Formation of the Milky Way and Its Neighbors

The study of the stars in the Milky Way (fig. 7) is a rich subject capable of revealing not only the physical structure of the Galaxy that we live in but also critical clues to its formation and evolutionary history. The current picture of galaxy formation favors the formation of large galaxies through hierarchical merging of smaller entities (fig. 8). Detailed studies of Galactic structure provide a unique context in which to test such theories and to probe the star formation and chemical enrichment history of galaxies, since in no other galaxy can we study in such detail the ages, kinematics, and metallicities of individual stars, which constitute the fossil record of galactic evolution. The Galaxy also provides a rich hunting ground for rare stellar populations of great astrophysical interest. For example, the ages of the oldest white dwarfs in the Galaxy can place stringent constraints on the age of the Universe, since the Universe must be older than the things in it.

  

Figure 7: The Milky Way as seen in starlight (top; Lund Observatory) and in the infrared (bottom; COBE/DIRBE). The ages, kinematics and abundances of individual stars constitute the fossil record of galactic evolution.

  

Figure 8: Simulations predict that large galaxies interact and merge with surrounding swarms of dwarf galaxies (top; Klypin 1999), while spectroscopic observations of our own galaxy have identified the remnant of one such merger in the Sgr dwarf (bottom; Ibata et al. 1994; Wyse 1997).

Since many Galactic structure questions are addressed statistically, there is an intrinsic need for large samples. For example, reconstructing the formation and evolutionary history of the Galaxy relies on robust statistical correlations between stellar age, metallicity, and kinematics, as a function of position in the Galaxy, a task that depends critically on large samples. Moreover, the properties of the background stellar distribution must be determined to high statistical accuracy in order to unambiguously identify substructure arising from discrete merger or star formation events. Since the Milky Way covers the entire sky, comprehensive studies of its structure and contents (especially rare populations) intrinsically require large areal coverage. Spectroscopy is critical since it provides the only robust means of obtaining stellar spectral types, radial velocities, and metallicities. Useful stellar spectral features are available throughout the optical and infrared, the longer wavelength diagnostics being critical for studies of extincted populations.