Scientific Requirements for Extremely Large Telescopes (ELTs)

Background:

The scientific and technical interest in 30 to 100m telescopes is increasing and many ELT design studies are under way. While the technical aspects are usually well covered in specialised SPIE meetings, the scientific aspects, supposed to be the drivers of such projects, are only discussed within the respective consortia. There is therefore a clear need to review in a large audience the scientific cases for ELTs, to see what constraints they impose on the designers (and which of their desiderata can be reasonably satisfied), while at the same time to analyse the lessons that can be learned from the current generation of 10m telescopes.

The scientific cases for ELTs cover the whole field of astrophysics and valuable documents have already been produced by the scientific panels in some of the largest projects, most notably the GSMT (panel chaired by R. Kudritski and S. Strom) and the European project (group chaired by I. Hook and P. Salinari under the Opticon network).

Astrophysical Drivers

Some of the main drivers (to quote only a few) are:

• the study of the first objects in the Universe and the re-ionization history
• establish the large scale structure and star formation history at high-redshift with various tracers, including SNe and GRB's;
• understand the formation of galaxies by studying their dynamics at various redshifts;
• follow the enrichment of the intergalactic medium by absorption line studies in galaxies;
• resolve stellar populations in galaxies up to Virgo to derive ages and merging history;
• derive the stellar mass function in those objects;
• explore other solar systems, from protoplanetary disks to spectroscopy of earth-like planets and signatures of life.

Impact on Fundamental Physics

ELT's will have a particular inpact on fundamental physics because they should allow observations of the faintest and most distant objects in the universe, therefore imposing constraints on (among others):

• The validity (or limits) of General Relativity through detailed studies of gravitationally lensed objects
• The big bang nucleosynthesis thanks to the assessment of the chemical composition of "primitive", nearly unprocessed material
• Possible changes of some fundamental constants through further studies of quasar absorption lines
• The dark matter content through studies of the nature and evolution of the large scale structures

Technical Considerations

Some questions to be asked are, for example:
• What angular resolution is needed (or sufficient) for cosmology?
• What field of view is needed for large scale structure studies?
• What sensitivity is required to study ISM lines in galaxies at z=6 or further out?
• How does the PSF affect the detection of SNe at z=6-8, or of exoplanets at 30 parsecs?
• Can the necessary contrast be achieved with large segmented dishes, or is it more/only feasible with interferometers?
• What kind of surveys do we need to feed these studies? Conducted where?
• What is the complementarity with other Large Scale Facilities (ground or space), and what is the appropriate timescale to benefit from this?
• What has been achieved with present 8-10m class telescopes in this respect (contrast, angular resolution,...) and how can one extrapolate from that?

2005 will be the International Year of Physics and the first occasion on which the International Union of Pure and Applied Physics (IUPAP) will hold its General Assembly in South Africa, or indeed, in Africa. Recognising the multidisciplinary character of the Symposium, IUPAP has agreed to provide some financial support for it.