2 The Messenger 121 – September 2005

Isobel Hook (University of Oxford) and the OPTICON ELT Science Working Grouphttp://www.astro-opticon.org/networking/elt.html

Astronomers around Europe are gearingup for the next generation of ground-based telescopes to follow on from thesuccess of the VLT and other 6–10-mtelescopes. All aspects of astronomywill be dramatically advanced by theenormous improvements attainable incollecting area and angular resolution:major new classes of astronomical ob-jects will become accessible to obser-vation for the first time. In July of thisyear a book1 was produced by a groupof European astronomers, which de-scribes the science achievable with atelescope of diameter 50–100 m. Herewe present some highlights from thisscience case, ranging from direct ob-servations of Earth-like planets outsideour own Solar System to the most dis-tant objects in the Universe.

Science with Extremely Large Telescopes

1 Hook, I. M. (Ed.), 2005, “The Science Case for theEuropean Extremely Large Telescope: The next stepin mankind’s quest for the Universe”. Printed copiesand CDs are available on request from SuzanneHoward (showard@ast.cam.ac.uk). PDF files can bedownloaded from http://www.astro-opticon.org/networking/elt.html

In the past half-century a new generationof telescopes and instruments allowedremarkable new discoveries: quasars,masers, black holes, gravitational arcs,extra-solar planets, gamma ray-bursts,the cosmic microwave background, darkmatter and dark energy have all beendiscovered through the development of asuccession of ever larger and moresophisticated telescopes. This progressposes new, and more fundamental,questions, the answers to some of whichwill perhaps unite astrophysics withelementary particle physics in a new ap-proach to the nature of matter, whileothers may give us insights as to the exis-tence (or otherwise) of other life-sup-porting planets in our Galaxy. As the cur-rent generation of telescopes continues to probe the universe and challenge ourunderstanding, the time has come to takethe next step.

Several projects are under way aroundthe world to design and construct the next generation of ground-based, Ex-tremely Large Telescopes (ELTs), whichwill provide astronomers with the ability toaddress the next generation of scientificquestions. Initial studies in the UnitedStates and Canada are concentrating onpotential designs in the 20–30-m range,such as the proposed Giant MagellanTelescope (GMT) and Thirty Meter Tele-

scope (TMT). In Europe the focus is oneven larger telescopes – preliminary stud-ies indicate that the technology to achievea quantum leap in telescope size is fea-sible, and a detailed design study is nowunder way in Europe (led by ESO) todevelop the technology needed to build a50–100-m telescope (see Figure 1).

A larger telescope is beneficial for twomain reasons – firstly, a larger collectingarea (proportional to the square of thediameter) allows fainter and more distantobjects to be observed. Secondly, theresolution achievable improves in propor-tion to diameter of the telescope, pro-vided that the telescope is equipped withan adaptive optics system that correctsfor the blurring effects of the Earth’s at-mosphere. Thus a 50-m telescope work-ing at its diffraction limit could in theoryproduce images over five times sharperthan the best images from today’s 6–10-mtelescopes. These two effects togetherhave a profound effect on the scientificobservations that can be made – from theability to resolve faint planets around other stars, to studying the most distantobject in the Universe.

Some examples are given below of thepotential scientific breakthroughs achiev-able with the vast improvement in sen-sitivity and precision allowed by the next

Figure 1: Concepts for 50–100-m ELTs. Left: theOWL (Over Whelmingly Large) Telescope, a design fora 100-m-class telescope being developed by ESO (Gilmozzi 2004, Dierickx et al. 2004). Right: TheEuro-50 concept (Andersen et al., 2003, 2004).

3The Messenger 121 – September 2005

step in technological capabilities, focus-ing on the science case for a 50–100-mtelescope, which is being developed inEurope. Additionally, as we have seen in the past, each new generation of facili-ties has advanced science by discover-ing the new and unexpected. Therefore itis likely that the major scientific impact of these new telescopes will be discover-ies beyond those we can predict today.

Are we alone? Planets beyond our Solar System

In 1995 the first planet around a normalstar other than the Sun was detected, by the Swiss astronomers Mayor andQueloz, using a small French telescopewith sophisticated instrumentation. Therate of announcement of new discov-eries of extra-solar planets currently ex-ceeds several tens per year, with dis-coveries dominated by indirect methods:either the motion of the parent star in-duced by the gravitational pull of the plan-et, or the light-loss resulting as the planettransits in front of its star, as seen by us. First claims of direct imaging of planetshave already been made using 8–10-mtelescopes (see Figure 2): it is only a mat-ter of time until several reliable detectionsare available. Quantitative studies willbecome possible with advanced adaptiveoptics, using coronographic techniques tosuppress the glare from the planet’sparent star. Studies of Earth-like planets,especially via spectroscopy, will howeverremain impossible.

