The science case for The European Extremely Large Telescope




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The science case for

The European Extremely Large Telescope

the essential next step in mankind’s direct observation of the nature of the universe, this will provide the description of reality which will underlie our developing understanding of its nature.
Index/contents

1 Executive summary 2

1.1 Astronomy with a 50metre–100metre telescope 5

2 Introduction 6

2.1 The power of Extremely Large Telescopes 6

2.2 Telescope design requirements 7

3 Planets and Stars 9

3.1 Exoplanets 10

3.1.1 Highlight Science Case: Terrestrial planets in habitable zones – “Exo-earths” 10

3.1.1.1 “Exo-earths” around Solar type stars 10

3.1.1.2 Spectroscopic signatures of life: biomarkers 11

3.1.2 Simulations of planet detection with ELTs 12

3.1.2.1 On-going simulations 13

3.1.3 Giant planets: evolution and characterisation 15

3.1.4 Mature gas giant planets 15

3.1.5 Earth-like moons in the habitable zone 16

3.1.5.1 Detection from reflex velocity measurements 17

3.1.5.2 Moon-induced astrometric wobble of the planet 17

3.1.5.3 Spectral detection of terrestrial moons of giant planets 18

3.1.5.4 Mutual planet/satellite shadows and eclipses 18

3.1.6 Rings around extrasolar planets 19

3.1.7 Planets around young stars in the solar neighbourhood 20

3.1.8 Free-floating planets in star clusters and in the field 21

3.2 Our solar system 22

3.2.1 Mapping planets, moons and asteroids 23

3.2.1.1 Large and nearby asteroids 23

3.2.1.2 Small asteroids 23

3.2.1.3 Major and minor moons 23

3.2.2 Transneptunian objects (TNOs) 24

3.2.3 Comets and the Oort cloud 24

3.2.4 Surface and atmospheric changes 26

3.3 Stars and circumstellar disks 28

3.3.1 Formation of stars and protoplanetary discs 28

3.3.1.1 Probing birthplaces 30

3.3.1.2 Structure in inner disks 31

3.3.1.3 Embedded young stellar objects 33

3.3.1.4 Jets and outflows: dynamics and moving shadows 33

3.3.1.5 Debris disks around other stars 34

3.3.2 The lives of massive stars 35

3.3.2.1 Early phases of evolution 35

3.3.2.2 Mature phase outflows 36

3.3.2.3 Normal and peculiar stars 37

3.3.2.4 Asteroseismology 38

3.3.2.5 Chemical composition: the challenge of chronometry 39

3.3.3 The death of stars 41

3.3.3.1 Mass function of black holes and neutron stars 41

3.3.3.2 Isolated neutron stars 41

3.3.3.3 Black holes in globular clusters 42

3.3.4 Microlenses: optical and near-infrared counterparts 45

4 Stars and Galaxies 46

4.1 The interstellar medium 46

4.1.1 Temperature and density probes in the thermal infrared 47

4.1.2 Fine structure in the ISM from ultrahigh signal-to-noise spectroscopy 47

4.1.3 The high redshift ISM 48

4.1.4 Measuring dust properties via polarimetry 48

4.1.5 Optical studies in heavily extinguished regions 49

4.2 Highlight Science Case: Resolved stellar populations 50

4.2.1 The Hubble Sequence: Understanding galaxy formation and evolution 52

4.2.2 Chemical evolution – spectroscopy of old stars 52

4.2.3 The resolved stellar population targets for the European Extremely Large Telescope 55

4.2.4 Technical issues and design requirements 56

4.3 Resolved stars in stellar clusters 59

4.3.1 Modelling and simulated observations of stellar clusters 60

4.3.1.1 Cluster photometry with adaptive optics 62

4.3.1.2 Analysis and results 62

4.3.1.3 Conclusions 64

4.3.2 Spectroscopic observations of star clusters 66

4.4 The stellar initial mass function 67

4.5 Extragalactic massive stars beyond the local group 70

4.6 Stellar kinematic archaeology 72

4.7 The intracluster stellar population 75

4.8 The cosmic star formation rate from supernovae 76

4.9 Young, massive star clusters 78

4.10 Black holes – studying the monsters in galactic nuclei 80

4.10.1 Introduction 80

4.10.2 The future of massive black hole astrophysics: new opportunities with an E-ELT 82

5 Galaxies and Cosmology 84

5.1 Cosmological parameters 85

5.1.1 Dark energy 85

5.1.1.1 Type la supernovae as distance indicators 86

5.1.1.2 Gamma-ray bursts as distance indicators 88

5.1.2 Expansion history 90

5.1.2.1 Cosmic expansion history from primary

distance indictors 90

5.1.2.2 Codex: the COsmic Differential EXpansion experiment 91

5.2 Highlight Science Case: First light – the first galaxies and the ionisation state of the early universe 93

