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Highlights from the Science Case for a 50-100m Extremely Large
Telescope
Isobel Hook, University of Oxford
On behalf of the OPTICON European ELT science working group
Keywords:Terrestrial Exoplanets; Galaxy evolution; Cosmology and cosmological constants
ABSTRACT
We present an overview of the science case for a 50-100m Extremely Large Telescope. This was the subject of a
meeting in Marseilles, France in November 2003, attended by about 50 European astronomers. Four key scientific
themes were identified by the participants: terrestrial planets in extra-solar systems; stellar populations across the
Universe; building galaxies since the darkest ages; the first objects and re-ionisation structure of the Universe. Although
by no means an exhaustive list of science areas in which ELT will have a great impact, these cases provide examples
where an ELT can make a dramatic advance in our understanding of the Universe around us. This paper describes these
highlighted science themes and the challenging demands they place on ELT performance. See http://wwwastro.physics.ox.ac.uk/~imh/ELT/ for more information, including the full list of participants in this work.
1. TERRESTRIAL PLANETS IN EXTRA-SOLAR SYSTEMS: ARE WE UNIQUE?
How common is life that we see around us on Earth? How many planetary systems resemble our own Solar
System? How common are terrestrial planets like the Earth? How many lie on orbits we would regard as habitable?
And on how many can biomarkers - the chemical signatures left by biological processes – be detected? We are
fortunate to be living at the first moment in the last 2000 years of human inquiry in which these fundamental
questions can be addressed scientifically.
A 50-100m European Large Telescope will make possible a detailed understanding of how planet formation is
related to the conditions believed to be required for life as we know it. The enormous light-gathering power of
such a telescope, combined with the ultra-sharp images expected from Adaptive Optics, will allow an
unprecedented level of discovery and understanding. Astronomers will be able to draw a continuous thread from
the earliest times of our own Solar nebula, through the gaseous disks of young stars that are the cradle of new
planets, to the end state of mature planetary systems around 1000 of the nearest stars in a search for Earth-like
analogues. Most of the breakthroughs in this ambitious endeavour require extremely high levels of contrast
suppression (106 – 1010) in order to detect faint planetary signatures in the vicinity of a bright parent star. Indeed,
confirming the feasibility of these scientific requirements is a major goal of the proposed European Large
Telescope design study.
With an ELT of 50-100m class it would become possible to not only directly detect (by imaging) earth-like planets
orbiting other stars, but also study large numbers of them in detail (via spectroscopy). The main elements of such a
study include the following (discussed in more detail below).
•
•
A large survey: A survey of the surroundings of about 1000 stars will give a large enough dataset to draw
meaningful conclusions on the rarity of terrestrial planets. To survey this many stars requires observing
out to distances of about 30 parsecs from us (~100 light years). At these distances the projected separation
between the star and its planets becomes very small (less than 0.1 arcsec) and an extremely large telescope
is needed to resolve them.
Measuring planet properties: By obtaining spectra of exo-planets can we determine their surface properties
(are they liquid or solid?) and search for "bio-markers" such as water, oxygen and carbon dioxide. Again
•
•
•
an extremely large telescope is needed to collect sufficient light from a faint planet to be able to analyse it
spectroscopically.
Measuring planet orbits : Multiple observations of each exo-planetary system will reveal the orbits of the
planets, i.e. their period and eccentricity. These parameters are also related to a planet's ability to support
life.
Studying entire exo-planetary systems : While searching for terrestrial planets, an ELT will also detect the
larger giant gas planets (equivalents of Saturn and Jupiter) in the exo-system. From this we can determine
how common are systems with multiple planets of varying sizes (like our own solar system). We can also
characterize the systems in different environments, such as around metal-poor stars, white dwarfs, massive
stars and brown dwarfs.
Disentangling the planet formation processes : An ELT could be used to study planets in the habitable
regions around newborn, low-mass "T Tauri" stars. These stars are typically located at distances of 150 pc
and beyond, so a resolution of 2 milliarcsecond is needed to properly sample their habitable regions.
.
1.1. Terrestrial Planets and their Planetary Environments
Only nine years ago, the first planet around a star other than the Sun was detected indirectly. Today, over 120
extra-solar planets are known, the vast majority of which are unlike the planets in our own Solar System. In the
next ten years, astronomers using current 8-10m class telescopes expect to perform the first direct detections of
gaseous giant planets, using advanced adaptive optics and coronagraphic techniques to suppress the glare from the
planets’ parent stars by factors of up to 107.
