In this Joint Discussion, we explore the motivation behind large surveys in Galactic astronomy. We focus our attention on surveys that measure the photometric, phase space or abundance properties of individual stars. In recent years, we have seen the release of optical and infrared all-sky photometric catalogues for ∼ 108 stars (SDSS: Gunn et al 1998; 2MASS: Cutri et al 2003), and new astrometric catalogues from the Hipparcos satellite and the US Naval Observatory (UCAC2: Zacharias et al 2004). The first major kinematic stellar survey of roughly 15,000 stars was recently completed by Nordstrom et al (2004), with two new much larger surveys now under way (RAVE: Steinmetz et al 2006; SEGUE: Rockosi 2005). There is on-going discussion of extending these million-star surveys to 8m class telescopes early in the next decade (WFMOS: Colless 2005). At that time, the European Space Agency is set to launch the Gaia astrometric satellite with a view to establishing phase space information for a billion stars (Perryman et al 2001; Wilkinson et al 2005). But why go to all this trouble?
Increasingly, we are required to defend big survey machines with rigorous and compelling science cases, and these must be argued carefully within the context of "Big Questions" that are posted on the web sites of major funding agencies. In fact, it is probable that only one of these questions bears directly on galactic stellar surveys − the formation and evolution of galaxies. But here, a good case can be made, as we discuss below.
It is no exaggeration to say that the study of galaxy formation and evolution will dominate observational cosmology and galactic studies for decades to come. This is because it is difficult to define a unique model for galaxy formation, assuming one even exists. A theoretical astrophysicist may be content to establish the existence of galactic "building blocks" at high redshift, and to demonstrate that numerical simulations can explain the properties of these objects, and the basic properties of galaxies at all epochs to the present age. But an applied physicist may argue that this is far from a complete picture where all of the salient microphysics is demonstrated self-consistently.
We have long known that galaxies form over cosmic time through the gradual build-up of dark matter and baryons within a vast hierarchy. This process, which can be studied in the near and far field, is likely to depend in part on the mean density field in the vicinity of the coalescing galaxy (e.g. Maulbetsch et al 2006). There are recent claims that large photometric surveys of galaxies argue against strong environmental factors (Blanton et al 2006), but this influence will be much clearer once the individual components are properly resolved.
In the near field, the Local Group is a relatively low-density region of the Universe; to study galaxies over a wide dynamic range in density contrast will require that we push our stellar surveys out to Virgo, something that is only conceivable with 30-40m class telescopes and the James Webb Space Telescope (JWST). This is made abundantly clear in Fig. 1 where we show the mean density of the 50 nearest galaxy groups.
Figure 1. The dependence of group density with distance from a sample of 50 galaxy groups taken from Tully (1987). Most groups are much like the Local Group in their average density.
The environmental dependence is not something that can be established easily in the far field much beyond the well known distinction of evolved galaxies in virialized clusters and generally younger stellar ages in the filaments between clusters (cf. Cucciati et al 2006). But a fact well known to galactic astronomers is that essentially all galaxies show evidence of an underlying old stellar component, something that is difficult to resolve out in the far field.
There are numerous ways in which large-scale stellar surveys can provide fundamental insight on the process of galaxy formation. But first we review what has been learnt in recent years.