stellar nucleosynthesis have allowed an estimate that it only took ***** yr to enrich up to * of solar metallicity. These results imply rapid star formation-remember that the freefall collapse time of a typical protogalaxy is ******** yr, which is a lower limit to the possible collapse time of the Galaxy, if it formed in a coherent collapse. A rapid collapse was proposed by Eggen, Lynden-Bell, and Sandage, fully 25 years prior to the inferences from studies of the elemental abundances. Additional Reading Eggen O.J., Lynden-Bell, D., and Sandage, A.(1962). Evidence from the motions of old stars that the Galaxy collapsed. Ap. J. 136 748. Gilmore, G., Wyse, R.F.G., and Kuijken, K.(1989). Kinematics, chemistry and structure of the Galaxy. Ann. Rev. Astron. 27 555. Norman, C.A., Renzini, A., and Tosi, M., eds.(1986). Stellar Populations. Cambridge University Press, Cambridge. O'Connell, D.J.K., ed.(1958). Stellar Populations (The Vatican Symposium 1957). North-Holland, Amsterdam. See also Galactic Structure, Large Scale; Galaxies, Formation; Galaxy, Dynamical Models.