It is the next generation of 50-100m extremely large telescopes, however, that will address critical issues associated
with details of gas giants similar to our own Jupiter and Saturn, and, importantly, questions about terrestrial (Earthlike) planets. Only these giant telescopes will have the light collecting power and the unprecedented resolution
(suppression levels of up to 1010) required to search many hundreds of nearby stars for the telltale light from faint
terrestrial planets. Specifically, such a telescope would be able to directly collect and analyse the light from
terrestrial planets orbiting nearby stars in ``habitable zones’’ (masses < 10MEARTH and temperatures~270-320K).
By imaging over several epochs, the orbits of these planets can be mapped. Variations in their photometric
properties then can be used to determine the albedos (reflectivities), and thereby the surface temperatures on
detected planets. For large gas giants, rings like those around Saturn would reveal themselves as changes in the
phase light curve of a planet (Fig. 1). On different timescales, the influence of planetary rotation and weather on
the surface brightness of exoplanets can be used to estimate variability in surface conditions, especially when
combined with spectra capable of detecting the signatures of molecular tracers associated with life in our Solar
System. Only the next generation of ELT will be able to detect the faint features indicative of rings, weather, and
rotation in planets similar to those in our own Solar System.
Assuming that ~1% of stars may have terrestrial planets in orbits undisrupted by eccentric gas-giant companions,
an ELT must be capable of searching stars as distant as 30pc for terrestrial planets in order to produce a
meaningfully large sample of detections. At 30pc, the Earth-Sun separation (1 AU) subtends an angle of 33 milliarcseconds (mas). To good approximation, the brightness difference between a star and habitable terrestrial planet
is 25 magnitudes in the visible, and slightly less in the near-infrared, requiring suppression ratios of 1010 over a 20300 mas orbital range. It is exciting to note that meeting this science requirement would allow the simultaneous
detection of all planets in a 30pc distant system identical to our own, excepting only Mercury and Pluto.
Fig. 1: Light reflected from a Saturn-analogue as a function of its position on its orbit, assuming that the inclination of the orbit
relative to the observer is 45 degrees. The solid line shows the phase light curve without rings; crosses denote the case with
rings. Note that rings generate complicated brightness and shadow variations as a function of orbit phase (shown in negative in
the right frame). The peak of the light curves for a Saturn analogue orbiting as close as 1 AU from its parent star would require
a brightness suppression of greater than 107 for detection. Credit: Dyudina et al, Australian National University.
More modest suppression ratios will allow the detection of signatures generated by planets just forming around
other stars. As material in the dusty proto-planetary disks known to encircle young stars begins to collapse
gravitationally to form planets, empty (and thus dark) circular gaps, or lanes, are created at discrete positions.
Telescopes with sufficient resolving power and moderately high suppression ratios, will be able to detect and these
planetary birthplaces at disk locations where habitable planets may be born. A suppression factor of 106 or more is
required to see a dark lane from a forming Earth-analogue against the background glare of its parent star (Fig 2).
Fig 2: Simulated signal generated by an Earth-analogue orbiting a small star as it forms in a disk of material
illuminated by the parent star. The disk brightness is shown on a logarithmic scale versus distance from the star; at
the position of the terrestrial planet (1 AU) a dark lane is present. Credit: Ryuichi Kurosawa and Tim Harris,
Exeter University.
1.2. What more will we learn?
In addition to the light curve and orbit mapping described above that will yield the planet’s orbital period and
eccentricity, surface brightness and rotation period, low-resolution spectroscopy with a 50-100m European Large
Telescope would allow astronomers and planetary scientists to obtain spectra from terrestrial planets in order to
examine their possible atmospheres.
In particular, broad molecular features are expected to be present in an atmosphere with temperatures of a few
hundred Kelvin; their absence in terrestrial exoplanets may rule out the presence of an atmosphere. The signatures
of O2, O3, H2O, CO, CO2, CH4 are accessible in the optical and near-infrared, and provide vital information on the
atmospheres, habitability, and indeed the presence of biospheres, of terrestrial planets. If the planet has no
atmosphere, allowing a direct glimpse at the surface, low-resolution spectra from an ELT would provide important
information on the mineralogy and geology of the planetary surface.