Extremely Large Telescopes offer spec-tacular advances in studying planetarysystems. In addition to the improved col-lecting area, needed for observing suchfaint objects as the smaller extra-solarplanets, the improved resolution allowscleaner separation of a planet from the image of its star. As a result, one ofthe most exciting new opportunities for Extremely Large Telescopes is the abil-ity directly to detect and to study largesamples of planets in other solar systems.

Planets of course come in a wide range of types, sizes and distances from theirparent stars. What sort of planets can bestudied with different types of telescope,and how many different planetary systemsmight one be able to detect? Simula-

Are we alone? Direct detection of earth-like planets in extra-solar systems and a first search for bio-markers (e.g water and oxygen)becomes feasible.

Direct study of planetary systems during their formation from proto-planetary disks will become possible for many nearby very youngstars. In mature planetary systems, detailed spectroscopic analysisof Jupiter-like planets, determining their composition and atmos-pheres, will be feasible. Imaging of the outer planets and asteroids inour Solar System will complement space missions.

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

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

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

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

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

Direct mapping of space-time, using the most distant possible trac-ers, is the key to defining the dominant form of energy in the Uni-verse. This is arguably the biggest single question facing physicalscience.

In the last decades astronomy has revolutionised our knowledge of the Universe, of its contents, and the nature of existence. The nextbig step is likely to be remembered for discovering the unimaginednew.

Are there Terrestrial planets orbitingother stars?

How typical is our Solar System? Whatare the planetary environments aroundother stars?

When did galaxies form their stars?

How many supermassive black holesexist?

When and where did the stars and thechemical elements form?

What were the first objects?

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

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

Extending the age of discovery

Table 1: Highlight science cases for a 50–100-m Extremely Large Telescope.

2MASSWJ1207334-393254

778 mas55 AU at 70 pc

N

E

Figure 2: Infrared image obtained withthe NACO adaptive optics facility onthe VLT of the young (~ 10 Myr) browndwarf 2M1207 (centre) in the nearbyTW Hydrae association (Chauvin et al.2004). The fainter object seen near it at an angular distance of 778 milliarc-sec has recently been confirmed to begravitationally associated with thebrown dwarf. Models suggest that it isa giant exoplanet with a mass about five times that of Jupiter. The source isvery young, is still liberating consider-able energy as it contracts and cools,and probably formed in a way unlikethat of planets in our Solar System. AnELT is essential to image fainter planetslike our Earth, particularly as they arelikely to be closer to their parent stars.

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Hook I. et al., Science with Extremely Large Telescopes

tions of observations of extra-solar plan-ets show that a 30-m telescope at a“standard” site, equipped with suitablysophisticated adaptive optics instrumen-tation, should be capable of studyingJupiter-like gas giant planets out to sever-al tens of light years, while only a muchlarger, 100-m-class, telescope would becapable of detecting and studying a sam-ple of Earth-like planets – the key here is the extremely high spatial resolutionneeded to observe an object that is about10 billion times fainter than, and veryclose to its parent star. Earth, for exam-ple, would appear only 0.1? from the sunif the Solar System were observed from a distance of 10 parsecs (~ 30 light years):see the simulation in Figure 3.

The habitable zone is the narrow region ina planetary system where water exists in liquid form: this is a prerequisite for lifeas we know it. Not all stars have plan-ets, and perhaps only a very few will haveplanets in the habitable zone, so the larg-est possible sample has to be surveyed if we are to be confident of identifying atrue Earth-twin. The number of stars thatcan be studied is approximately propor-tional to the spatial resolution to the cube(i.e. to D3, where D is the telescopediameter). A 100-m telescope can in prin-ciple detect an Earth-like planet around asolar-type star out to a distance of 100 light years. This distance limit meansthat there are about 1000 candidate Sun-like stars to be observed. The corre-sponding numbers are about 200 starsfor a 50-m telescope and 30 stars for a30-m telescope.

The large telescope collecting area, whichis the key to achieving the challenginggoal of detection of an Earth-like planet in a habitable zone, will automaticallyallow substantial extra analysis, beyond‘just’ detection: it will characterise plan-etary surfaces and atmospheres.The search for biomarkers in the planetatmosphere has the potential to discoverlife beyond our Solar System.