5.2.1 Introduction 93

5.2.2 The highest redshift galaxies (z>10) 95

5.2.3 Galaxies and AGN at the end of re-ionisation (510) 97

5.2.4 Probing the re-ionisation history 101

5.2.5 Early chemical evolution of the IGM 103

5.3 Evolution of galaxies 105

5.3.1 Introduction 105

5.3.2 Physics of high redshift galaxies 107

5.3.3 The assembly of galaxy haloes 109

5.3.4 The star formation rate over the history of the universe 115

5.4 Fundamental constants 118

Annex A Summary of the dependence of the science cases on telescope aperture 121

A1.1 Exoplanet detection from ground-based ELTs 121

A1.2 Resolved stellar populations 123

A1.3 The very high redshift universe 124

A1.4 Summary 126

Annex B New scientific opportunities in the extremely large telescope era 127

B1.1 The physics – astrophysics connection 127

B1.2 The next generation of ground-based astronomical and related facilities 128

B1.3 Future astronimical space missions 130

B1.3.1 Comparison with JWST imaging sensitivity 131

B1.4 Exoplanet detection from space 132

B1.5 Supporting multi-wavelength science via the virtual observatory 133

B1.6 Developments in instrumentation 134

B1.6.1 Adaptive optics modes for ELTs 134

B1.6.2 The use of ELTs at mid-infrared wavelengths 134

B1.6.2.1 Design considerations for an ELT operating in the mid-IR 135

B1.6.3 The use of ELTs at sub-mm wavelengths 135

B1.6.3.1 Design considerations for an ELT operating in the submillimetre 136

B1.6.4 The potential of astronomical quantum optics 136

Credits 139

References 140

Section authors and general contributors 144
1 Executive summary

Science Case for the European Extremely Large Telescope

Astronomy is in its golden age. Since the invention of the telescope, astronomers have expanded mankind’s intellectual horizons, moving our perception of the Earth from an unmoving centre of the Universe to being one of several small planets around a typical small star in the outskirts of just one of billions of galaxies, all evolving in an expanding Universe in which planets are common.

The nuclear energy sources which provide starlight are identified, and we know that the chemical elements of which we are made are the ash of that process: stardust. Exotic states of matter are known: neutron stars, black holes, quasars, pulsars. We can show that the Universe started in an event, the Big Bang, and see the heat remnant of that origin in the Cosmic Microwave Background. Tiny ripples in that background trace the first minute inhomogeneities from which the stars and galaxies around us grew. By comparing the weight of galaxies with the weight of all the visible matter, astronomers have proven that the matter of which we, the planets, the stars, and the galaxies, are made is only a tiny part of all the matter which exists: most matter is some exotic stuff, unlike what we are made of, not yet detected directly, but whose weight controls the movements of stars in our Galaxy. This ‘dark matter’ or ‘unseen mass’, whatever it is made of, is five times more abundant than are the types of matter of which we are made. Perhaps most exotic of all, some new force seems to be stretching space-time, accelerating the expansion of the Universe. The nature of this force, which controls the future of the Universe, remains quite unknown.

Astronomy is a technology-enabled science: progress in astronomy demands new technologies and new facilities. Astronomical telescopes and associated instrumentation are the essential tools allowing access to the widest and most comprehensive laboratory of all, the Universe we live in. Telescopes allow discovery of the new, and subsequent exploration of the whole range of known phenomena, from Solar System objects – planets, comets and asteroids, to the formation of stars and galaxies, extreme states of matter and space (e.g. around black holes) and finally to determine the global matter-energy content of our Universe. In the past half-century a new generation of telescopes and instruments allowed a golden age of remarkable new discoveries: quasars, masers, black holes, gravitational arcs, extrasolar planets, gamma ray bursts, the cosmic microwave background, dark matter and dark energy have all been discovered through the development of a succession of ever larger and more sophisticated telescopes.