In summary, a critical goal for a European Large Telescope is to directly detect light from planets similar to those
in our own Solar System, including Earth-analogues. If the required technical challenges can be met, the prospects
for research of international scientific importance, spreading across multiple disciplines – astronomy, planetary
science, optics, astrobiology and biology – are enormous. Indeed, entirely new fields in extra-Solar Planetary
Science and Astrobiology will flourish, giving humankind a new perspective on the Universe and its own position
within it.
2. STELLAR POPULATIONS ACROSS THE UNIVERSE
2.1. Formation of stars across the Universe
When did stars form? To answer this basic question we can make use of the fact that every star must eventually die.
Indeed the more massive stars die in spectacular supernova explosions that can outshine a whole galaxy. With an
ELT these explosions can be seen to such vast distances (corresponding to redshift about 10) that their light has
taken most of the age of the Universe to reach us. The frequency of these explosions is directly related to the
number of stars that have formed - the rate of "core-collapse" Supernovae (those of types II and Ib/c) gives a direct
measurement of the death rate of stars more massive than 8 solar masses, and thermonuclear supernovae (Type Ia)
are the result of the death of moderate mass stars (3-8 solar masses). Measuring the rate of supernova explosions
across the Universe is a direct way to determine when stars form and in what quantities.
Current methods to measure the star-formation history of the universe often rely on measurements of ultraviolet
emission from stars or optical emission lines (such as H-alpha), which are produced by only the most massive stars,
larger than about 40 solar masses. However the supernova method is sensitive to more normal-mass stars which
make up the majority.
2.1.1. An example supernova observing campaign with an ELT
In the following example we show how an ELT could be used to discover and follow distant supernovae. For this
example we assume the design parameters of the 100m OWL telescope1. First we calculate the expected number of
SNe in a typical OWL field (2arcmin x 2arcmin). Based on previous work,2,3 the a priori expectation of SN
explosions is 8 (+/-4) per yr up to z~10. Since the typical light-curve width around maximum in the rest-frame is
about 15-20 days and most SNe will occur at z<6, the typical light-curve width in the observer frame will be about
100-140 days. Therefore, taking 4 exposures at intervals of about 3 months, one would expect to detect about 6-8
SNe in a typical OWL field. In addition, from the computed rate4 of 3.9x10-6 pair-instability supernovae per second
per square degree (at z=20), we estimate to detect, over a 50 fields survey, up to 7 SNe originating from the very
first generation of stars (Pop III star population).
Next we calculate the time needed to complete the search and follow-up campaign. A 50-field survey is suitable to
obtain a statistically significant number of SNe for each z-bin. The survey would consist of imaging 50 fields (200
sq arcmin) in the J(1h each), H(1h each) and K(1h each) bands at 4 different epochs for the `Supernova search',
complemented by 3 epochs in the K band for the photometric follow-up of about 350 SNe up to z ~ 10. An
additional 4h would be needed to obtain the spectroscopic classification up to z~ 4.5/5. The Grand Total time is
therefore 4 x 3h x 50(=600h) + 3hx50(=150h) + 4hx50(=200h)= 950h+10% for overheads ~1050h or 130 nights
(equivalent to 4 months) to study 400 SNe (200 of which would be followed spectroscopically). This is
comparable to the size of current HST Treasury Programs (~ 450 orbits).
Fig 3. Simulated Hubble diagram, normalised to a cosmological model for an empty Universe, for supernovae out to redshift 10.
The points at z<2 are taken from recent Type Ia Supernova studies5,6. The points at z>1 represent simulated observations with a
100m ground-based ELT. Black circles represent core-collapse supernovae, visible to z~10, while pink squares show how Type
Ia supernovae populate the diagram out to z~4.5. The dispersion of the simulated observations is representative of the
photometric errors and it does not include the intrinsic scatter in the luminosity at maximum of the respective SN populations. In
addition to providing a measurement of the Star-formation history of the universe as described in the text, distant Type Ia
supernovae (and also TypeII SNe if calibrated by the expanding photoshere method) can be used as standard candles to
measure cosmological parameters including the equation of state of the dark energy component, which is causing the current
acceleration of the Universe. The most distant supernovae can also be used as bright background sources to study the ionisation
history of the universe (see section 4). Figure credit: Massimo Della Valle and Roberto Gilmozzi.