Massive planets

A limitation in studies of our own SolarSystem is that we have only one example:is what we see typical? unique? tran-sient? It is clear that a telescope and in-

strumentation which could detect Earth-like planets would with ease detect largerplanets, and planets with larger separa-tion from their star. Imaging of entire plan-etary systems will become possible. Such data will define the outcome of theformation of planetary systems, by dis-covering and defining the types of sys-tems which form and survive. Basic ques-tions which remain unanswered to dateinclude which stars have which types ofplanets, what conditions are required toform the various types of planet, what arethe special properties, if any, of the parentstars and are there planets around raretypes of stars (e.g. white dwarfs, very oldhalo stars: planets near neutron stars arealready known).

By repeated imaging, planets will be fol-lowed around their orbits. Variations in their apparent brightness during thisprocess then can be used to determinemany properties. For example, theirreflectivities (albedo) determine their sur-face temperatures. For larger planets,rings like those around Saturn, and thepresence of moons may be inferredindirectly from the small deviations theyproduce in brightness, position of the planet and its velocity over time.

Worlds in formation

At least as important as determining thediversity of mature planetary systems isunderstanding the formation and earlyevolution processes. Is planet formationubiquitous but survival unlikely? Or vice-versa? How long does planet formationtake? How is it terminated? What hap-pens to a planet after it forms? All these,and many related, questions requiredetection of the observable effects gener-

ated by on going planet formation aroundyoung stars. Current models, as yetuntested by direct observations, suggestthat planets form from condensations in adusty disc encircling a young star, andsubsequently create circular gaps at dis-crete positions in the disc; Figure 4 showsa simulation of this process. A telescopewith sufficient resolving power and a coro-nagraph to suppress light from the centralyoung star will be able to detect theseplanetary birthplaces, even at the innerdisc locations where habitable planetsshould form.

A sub-millimetre detection capability on asuitably large telescope would even per-mit the mapping of the colder, outerregions of protoplanetary systems out totheir Kuiper Belts, where the debris ofplanetary formation is believed to accu-mulate and survive.

Figure 3: A simulated time-series image of a solar-system analogue, containing a Jupiter-like and anEarth-like planet at a distance of 10 pc. The systemhas been “observed” at number of epochs as theplanets go around in the 15-degree obliquity orbits toillustrate the phase effect. Each epoch is representedby a 100-ksec exposure in the V-band with the OWL100-m telescope, based on adaptive-optics simula-tions. The PSF of the central star has been subtractedfrom the image. (From Hainaut, Rahoui, & Gilmozzi2005.)

0.1 arcsec

Figure 4: Simulation showing four stages in the for-mation of gas giant planets via fragmentation of proto-planetary discs (from Mayer et al. 2004). As the plan-ets form, gaps are carved in the disc. An ELT has thepotential to detect such gaps.

5The Messenger 121 – September 2005

Solar System astronomy

An Extremely Large Telescope provides a natural and valuable complement todedicated spacecraft. It would be capableof assembling a unique atlas of thesurfaces of hundreds of solar system ob-jects. Of unique value will be an ExtremelyLarge Telescope’s ability to make re-peated highly-resolved imaging and spec-troscopic observations of planets andmoons with evolving surfaces and atmos-pheres. Detailed and continuing observa-tions of this kind cannot otherwise be ob-tained except by dedicated (single-target)orbiters, none of which have yet beensent to the outer Solar System. Figure 5shows an image of Jupiter’s moon Iotaken during a fly-by of the Jupiter orbiterGalileo: a 100-m telescope for examplewould have a diffraction-limited resolutionof about 8 km (at a wavelength of 1µm) at the distance of Io, allowing detailedsurface maps to be made and changes tobe monitored (such as the volcanic activi-ty shown in Figure 5). Thermal-infraredimages at these resolutions have neverbeen secured.

Such a systematic series of imaging andspectroscopic observations would alsoallow us to follow the seasonal and long-term variability of Titan’s dense haze lay-ers, allow studies of the structures gener-ated by the gas geysers on Triton andpermit the monitoring of the evolution ofthe atmosphere of Pluto as it recedesfrom the sun.

When and where did the stars form?

When did the stars form? This basicquestion is a key puzzle in astronomy andis only partly answered: young stars arebeing born today in our and other galax-ies, but at a very low rate. Most starswere formed long ago. But when were thestars that make up the giant ellipticalgalaxies and the central bulges of spiralslike our own Milky Way formed? Toanswer this we can make use of the factthat massive stars die young. Indeedmany explode only a few million yearsafter their birth, in spectacular supernovaeexplosions, whose flash can outshinewhole galaxies. With an Extremely LargeTelescope, such supernovae could beseen to vast distances, corresponding to

Figure 5: Changes in the surface of Io ob-served by the Galileospacecraft. The imageswere taken on April 4and September 17 1997.(Credit NASA /JPL).Ground-based ELTs willallow repeated imagingand monitoring of suchevents.