In the last decade, satellite observatories and the new generation of 8- to 10-metre diameter ground based telescopes, have created a new view of our Universe, one dominated by poorly understood dark matter and a mysterious vacuum energy density. This progress poses new, and more fundamental, questions, the answers to some of which will perhaps unite astrophysics with elementary particle physics in a new approach to the nature of matter. Some discoveries, made using relatively modest technologies, will require vast increases in technology to take the next step to direct study. Each new generation of facilities is designed to answer the questions raised by the previous one, and yet most advance science by discovering the new and unexpected. As the current generation of telescopes continues to probe the Universe and challenge our understanding, the time has come to take the next step.

In the words of the Astronomer Royal for England, Sir Martin Rees,

“Cosmologists can now proclaim with confidence (but with some surprise too) that in round numbers, our Universe consists of 5percent baryons, 25percent dark matter, and 70percent dark energy. It is indeed embarrassing that 95percent of the Universe is unaccounted for: even the dark matter is of quite uncertain nature, and the dark energy is a complete mystery.”

A small step in telescope size will not progress these fundamental questions. Fortunately, preliminary studies indicate that the technology to achieve a quantum leap in telescope size is feasible. A telescope of 50-metre to 100-metre diameter can be built, and will provide astronomers with the ability to address the next generation of scientific questions.

Simulation showing stages in the formation of the galaxies in the Local Group in a cold dark matter scenario. Snapshots are shown at various times from the early Universe (z=50) to the present day (z=0).


Primary science cases

for a 50metre-100metre Extremely Large Telescope

Are there terrestrial planets orbiting other stars? Are we alone?

Direct detection of earth-like planets in extrasolar Systems and a first search for bio-markers (e.g. water and oxygen) becomes feasible.


How typical is our Solar System? What are the planetary environments around other stars?

Direct study of planetary systems during their formation from proto-planetary disks will become possible for many nearby very young stars. In mature planetary systems, detailed spectroscopic analysis of Jupiter-like planets, determining their composition and atmospheres, will be feasible. Imaging of the outer planets and asteroids in our Solar System will complement space missions.


When did galaxies form their stars?

When and where did the stars now in galaxies form? Precision studies of individual stars determine ages and the distribution of the chemical elements, keys to understanding galaxy assembly and evolution. Extension of such analyses to a representative section of the Universe is the next great challenge in understanding galaxies.


How many super-massive black holes exist?

Do all galaxies host monsters? Why are super-massive black holes in the nuclei of galaxies apparently related to the whole galaxy? When and how do they form and evolve? Extreme resolution and sensitivity is needed to extend studies to normal and low-mass galaxies to address these key puzzles.


When and where did the stars and the chemical elements form?

Can we meet the grand challenge to trace star formation back to the very first star ever formed? By discovering and analysing distant galaxies, gas clouds, and supernovae, the history of star formation, and the creation history of the chemical elements can be quantified.


What were the first objects?

Were stars the first objects to form? Were the first stars the source of the ultraviolet photons which re-ionised the Universe some 200million years after the Big Bang, and made it transparent? These objects may be visible through their supernovae, or their ionisation zones.


How many types of matter exist? What is dark matter? Where is it?

Most matter is transparent, and is detectable only through its gravitational effect on moving things. By mapping the detailed growth and kinematics of galaxies out to high redshifts, we can observe dark matter structures in the process of formation.


What is dark energy? Does it evolve? How many types are there?

Direct mapping of space-time, using the most distant possible tracers, is the key to defining the dominant form of energy in the Universe. This is arguably the biggest single question facing physical science.


Extending the age of discovery...

In the last decades astronomy has revolutionised our knowledge of the Universe, of its contents, and the nature of existence. The next big step may well also be remembered for discovering the unimagined new.


