POPS Scientific Background
In the last decade, cosmology has entered a golden age. Large optical imaging and redshift surveys (e.g. APM, 2dF, SDSS), cosmic microwave background experiments (e.g. COBE, BOOMERANG) and X-ray surveys (e.g. ROSAT, XMM) have been providing increasingly stringent constraints on the basic parameters of the world model. With the geometry of the universe finally settled and the global parameters of galaxies approximately known, testing the hierarchical paradigm, i.e., understanding the actual physical processes governing the formation of galaxies, the growth of their masses and the origin of their structure and morphology, will become the major challenge of astrophysical research in the next decade. Coupled with the fundamental issue of early galaxy evolution, is the question of how massive black holes formed and how their evolution is connected to that of their host galaxies. Although the last few years have also seen significant progress on these questions, the most important issues are still open. Two requirements follow if we want to make progress in answering these questions:
First, it is essential to obtain spatially resolved ('integral field') spectroscopic information about galaxies at medium and high redshift - if possible using adaptive optics to improve the spatial resolution. Galaxies at higher redshifts typically have very complex morphologies. As indicated by the recent serendipitous discoveries of Lyman-a emitters, integral field spectroscopy will also be especially valuable for finding and exploring new objects at the highest redshifts. In order to probe statistically meaningful samples of objects, it is necessary that data on a large number of objects are obtained simultaneously. Sampling galaxy kinematics also requires moderately high spectral resolving powers (l/Dl>3000). The currently tightest constraints on the evolution of galaxies at z<1 could not have been obtained without high resolution imaging (mostly with HST) and spatially resolved spectroscopy under very good seeing conditions. The latter has yielded rotation curves, velocity dispersion and stellar population profiles which have allowed the accurate analysis of the evolution of the Tully-Fisher, Fundamental Plane and Mg-s relations, as well as the nature of E+A galaxies. These measurements confirm that spirals and ellipticals with present day masses and morphologies were largely in place already at z~1, and only irregulars show significant evolution at low redshifts. A major goal of these studies will be to extend these preliminary findings to z>1 when the bulk of the assembly of large galaxies took place.
Second, it is important to extend this multi-object survey capability to the near infrared. Up to now, ground based extragalactic research has been driven mainly by optical observations. However, optical observations alone are not able to study the assembly of the present-day galaxy population much beyond z~1 because crucial spectral features for the analysis of their old stellar populations (e.g. via the 4000Å break) and their kinematics (key emission lines like [OII] and Ha) are shifted into the near-IR. At z~1-2, infrared spectroscopy will permit analysis of the masses, the stellar populations, and star-formation rates of galaxies. At z>2, infrared observations provide estimates of masses and metallicities of galaxies from their rest-frame optical emission lines. At z>6, prominent UV emission lines, such as HeII and Lyman-a move into the near-IR. Finally, many crucial events in the life of a galaxy, like mergers, or the outburst of nuclear activity are enshrouded in dust and can only be observed in the infrared. For observations of key spectroscopic indicators throughout the z~1-3 redshift range, infrared spectroscopy should cover the J, H and K windows, thus requiring a cryogenic spectrograph.
The above scientific requirements indicate that an instrument capable of addressing these questions must be capable of multiple integral-field spectroscopy. The surface densities of interesting targets are from 1-5/sq. arcmin indicating that multiplex advantages of 10-100 could in principle be exploited assuming typical Cassegrain or Nasmyth field sizes. A very efficient method of delivering this capability is to use a pick-off mirror system to steer the desired objects onto a fixed array of image slicers which produce a resolved 2-dimensional spectrum of the object at the detector. Two such instruments employing this capability are currently under study for Gemini1 and the ESO-VLT2. One of they key technological challenges in the design of such an instrument is the construction of a robust opto-mechanical pickoff system capable of working at cryogenic temperatures. This is the focus of the PoPS project.
1. Wright, G., Ivison, R., Hastings, P., Wells, M., Sharples, R.M., Allington-Smith, J.R. and Content, R. 2001, in ESO Workshop on Scientific Drivers for ESO future VLT/VLTI Instrumentation, p. 128-135.
2. R.M.Sharples, R. Bender, R. Hofmann, R. Genzel and R.J. Ivison. 2002, SPIE 4841 (in press).