redshifts up to ten in the case of a 100-mtelescope (see Figure 6) and possibly toredshift 20 for supernovae from the veryfirst (population III) stars. Redshift ten cor-responds to direct observation back to500 million years after the Big Bang, bare-ly 3 per cent of the present age of theUniverse. The frequency of supernovae atdifferent times in the history of the Uni-verse is directly related to the number ofstars that formed at that particular cosmicepoch. Measuring the rate of supernovaexplosions across the Universe can there-fore tell us when stars formed and at whatrate. Simulations suggest that a 100-mtelescope would require about 130 nightsboth to discover ~ 400 supernovae (using

Figure 6 (below): Hubble diagram, normalised to acosmological model for an empty Universe, for super-novae out to redshift 20 (from Della Valle et al. 2005).Pink dots are simulated Type Ia SNe, black dots Type II (+Ib/c), blue and green dots are Ia SNe actuallydiscovered by groundbased telescopes (Perlmutter etal. 1998, 1999; Riess et al. 1998; Knop et al. 2003,Tonry et al. 2003) and from HST (Riess et al. 2004).The SNe have been distributed around the track ΩM = 0.3, ΩΛ = 0.7 after taking into account the intrin-sic dispersion of the peak of the luminosity of Type Iaand II SN populations, while the photometric errorshave been derived from the S/N ratio that has beencomputed for each simulated observation, assuminga 100-m telescope. Red dots represent SNe fromPop III star population. A 100-m ELT could generatesuch a sample of supernovae in about 130 nights ofobserving, including crucial measurements of spec-troscopic redshifts and determination of the superno-va types.

Ωm = 0 ΩΛ = 0

∆m

Ωm = 1 ΩΛ = 0

Ωm = 0.3 ΩΛ = 0.7

Ωm = 0 ΩΛ = 1

0

-4

-3

-2

-1

0

1

5 10z

15 20

6 The Messenger 121 – September 2005

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near-infrared imaging in the J-, H- and K-bands) and to carry out spectroscopyto confirm their nature, redshift andproperties. Such a sample will provide areliable measure of the star-forming his-tory of the Universe back to a time whenthe Universe was a few per cent of itspresent age.

When did the stars assemble into today’sgalaxies?

How did the galaxies that we observearound us come to be formed? Thisremains one of the outstanding questionsin modern astronomy. The current bestmodel suggests that a hierarchical se-quence of mergers of smaller componentgalaxies built up most of the galaxies wesee today. Indeed, recent studies of our own Milky Way galaxy have revealed afew small galaxies currently merging intothe Milky Way, while similar behaviour isapparent in our neighbour the Andromedagalaxy M31. Detailed analysis of merg-er events gives clues as to the timing of the main mergers in a galaxy’s history andthrough this, the role of the mysteriousdark matter, which must play an importantpart in galaxy formation through its domi-nant gravitational effect.

Up until now these studies have been lim-ited to our own Galaxy and its nearestneighbours. But do all galaxy types havesimilar merger histories? How important is environment? To study a representativesection of the Universe requires reachingat least the nearest large galaxy clus-ters which contain large elliptical galaxies.This means observing galaxies in theVirgo or Fornax clusters at distances of 16or 20 mega-parsecs respectively. Initialfeasibility studies look very promising –simulations show that a 100-m-class tele-scope should be able to resolve individ-ual stars in galaxies in the Virgo cluster,and obtain sufficiently accurate photome-try to determine their ages and composi-tion, even for the oldest, hence faintest,unevolved stars. Spectroscopic observa-tions of the brighter stars will also be pos-sible, allowing measurements of the kine-matic motion of the stars and accuratedetermination of their chemical composi-tion. From these a detailed picture will bederived of the process by which (and,

-1

-8

-6

-4

-2

0

2

40 1

AGB

(V–I)

MI

2 3

RC

RGB

MS

HB

BL

indeed, of the components from which)the target galaxies were assembled, andthe role of dark matter in this process.

The physics of galaxy formation

To understand the creation and evolutionof galaxies in general we must addresswhat is one of the major goals of futureastrophysics: to map the distribution andgrowth of both the baryonic (normalmatter) and dark matter components ofgalaxies at moderate to high redshift (z = 1–5), a key epoch for galaxy forma-tion. Although individual stars cannot be resolved at these cosmological dis-tances, a 50-m to 100-m Extremely LargeTelescope will not only resolve the dis-tant galaxies into their luminous compo-nents, but will be able to characterisethese individual components.

