3.1. The case for extending beyond the Local Group
The observations to date provide evidence of a complex tapestry that is only now coming into focus. We will need to study this tapestry in far more detail before we can begin to unravel its origins, and the processes by which it came into being. In the next decade, it will be possible to study the star formation histories in hundreds of nearby galaxies, both in dense and loose groups, and for distinct galactic subcomponents. With these observations, we can choose to compare individual galaxies, or compare the volume-averaged star formation histories between groups in order to answer these questions.
One thing is clear: we must extend these large-scale surveys beyond the Local Group, particularly in an era of ELTs and JWST (see Section 1). These telescopes promise diffraction limited performance in infrared bands, which is good news for crowded fields. But we will also need optical bands for metallicity sensitivity in warm stars. We will need to reach down to the main-sequence turn-off to derive accurate ages. Remarkably, high Strehl ratios may not be required in crowded fields (Olsen et al 2003). But just how well can one achieve accurate photometry when using adaptive optics in long exposures? It is hard to guess at what the future has in store without an answer to this troubling question.
3.2. The origin of the thick and thin disk
Freeman & Bland-Hawthorn (2002) argue that establishing a theory of galaxy formation is largely about understanding the processes involved in forming disks in the early universe. The ancient thick disk is a particularly attractive target because it takes us back to an early epoch when large disks were forming for the first time.
The origin of the thick disk remains controversial. Various scenarios have been suggested: (i) snap-frozen relic of the old thin disk heated by an ancient merger event; (ii) material from one or more major merger events; (iii) dissipational collapse; (iii) the byproduct of unbound star clusters in the early universe (Kroupa 2002). Interestingly, Elmegreen & Elmegreen (2006) measure the scale heights of large star forming complexes in the Hubble Ultra Deep Field (HUDF) and find these are broadly consistent with the thick disk, in support of Kroupa's model.
We often think distribution functions of dominant stellar components as being smooth for the most part. But is that really true in practice? In light of Juric et al (2005), important clues on disk formation may be revealed in the substructure. Simulations by Abadi et al (2003) suggest that substructure could survive throughout the disk, presumably because much of this material spends much of its time out of the plane. Strong scattering near resonances may wash out some of the substructure, but equally one can imagine fossil structures that are trapped in resonances.
In Section 2.2, we discussed new results on the disks of spirals. Does the extent and the specific angular momentum of the outer stellar disk reflect certain properties of the collapsing protocloud? If the answer is no, then is the outer stellar disk still forming? If the answer is yes, what is the origin and role of the HI that extends beyond the stellar disk in most instances? Where stellar disks are seen to extend far into the HI, the inferred Q values are much too high to form stars, so how is this possible?
In our view, a major shortcoming of contemporary Galactic studies are accurate stellar ages, especially on timescales of billions of years. One can envisage experiments to rectify this problem for ∼ 105 stars, an issue we discuss elsewhere (Freeman & Bland-Hawthorn 2002). The present resurgence of interest in open clusters is partly due to the prospect of a decent age for the stellar ensemble. As a result, the chemistry of stellar clusters may allow us to determine how large-scale enrichment took place in the Galaxy over cosmic time. Early results seem to indicate that the chemical gradient today is flatter than it was 10 Gyr ago (Section 2.3). An alternative scenario to gradual disk accretion is that Galaxy-wide enrichment events from the nucleus has gradually built up the chemical elements in the outer disk, much like a volcano building up its ramparts from successive events over many dynamical times.
3.3. The origin of the bulge and halo: ancient stars
An area of intense future interest is expected to be the spheroidal components of M31 and the Galaxy. Just how did these form and what is their relation to the rest of the galaxy? The highest resolution simulations tell us that we can expect to find far more evidence of substructure throughout the inner 10 kpc or so (Gao & White 2006). These surveys are likely to turn up an ancient stellar populations over the entire metallicity spread, from ultra metal poor to extremely metal rich stars.
A rare population of ancient ultra-metal-poor stars in the Galactic halo provides critical information on the chemical yields of the first generation of massive stars (Beers & Christlieb 2005; Tumlinson 2006). Larger concentrations of ancient stars may be hiding in the centres of galaxies where the mass density is high and conditions likely first favoured star formation. Future instruments will search for these ancient stars, but once again, dealing with crowding and dust obscuration will require a high spectroscopic resolution, near-diffraction limited spectrograph working at infrared wavelengths.
A new generation of multi-object survey instruments will be critical to unravelling some of the biggest questions in modern cosmology today. In addition to the fully-funded Gaia mission in 2012, the Japanese are expected launch the JASMINE explorer in 2015 to obtain infrared astrometry on 10 million bulge stars (Gouda et al 2005). Seidelmann & Monet (2005) summarise a number of related missions in the next decade. There is a great deal to be learnt on the nature of galaxies from explorations of this kind.