1.1 Astronomy with a 50metre – 100metre telescope

The science case for 50m–100m diameter telescopes is spectacular. All aspects of astronomy, from studies of our own Solar System to the furthest observable objects at the edge of the visible Universe, will be dramatically advanced by the enormous improvements attainable in collecting area and angular resolution: major new classes of astronomical objects will become accessible to observation for the first time. Several examples are outlined in the following sections. Furthermore, experience tells us that many of the new telescope’s most exciting astronomical discoveries will be unexpected: indeed the majority of the science highlights of the first ten years of the first 10m telescope, the Keck, such as its part in the discovery and study of very high redshift, young ‘Lyman-break’ galaxies, were entirely new, violated received wisdom, and, being unknown, were not featured in the list of science objectives prior to the telescope’s construction.

The vast improvement in sensitivity and precision allowed by the next step in technological capabilities, from today’s 6–10m telescopes to the new generation of 50–100m telescopes with integrated adaptive optics capability, will be the largest such enhancement in the history of telescopic astronomy. It is likely that the major scientific impact of these new telescopes will be discoveries we cannot predict, so that their scientific legacy will also vastly exceed even that bounty which we can predict today.
fig.1.1

Concepts for 50-100m ELTs. Above: the OWL (OverWhelmingly Large) Telescope, a design for a 100m-class telescope proposed by ESO (Gilmozzi 2004, Dierickx et al 2004).


fig.1.2

The Euro-50 concept (Andersen et al., 2003, 2004).


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.


fig.2.1

The theoretical diffraction limits (1.2 l/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 : (VM, VL)=(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-to-noise 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).
Image size 8m 30m 100m Comments

0.5 arcsec 0.0 1.4 2.7 Seeing limited

0.2 arcsec 1.0 2.4 3.7

0.1 arcsec 1.7 3.2 4.5

Strehl (K) = 0.2 0.6 3.5 6.1 e.g. Multi-conjugate AO

Strehl (K) = 1.0 2.4 5.2 7.8 Theoretical limit


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 arcsec images.


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.
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 exoplanets (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.
3 Planets and Stars

Introduction

Stars have focused the interest of astronomers for centuries. A great variety of observations have driven our knowledge of the processes leading to star formation, of how the interplay between gravity and nuclear reactions determine stellar evolution, and ultimately, the physical principles that explain the existence of some of the most exotic states of matter in the Universe: neutron stars and black holes. In spite of the undeniable progress made during the last few decades in understanding how stars form and evolve, essential questions remain for which the collecting area and angular resolution of an extremely large optical/infrared telescope will prove decisive. Many of these questions deal with the earliest and the latest stages of stellar evolution, plagued by significant unknowns.

Determining the entire stellar mass spectrum in the cores of molecular clouds or measuring the dynamical mass spectrum of black hole/neutron stars in binary systems are areas in which major changes may be required in the currently accepted scenario for star formation and evolution, and which present great observational challenges for the current generation of very large telescopes. Current limitations in the present understanding of star formation are directly related to the lack of high sensitivity, high spatial resolution observations in star forming regions, where 1 AU in the nearest regions at 100 pc corresponds to just 10 milli-arcsec. Probing such regions in the optical and infrared with the few milli-arcsec resolution provided by an E-ELT will most likely unveil the nature of processes that form stars in the cores of molecular clouds. It will also probe the conditions for formation of protoplanetary disks and the ubiquity of planet formation.

The recent discovery that at least 7% of Solar-type stars host giant planets at separations of less than 5 AU has opened a new domain for research. With current very large telescopes, it may well be possible to directly image massive giant planets, but only around a few specific types of objects, such as very young stars or brown dwarfs. However, to establish a complete picture of the formation of planetary systems, direct imaging and characterisation of planets around stars in various evolutionary stages is needed. In particular, in order to understand whether or not the architecture of our own Solar System is a common occurrence, detailed observations of stars similar in mass and age to the Sun must be obtained. These observations will require the higher angular resolution and collecting area of an E-ELT. Such a telescope will not only be a crucial tool for our understanding of the formation and evolution of planetary systems, but also for the exploration of our own Solar System, where planets and their moons will be studied with resolution comparable only to those of space missions.

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