Using techniques such as integral-fieldspectroscopy, in which spectra are ob-tained at thousands of locations across a(proto-) galaxy simultaneously, it will bepossible to determine the relative star for-mation rate, the mass of stars and thechemical composition at these differentlocations within each galaxy. This willshed light on the “feedback” mechanismsbelieved to affect the formation of galax-ies, such as the effects of a newly-formedactive galaxy nucleus, or supernova ex-plosions, on surrounding star formation.

In addition, the bulk motions of gas andstars inside galaxy could be determined,thus allowing one to map the dark mattercontent of individual galaxies at a range ofredshifts, corresponding to epochs whenthe galaxies were in the process of as-sembly. Then, measuring the kinematicsof their satellite objects, both internal andrelative to their more massive partners,we can estimate the amount of and inferthe distribution of the mass present in thegalaxy’s halo, which is one of the fewways we have of detecting and examiningthe dark matter and its distribution.

This will provide astronomers with a de-tailed evolutionary history of the clump-ing of dark matter (Figure 8). We will “see”galaxy formation in all its glory from for-mation to maturity, and so directly test ourunderstanding of the basic evolutionaryprocesses in the Universe.

Supermassive black holes

The centres of most, perhaps all, galaxiesharbour supermassive black holes. Theseexotic objects are usually discovered indi-rectly, as extreme radio- or X-ray-lumi-nous sources, quasars and Active Galac-tic Nuclei. Direct studies, critical forreliable mass determination, and essentialwhen the hole is not active, are possi-ble only when precision studies of thevery local region of the galactic nucleusare feasible: only (relatively) close to a

t ≤ 0.10.1 < t ≤ 0.40.4 < t ≤ 1.01.0 < t ≤ 3.03.0 < t ≤ 6.06.0 < t ≤ 10.013.0 < t ≤ 10.0

Labels indicate differentstellar evolutionaryphases: BL – Blue LoopHB – Horizontal BranchRC – Red ClumpRGB – Red Giant BranchAGB – Asymptotic giant

branchMS – Main Sequence

Figure 7: A predicted distribution of stars as a func-tion of magnitude (vertical axis), colour (horizontal axis)and age, for a plausible galaxy model (from Aparicio &Gallart 2004). The colour coding represents stars in different age intervals, with units in billions of years. A 100-m-class ELT would have the resolving powerand collecting area needed to place individual starson such a diagram, and hence measure the age of thestellar populations, in galaxies as distant as the Virgocluster.

7The Messenger 121 – September 2005

z = 4.71 z = 4.00 z = 2.64

z = 2.08 z = 1.56 z = 1.35

z = 0.90 z = 0.38 z = 0.00

Figure 8: Simulations showing Dark Matter particleswithin a cube of 320 physical kpc on a side, shown at various redshifts and projected so that the luminousgalaxy at z = 0 is seen edge-on (from Abadi et al.2003). The bottom right panel zooms into the inner-most 40 kpc of the system. Each particle is colouredaccording to the logarithm of the local dark matterdensity using a palette that runs from red to blue: redand blue correspond to ρdm greater than and less than about 1010 MA kpc–3 respectively. An ELT has thepotential to observe such galaxy haloes in the pro-cess of formation: observations of the motion of thecentral galaxy and its satellites will provide a mea-surement of the distribution of dark matter in the ex-tended galaxy halo.

black hole is the gravity of the whole gal-axy dominated by the mass of the black hole, so that the black hole’s pres-ence can be deduced. The methodology has been proven by observations at ESOover many years proving the existence of a massive (~ 3 × 106 MA) black hole in the core of the Milky Way galaxy. Directmeasurements of the speed at whichstars and gas clouds are orbiting the cen-tre of a galaxy are required. The closer to the centre these can be measured, themore reliable is the evidence for, and the determination of the mass of, theblack hole.

For reasons which are not understood,the evolution and mass of the supermas-sive black holes is apparently very close-ly related to the properties of the verymuch larger host galaxy. Understandingthis, and determining if it is indeed ubiq-uitous, would be the first clue relating thenuclei of galaxies to their major parts, and the first link between the exotic andthe typical in galaxies. How do the blackholes first form? How do they grow, and at what rate? Are growing black holesalways active? How does a central black hole “know” the properties of thelarger galaxy in which it resides? Doesevery galaxy have a massive black hole?

All these questions require for progressdetailed study of the masses and ubiquityof central black holes. This requires thehighest possible spatial resolution andfaint-object spectroscopy, attainable onlywith an extremely large telescope. Forexample, Figure 9 shows that a 100 mtelescope working at its diffraction limitcan in principle resolve the sphere of influ-ence of a supermassive (109 MA) blackhole at all redshifts across the Universe(provided they exist of course!) and evendetect the influence of a modest 100 MA

black hole in the local Universe, out toabout 1 Mpc from us. It will then be possi-ble to carry out a systematic census ofblack holes as a function of cosmic epochand begin to understand their formation inrelation to the galaxies around them.

