16/02/2016
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THE SCIENCE CASE FOR
THE EUROPEAN
EXTREMELY LARGE TELESCOPE
2 Introduction
In the following three chapters we describe the science case for a 50m-100m aperture
Extremely Large Telescope. We divide the extensive science case into three broad
areas: Planets and Stars (Chapter 3), Stars and Galaxies (Chapter 4) and Galaxies and
Cosmology (Chapter 5). These divisions were chosen to reflect a change in the way
astrophysics will be approached in the era of Extremely Large Telescopes. The
traditional astronomical categories appropriate to currently available facilities divide
the subject into three almost distinct disciplines, broadly Planets, Stars and Galaxies.
In the Extremely Large Telescope era we will study planets around stars other than
our own Sun, study individual stars in galaxies far beyond our own Milky Way, and
begin detailed study of galaxies at cosmological distances. The context of these
studies, in an era when particle physics, fundamental physics and astrophysics are
increasingly merging into a single Grand Challenge, is discussed in Annex B.
Before describing these exciting possibilities, we first give a brief summary of the
technological performance of Extremely Large Telescopes, which must be delivered
to make these exciting scientific advances feasible.
2.1 The Power of Extremely Large Telescopes
Current Adaptive Optics systems on 8-m class telescopes have recently demonstrated
performance close to the theoretical diffraction limit. Figure 2.1 shows the diffraction
limits for 8m, 30m and 100m telescopes compared to the typical sizes of astronomical
objects. While 8m telescopes can resolve large regions within galaxies (between 300
and 1000pc in size) at redshifts around unity, Extremely Large telescopes can, given
appropriate adaptive optics capability, resolve structures of a few tens of parsecs in
size, the approximate size of a major star-forming region, at similar redshifts.
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Figure 2.1 The theoretical diffraction limits (/D) for 8m, 30m and 100m telescopes are
plotted at three wavelength values corresponding approximately to the J, H and K
infrared bands (horizontal bars). Also plotted are curves of projected angular size as a
function of redshift for objects of various physical sizes (10pc, 50pc, 300pc and 1kpc) for
two sets of cosmological parameters : (M, )=(0,0) and (0.3,0.7) for the lower and
upper curves respectively.
A smaller diffraction limit combined with increased light-collecting aperture
translates into great gains in sensitivity as telescope diameter is increased, particularly
for unresolved point sources. This is largely because of the reduced contribution from
sky noise when the size of the image is reduced. For example a 100m telescope with
perfect diffraction-limited images would reach about 8 magnitudes fainter for point
sources than an 8m telescope that delivers 0.5 arcsec images, for the same signal-tonoise and exposure time (in the near IR). In this simple scaling argument, we have
assumed perfect diffraction-limited images (Strehl = 1). Even with a moderate
Adaptive Optics (AO) correction that results in the majority of the light falling inside
a 0.1 arcsec aperture, a 100m telescope would give a gain of 4.5 magnitudes for point
sources compared to an 8m telescope producing 0.5 arcsec images, a factor of 60 in
intensity (see Table 2.1).
Table 2.1 Gains in magnitude for the same signal-to-noise and exposure time when
observing unresolved sources in the background-limited regime. The gains are shown
relative to an 8m telescope delivering 0.5” images.
Image size
0.5 arcsec
0.2 arcsec
0.1 arcsec
Strehl (K) = 0.2
Strehl (K) = 1.0
8m
0.0
1.0
1.7
0.6
2.4
30m
1.4
2.4
3.2
3.5
5.2
100m
2.7
3.7
4.5
6.1
7.8
Comments
Seeing limited
e.g. Ground-layer AO
e.g. Multi-conjugate AO
Theoretical limit
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2.2 Telescope Design requirements
This science case is being developed in parallel with a European Extremely Large
Telescope design study (a four year program which began in March 2005), and the
relevant OPTICON Joint Research Activities, which are developing the appropriate
technological capabilities. We have listed design requirements for the telescope
resulting from each part of the science case throughout the document, and where
possible we have included simulations to demonstrate the potential of Extremely
Large Telescopes for specific science applications. Naturally the science case and
requirements will continue to evolve as the scientific and technical studies progress.
In many cases we have been able to consider the dependence of the science
achievable on telescope aperture. These results are summarised for a few key science
cases in Annex A.
2.3 Highlight Science Cases for a 50m-100m Extremely
Large Telescope
The science case presented in this document demonstrates the very wide range of
applications for an ELT. Of these a few stand out as “highlights” and have generated
particularly high levels of enthusiasm and discussion among the European ELT
science group. These highlighted cases are
(1) Terrestrial exo-planets (Section 3.1.1)
(2) Resolved stellar populations in a representative section of the Universe
(Sections 4.2 and 4.3)
(3) First light and the re-ionisation history of the Universe (Section 5.2)
These are seen as some of the most exciting prospects for ELTs precisely because
they push the limits of what can be achieved, and they will provide some of the most
technically challenging specifications on telescope design. The boundaries of what is
achievable in these scientific areas (and others) will not be known exactly until the
ELT is in operation, although more precise feasibility assessments will be possible
when the technical studies described above are complete. We now present the science
case that we believe is within range of a 50-100m ELT based on our current
understanding of the technical issues.