|Annu. Rev. Astron. Astrophys. 2002. 40:487-537
Copyright © 2002 by Annual Reviews. All rights reserved
Throughout this review, we have identified fossil signatures of galaxy formation and evolution which are accessible within the Galaxy. These signatures allow us to probe back to early epochs. We believe that the near-field universe has the same level of importance as the far-field universe for a comprehensive understanding of galaxy formation and evolution.
We have argued that understanding galaxy formation is primarily about understanding baryon dissipation within the CDM hierarchy; to a large extent, this means understanding the formation of disks. The question we seek to address is whether this can ever be unravelled in the near or far field. Dynamical information was certainly lost at several stages of this process, but we should look for preserved signatures of the different phases of galaxy formation.
Far-field cosmology can show how the light-weighted, integrated properties of disks change with cosmic time. While light-weighted properties provide some constraint on simulations of the future, they obscure some of the key processes during dissipation. The great advantage of near field studies is the ability to derive ages and detailed abundances for individual stars within galaxies of the Local Group.
We have addressed the issue of information content within the Gaiasphere. The detailed information that is possible on ages, kinematics, and chemical properties for a billion stars – which we see as the limit of observational knowledge over the next two decades – may reveal vast complexity throughout the disk. It may not be possible to perceive the sequence of events directly. However, we are optimistic that future dissipational models may provide unique connections with the observed complexity.
It is clear that detailed high resolution abundance studies of large samples of galactic stars will be crucial for the future of fossil astronomy. Christlieb et al. (2000) find that strong r-process enhanced stars can be identified with R = 20,000 and SNR = 30 pix-1 from the Eu lines. Both UVES and HDS can reach this sensitivity for a B = 15 star in just 20 min. But the detailed abundance work requires a substantial increase in the resolving power. Cayrel et al. (2001) and Hill et al. (2002) demonstrate the exquisite quality and capability of high resolution spectroscopy for CS 31082-001 where they achieve a SNR ≃ 300 in just four hours with UVES at R ≃ 60,000. (See Figure 12 for another excellent example.) But these are bright stars with some of the most extreme overabundances of r-process elements observed to date.
Gaia will provide accurate distances, ages and space motions for a vast number of stars, separate with great precision the various Galactic components, and identify most of the substructure in the outer bulge and halo. High resolution spectrographs like UVES on the VLT, HDS on Subaru, and HIRES on Keck are starting to reveal the rich seam of information in stellar abundances.
We must stress that in order to access a representative sample of the Gaiasphere, this will require a new generation of ground-based instruments, in particular, a multi-object echelle spectrograph with good blue response on a large aperture telescope. We close with a brief discussion of what is required.
As an example, the FGK sub-giants and giants are a characteristic population which could be studied over the full extent of the Gaiasphere, as discussed in the previous section. Typical stars will have magnitudes around 17−18, which is at the limit of the state-of-the-art spectrometer UVES at R ≃ 60,000.
We now consider what it would take to achieve high resolution spectroscopy for a representative sample of stars within the Gaiasphere. Our baseline instrument UVES achieves cross-dispersed echelle spectroscopy in two wavelength ranges (300−500 nm, 420−1100 nm). For a limiting resolution of R ≃ 60,000 for a single night exposure, the sensitivity limit is U ≈ 18.0 and V ≈ 19.5 in the blue and red arms. UVES now allows multi-object echelle spectroscopy (red arm) from fiber inputs provided by the Fiber Large Array Multi-Element Spectrograph (FLAMES). This will enable the simultaneous observation of eight objects over a 25' field of view.
Existing multi-object spectrographs are mostly used redward of 450 nm because of the fundamental limits of conventional optical fibers. Normal fibers transmit light through total internal reflection but blue light is Rayleigh scattered below 450 nm. Recently, photonic crystal (microstructured) fibers threaded with air channels (Cregan et al. 1999) have been shown to be highly transmissive down to the atmospheric cut-off. This is a technical breakthrough for blue multi-object spectroscopy.
We believe there is a real need for a high-resolution spectrograph that can reach hundreds or even thousands of stars in a square degree or more. The Gemini Wide Field proposal currently under discussion provides an opportunity for this kind of instrument (S. Barden, personal communication). Such an instrument will be expensive and technically challenging, but we believe this must be tackled if we are to ever unravel the formation of the Galaxy.
The philosophy behind this review has emerged from discussions dating back to the spring of 1988 when KCF and JBH were visiting the Institute of Advanced Study at Princeton. At that time, there was a quorum of galaxy dynamicists at the IAS whose work continues to inspire and excite us. Our thanks go to John Bahcall for this opportunity. We thank Michael Perryman and the Gaia team for the inspiration of the Gaia science mission. We have greatly benefitted from excellent reviews by E. Friel, J. Sellwood, and G. Wallerstein and collaborators. Most recently, we acknowledge the inspiration of colleagues at the 2001 Dunk Island conference, in particular, Tim de Zeeuw, Mike Fall, Ivan King, John Kormendy, John Norris, Jerry Sellwood, Pieter van der Kruit, and Ewine van Dishoeck. We have benefited from discussions with Vladimir Avila-Reese, Rainer Beck, Bob Kurucz, Ruth Peterson, Tomek Plewa, and Jason Prochaska. We are indebted to Allan Sandage for many constructive comments. Finally, we thank the editor for suggesting the main title New Galaxy for this review.