The ionisation of the Universe

The early Universe was hot (ionised) andtransparent. With time, the gas cooled.The aftermath of the Big Bang left the

early Universe an opaque gas of hydrogenand helium. Some time later, the first ob-jects heated the hydrogen and helium,making it (again) transparent – the “era of(re-)ionisation’’. A key goal of astrophysicsis to understand how and when the firstluminous objects in the universe formedfrom the primordial gas, what they were,and how they contributed to ionising andenriching the gas with heavy elements.

Tantalising questions about the re-ionisa-tion history of the universe are raised byrecent results. Those from the Wilkinson-MAP Cosmic Microwave Background sat-ellite probe, when combined with ground-based surveys of the large-scale structureof the Universe today, suggest that thegas was re-ionised by about 180 millionyears after the Big Bang (redshift ~ 17)while observations of the highest redshiftquasars at about 700 million years (red-shift ~ 6) demonstrate that enough of theintergalactic medium remained un-ion-ised at that time to absorb almost com-

pletely all radiation bluer than the Lyman αrecombination line of HI (Figure 10). Whatis the solution to this apparent quanda-ry? It may be that there were two re-ioni-sation epochs, an earlier caused by thefirst generation of massive stars, followedby cooling, and then one later by the first quasars and galaxies. Alternatively, aslower, highly inhomogeneous re-ioni-sation process may have occurred over the period between the two epochs.

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Figure 9: Left: Artist’s conception of an AGN with theblack hole surrounded by accreting material andejecting jet at relativistic velocities. Right: The impactof a 100-m-class telescope on studies of interme-diate and massive black holes would be considerable.Shown here are the distances to which the sphere of influence can be resolved (for comparison, the res-olution of a 30-m telescope is also shown). With a100-m ELT we be able to detect 109 MA black holesat all redshifts (where they exist). Here we assume a cosmology of H0 = 70 km s–1Mpc–1, ΩM = 0.3 andΩΛ = 0.7. The point spread function is given by1.22 λ/D, where D is 30 m or 100 m and λ = 1 µm.(Figure credits: Left: GLAST/NASA, Courtesy Aurore Simonnet, Sonoma State University, Right: M. Hughes.)

Figure 10: Spectra of quasars of increasing redshiftillustrating the increase in absorption due to inter-vening neutral gas with increasing redshift. At thehighest redshifts these show a “Gunn-Peterson” ab-sorption trough – the complete absorption of light at wavelengths shortward of the Lyman α line ofatomic hydrogen – implying that the re-ionisationepoch which began at redshift ~ 20 must have contin-ued until redshift ~ 6. An ELT’s supreme sensitivity to point sources (such as quasars, gamma ray burstsand supernovae) will allow observations of the ionisa-tion state of the Universe to very high redshift, possi-bly all the way back to when the re-ionisation processbegan. (Figure shows observations by W. Sargent & M. Rauch; Tae-Sun Kim; M. Pettini. Compilation by R. Carswell.)

Supermassive black holes

log(

RB

H)[

arcs

ec]

1-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

2 31+z

5 64

100-m ELT30-m ELT109 M

A

108 MA

107 MA

106 MA

These models, together with other morecomplex possibilities, could be tested ifwe can observe the ionisation state of thehigh-redshift Universe directly: this isfeasible, through analysis of the absorp-tion features produced in the spectra of suitably-luminous very distant “back-ground” objects. There are a few popu-lations of sources that could be observedat such very high redshift with an ELT. The short-lived gamma-ray bursts areextremely bright for a short time, so muchso that they should be detectable up toredshift ~ 15–20. Supernova explosionsof the first stars to form, though not yet detected, would probably be fainterthan this, but could still be used to probethe state of the gas at redshifts up to 10.This population of “first supernovae” maywell disappear once the local heavy-element enrichment becomes higher than1/10000 of the solar value. Testing thisprediction will itself be a major challenge,and discovery.

Quasars are currently used as powerfulbackground sources, and will continue to be useful in future, if they exist at high-

er redshift. Although the epoch of firstquasar formation remains an open ques-tion, the quasars being found at redshiftsaround 6 are (presumably!) powered bysupermassive black holes, so we inferthat intermediate-mass black holes (cor-responding to quasars of intermediateluminosity) must have existed at earlierepochs, up to at least redshifts of about10. Probing the physics of the gas in theearly Universe requires intermediate/high-resolution spectroscopy of these “back-ground” sources in the near infrared, thenatural domain of ground-based Extreme-ly Large Telescopes. Apart from the veryrare extreme gamma-ray bursters (and/orbursts caught very early), which could beobserved with a 30-m-class telescope,spectroscopic observations of these faintbackground objects can only be carriedout with telescopes of the 60–100-m class.

