How do galaxies form and evolve? Primeval galaxies undergoing their first episodes of star formation have remained elusive, either because these nascent systems are shrouded in dust or because galaxy formation is a slow hierarchical process wherein galaxies are assembled over time from small building blocks. The aging of galaxies has also remained a puzzle: so many factors contribute to the chemical, dynamical and morphological development of a galaxy, that the small observational data sets painstakingly obtained barely provide us with clues even to the most global issues (e.g., the evolution of the luminosity function) over a large range in lookback time.
Our investigation of the processes of galaxy formation and evolution is currently restricted to theoretical simulations and small observational data sets. Current simulations suggest that galaxy assembly is a hierarchical process, wherein mergers and interactions play a significant role in determining the present-day morphologies and stellar constituents of galaxies. At present, the observational data are ambiguous, and results suffer from selection effects and small number statistics. The pioneering studies during the last decade have been largely restricted to small pencil beam surveys or shallow surveys of the low-redshift galaxy population (e.g., CFRS - Lilly et al. 1996; LDSS/Autofib - Ellis et al. 1996; various Keck surveys - Cowie et al. 1996, Koo et al. 1996, Cohen et al. 1999). More recently, observations with HST and the Keck telescopes have demonstrated the existence of star-forming galaxies at redshifts beyond 3 (Steidel et al. 1996, 1999). These galaxies are believed to be the building blocks of the present-day galaxies, but their properties (masses, chemical composition, stellar content, ages and evolutionary histories) are almost completely unknown.
In order to address the question of galaxy formation and evolution for the entire population, we would ideally want to trace the evolutionary history of galaxies (i.e., their star-forming history, chemical evolution, merging and morphological evolution) as a function of mass, redshift and environment. It is critical to understand and interpret the formation of galaxies in the context of structure formation and evolution. These astrophysical problems are inextricably linked, since the large-scale environment plays a crucial role both in the assembly of galaxies and in their evolution (through merging, exclusion, harassment, etc.).
Need for Large Area / Depth / Large Samples: To trace the evolutionary history of galaxies as a function of environment, we need to sample galaxies over the entire range of environments: from the lowest density regions (voids) to the rarest high-density environments (cores of rich clusters). Surveys over large volumes (100 sq.deg. -- see previous section) are needed to accomplish this.
In order to explore the early evolutionary history of galaxies (z~4-6), deep spectroscopic surveys (to ~25 AB mag) are essential. These depths are needed not only to probe the highest redshifts, but also to ensure that the samples are not restricted to the rarest, most luminous objects, and instead sample more typical objects as well (i.e., as much of the luminosity function as possible).
Galaxies in the present epoch exhibit a large range in physical properties (e.g., masses, chemical abundances, star forming histories, morphologies, gas and stellar content). Since the evolutionary processes responsible for these properties are numerous and complex (e.g., star formation, mergers and interactions, infall), an obserational program to unravel the formation histories of present-day galaxies inherently requires large samples (~106 galaxies). For example, in order to trace the evolutionary history of galaxies as a function of mass, redshift and environment, we would need at least 5 redshift bins (1<z<6), 4 mass bins (logarithmic intervals spanning 108-1012 Msun), 5 bins in mean stellar age or star-formation rate (0.1 - 10Gyr or 0-103Msun/yr), 4 bins in morphology (E/S0, S, Irr, multi-component), 4 bins in mean chemical abundance (0.002-2 Zsun), and 3 bins in environmental density (field, groups, clusters). With at least 100 galaxies per bin, this implies a total sample size of at least 500,000 galaxies. This is truly a lower limit since we have required that the bins of rare objects are also well populated. Populating such rare bins is critical in order to address questions such as the formation history of the most massive galaxies, or the evolutionary history of the most metal-poor galaxies.
A Representative SWIFT Project
The surface densities on the sky of R<25AB mag z~3 and I<25AB mag z~4 galaxies are ~4000 and 800/sq.deg. respectively. A survey of 5x105 galaxies over 100 sq.deg. requires a wide-field, highly multiplexed multi-object capability. We are only beginning to scratch the surface with current studies: a typical Keck+LRIS campaign results in a dozen redshifts of R~24-25 galaxies per night. The low-resolution spectra that are obtained probe only a restricted class of objects (mostly low-extinction, star-forming systems) and are generally only sufficient for measuring redshifts. Detailed investigations of the physical properties of these objects and their evolution requires spectroscopic capabilities which are neither available nor planned.
For the detailed spectroscopic studies described here, we need a minimum resolution of approximately 2000 and signal-to-noise ratios per resolution element of ~20 (e.g., figure 6). The redshift range targeted by this survey requires spectroscopy at both optical and infrared wavelengths. In 0.7 arcsec seeing with DEIMOS on Keck, this requires an exposure time of approximately 1 night per 80-object mask; hence it will take 6250 clear nights to obtain spectra of 5x105 galaxies, or a minimum of 17 clear observing years with Keck and DEIMOS dedicated to this one project. In comparison, SWIFT would execute this project in only clear nights. A large spectroscopic study of this nature would yield a very large slit survey of `blank' sky, resulting in a high potential for serendipitous discoveries (e.g., of extremely high redshift objects not directly targeted by the study).