The first galaxies

The first galaxies, being the places of for-mation of the first stars (a prediction wellworth verifying!) compete with the firstquasars for the re-ionisation of the gas inthe early Universe. Candidate star-forminggalaxies out to redshift about 6 havealready been discovered and a few havebeen confirmed spectroscopically. Equiv-alent objects are expected to exist out to redshifts greater than 10 for severalreasons. Firstly, the analyses of the fluctu-ations in the Cosmic Microwave Back-ground indicate ionisation of the universeat redshifts > 10, presumably by ultra-violet emission from the first objects. Sec-ondly, SPITZER satellite observations ofthe higest redshift galaxies known to dateshow evidence for old stellar populations– indicating that these galaxies formed

9The Messenger 121 – September 2005

5011 5225 5296 5307 5788

2399 3250 3262 3968 4173

443 712 1276 1392 2225

Figure 11: HST/ACS images of high-redshift (z ~ 5–6)galaxies from Bremer & Lehnert (2005) showing thesmall but resolved nature of the galaxies, with typicalhalf-light radii of 0.1–0.2 arcsec (1–2 kpc). A 50 to100-m ELT will not only measure detailed physicalproperties of such galaxies but will also be capable offinding and confirming (with spectroscopy) galaxies atsignificantly higher redshifts (10 and beyond), possiblyincluding the first galaxies to form in the Universe.

much earlier. Very high-redshift star-form-ing galaxies will probably be detectable inconsiderable numbers with future space-craft (James Webb Space Telescope) andground-based (ALMA) facilities. Howevera 100-m-class Extremely Large Telescopewill be needed to provide the desired di-agnostics of the astrophysics of both thegaseous interstellar medium and the earlystellar populations in these galaxies.

Furthermore, a sub-millimetre capabilityon an Extremely Large Telescope, if it were at a suitable site, would allow a large-scale survey (with mapping speedthousands of times faster than ALMA at 850 µm) that would detect the millionsof dusty high-redshift galaxies which(probably) contribute the cosmic far-infra-red and sub-mm background, resolveddown to quite faint levels throughout theUniverse. With redshift estimates fromsub-mm flux ratios, such a survey wouldyield a treasure-trove of information on large-scale structure from very earlyepochs to the recent past.

Dark energy and fundamental physics

The recent discovery of the acceleratingexpansion of the Universe has led to anurgent need to understand the nature ofthe mysterious “dark energy” which isdriving this expansion. The dark energy isbelieved to account for about 70 per centof the energy budget of the Universe (Figure 12, see next page) and yet itsnature is completely unknown. One po-tential candidate is the vacuum energyimplied by the “cosmological constant”term in Einstein’s field equations (whosesolutions represent global pictures of the Universe). However measurements ofthe effects of dark energy on cosmo-logical scales constrain its contribution tobe many orders of magnitude smaller than the vacuum energy scale predictedby particle physics theories.

The direct measurement of the dynamicalexpansion history of the Universe via Type Ia supernovae has shown that thedark energy exerts a negative pressureand hence accelerates the universalexpansion. Direct analysis of the expan-sion rates of the Universe across space-time is needed to investigate this remark-able form of energy. Intriguingly, most

of the effects of dark energy are apparentat relatively low redshifts (less than aboutz = 1), although equivalent studies at highredshift, when feasible, may well have theirown surprises in store.

An Extremely Large Telescope can deter-mine the expansion history of the Uni-verse using several different and comple-mentary astrophysical objects, thusdecreasing any dependency on possiblyunknown systematic effects. The well-understood primary distance calibrators,pulsating Cepheid stars, globular clusters,planetary nebulae and novae, could all inprinciple be observed to cosmologicaldistances where the effect of dark energyis dominant in the Universe. The exquisitesensitivity to point sources of an Extreme-ly Large Telescope with appropriate adap-tive optics capability, combined with itsimpressive collecting area, will allow it todetect Type Ia supernovae to redshift of about 4, and Type II supernovae (whichcan also be used as distance indicatorsvia the expanding photosphere method)possibly all the way to redshifts of aboutten (see Figure 6).

By mapping the geometry of the Universeon the largest scales and accuratelydetermining any variations of the strengthof dark energy with time, astronomers can answer the fundamental question ofwhether dark energy corresponds to Einstein’s cosmological constant or to some “quintessence field” as sug-

gested by modern versions of quantumfield theories. The need for these obser-vations is critical and the implications forall of physics and cosmology are vast.

Furthermore, ELTs offer the intriguing pos-sibility of observing the expansion of the Universe directly – by observing theminute increase in redshift of absorp-tion lines in quasar spectra over a periodof about 10 years it will be possible towatch the Universe expanding in “realtime”! Such detailed and direct measure-ments will provide an important test ofcosmological models and also allow testsfor the constancy of fundamental parame-ters such as the fine structure constant.

Cosmological observations have nowbecome the only way to characterise sev-eral of the most promising unexploredsectors beyond the Standard Model of particle physics. The discovery and de-scription of dark energy is possible onlywith cosmological-scale observations: nosmall-scale effects are yet known. Howev-er, dark energy and dark matter must be part of the process towards under-standing the next generation of theory inPhysics: they are related to super-sym-metric particles, string theory, theories ofgravity and quantum gravity, theories of higher dimensions, and the constan-cies of the fundamental constants. AnExtremely Large Telescope is the next bigstep in direct observation of the natureof the Universe.

10 The Messenger 121 – September 2005

Figure 12: Our current understanding of the mass-energy content of the Universe. All “normal” matter isa minor contribution of only about 4 %. Dark matter, of unknown nature dominates mass. Dark energy, ofunknown identities, dominates the Universe. What is it? ELTs can tackle this question by measuring theexpansion history of the Universe using a range ofindependent techniques.

Why an ELT now?

The relatively large apertures which areaffordable and technically feasible for groundbased telescopes means thatthese facilities are the natural means to provide maximal light-gathering power.Natural complementarity exists betweenthese and orbiting observatories which,although considerably more expensive forthe same size of telescope, benefit frombeing clear of the thermal backgroundand the seeing effects of the Earth’s at-mosphere.

For example, routine images from theHubble Space Telescope’s AdvancedCamera for Surveys reveal objects whichare so faint the largest existing teles-copes are unable to acquire their spectra.Without spectroscopic information we can learn only a limited amount about thebasic nature and properties of an astro-physical object. The advent of the JamesWebb Space Telescope, currently sched-uled for launch in 2012, will increase thisimbalance. Until the astronomical com-munity acquires complementary ground-based facilities which are much largerthan those available at present, the major-ity of future discoveries will be beyond our spectroscopic reach and detailed un-derstanding. This is a major reason why astronomers are urgently seeking to begin construction of the first ground-based Extremely Large Telescopes.

Space observatories which are designedfor observations at wavelengths inacces-sible from the ground (because of absorp-tion by the Earth’s atmosphere), such as the flagship X-ray facilities XMM-New-ton and Chandra, regularly discoversources which are too faint in the wave-length range readily accessible to theground, the optical and near infrared, tobe detected or investigated by existingtelescopes. Planned next-generation mis-sions will further increase the need for a major enhancement in the performanceof our large optical/near infrared tele-scopes if the new phenomena which theyreveal are to be understood.

New radio and sub-millimetre astronomygroundbased facilities are being built that will also naturally complement an Ex-tremely Large Telescope’s optical and infrared capabilities, and will discover

sources which will demand further studyat other wavelengths. For example the Low-Frequency Array (LOFAR), duefor completion in 2008, will operate atlong radio wavelengths, and a more ambi-tious project, the Square Kilometer Arrayis being proposed to follow as the next-generation radio facility. In the sub-mmbands the Atacama Large Millimeter Array(ALMA), an interferometer currently in the initial stages of construction and a keyelement of ESO’s scientific strategy, will provide very high sensitivity and spa-tial resolution beyond the limits of cur-rent ground-based telescopes. ALMA isdue to be fully operational by 2012 andwill cover a very wide range of science,detecting both thermal continuum emis-sion from dust and line emission in objects from the nearest star-forming re-gions to luminous galaxies at very highredshift. Groundbased Extremely LargeTelescopes will be ideally matched to pro-vide imaging and spectroscopic follow-upof these sources at optical to mid-infra-red wavelengths, with matched angularresolution.

Science Case Development

This summary is based on a full sciencecase document developed at a series of meetings over four years involving over100 astronomers. The work is sponsoredby the EC network OPTICON, and main-tained as part of the European ExtremelyLarge Telescope Design Study, funded inpart by the EC FP6 programme, by ESO,and by many European national agenciesand organisations.

WWW sites for further information:OPTICON European ELT science case

work: http://www.astro-opticon.org/networking/elt.html

Euro-50 telescope web site:http://www.astro.lu.se/~torben/euro50/

OWL telescope web site:http://www.eso.org/projects/owl/

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Dark energy73 %

Atoms4 %

Cold dark matter23 %

Hook I. et al., Science with Extremely Large Telescopes