In summary, a supernova sample measured with a 100m class ELT provides a measurement of the star formation
rate up to z~10 which is
• independent from other possible determinations,
• More direct, because the initial mass function extrapolation is much smaller, and, possibly,
• More reliable, because it is based on counting SN explosions, i.e. explosion of STARS, rather than relying
on identifying and measuring correctly the source of ionization (if using the H-alpha flux) or the source of
UV continuum.
2.2. Resolved stellar populations in a representative sample of the Universe
How did the beautiful galaxies that we observe around us come to be formed? This remains one of the outstanding
questions in modern astronomy. It is now believed likely that mergers between galaxies play an important part in
the build-up of the galaxies we see today. If so, we would expect to see evidence of these past mergers.
Recent studies of individual stars in our own Milky Way galaxy have revealed populations of stars with distinct
ages and composition. These distinct populations are thought to be remnants of previous mergers, and give clues as
to when the main mergers in the Milky-Way's history happened. Up until now these studies have been limited to
our own Galaxy and its nearest neighbours. However it is unknown whether these are special cases and whether the
merger history is similar for all galaxy types. In particular, our own galaxy is a spiral galaxy, and no examples of
large elliptical galaxies are within reach of current telescopes for this type of study.
To study a representative section of the Universe requires reaching at least the nearest large galaxy clusters which
contain large elliptical galaxies. This means observing galaxies in the Virgo or Fornax clusters at distances of 16 or
20 Mega-parsecs respectively. The challenge here is twofold. Firstly individual stars at these distances appear very
faint (about V=35 magnitudes). Secondly the stars must be individually resolved from each other in order to
determine their ages and chemical composition. As in the case of detecting faint planets, both these challenges are
addressed in parallel with an extremely large telescope - the sheer collecting area allows the colours (hence ages
and chemical composition) of very faint stars to be measured (by imaging), and the diameter of the telescope allows
the image of each star to be sufficiently sharp that they can be separated even in crowded regions, provided the
telescope is equipped with an adaptive optics system that allows it to observe close to the diffraction limit.
Simulations by P. Linde and by Frayn7 demonstrate the accuracy with which 50m and 100m ground-based
telescopes respectively are able to recover the colour magnitude diagram of individual stars observed in distant
galaxies (see Figure 4). One of the most important age-metallicity indicators is the position of the main-sequence
turnoff in in the colour-magnitude diagram. Figure 4 (right panel) shows the distance to which the main sequence
turn-off could be observed at as a function of the age of the stellar population (older populations have fainter mainsequence turnoff). The thick lines show the expected range of a 30m (lower line) and 100m telescope (upper line).
Since the distance modulus of the Virgo cluster is approximately m-M=+31.0 mag, this diagram shows that a 30m
telescope could detect the main-sequence turnoff at Virgo distances for only the youngest stars. To observe this in
populations of any age requires a larger, 100m telescope.
Figure 2 Left : simulation by P. Linde of the colour-magnitude diagram of a stellar population at a distance of 5Mpc, as
observed through narrow-band filters with a 50m ground-based telescope. Right: Maximum distance to which the main
sequence turn-off can just be resolved as a function of population age and limiting magnitude7. The thicker lines represent the
limiting magnitude that can be reached with a 30m (lower line) and 100m (upper line). Thus to detect the main sequence turn off
for all stellar ages at the distance of Virgo (distance modulus of approximately m-M=+31.0) requires a 100m class telescope.
3. BUILDING GALAXIES SINCE THE DARKEST AGES: THE GROWTH AND EVOLUTION
OF HIGH REDSHIFT GALAXIES
How have galaxies grown from the first cosmological seeds? Our current understanding of this process relies on
measuring the star-formation history of the Universe. However, because this is an integral quantity, it does not tell
us much about how galaxy growth and evolution relates to their final or current mass. Predicting star-formation
rates does not involve any details of how star-formation proceeded within any individual galaxy or class of
galaxies, and indeed several radically different kinds of models for the growth of galaxies can provide good fits to
these data. Today, we do not know much of the detailed physics of galaxies in the distant Universe or how starformation proceeded within any individual galaxy.
More fundamentally, simply studying the baryonic component of galaxies is inadequate since the mass of any
structure in the Universe is dominated by dark matter, and because of the complexity of the physical processes that
control the growth of galaxies (shocks, feedback from massive stars and AGN, merging, interplay between dark
and baryonic matter etc.). One of the major goals for the future of astrophysics is to map the distribution and
growth of both the baryonic and dark matter components of galaxies at moderate to high redshift (z=1.5-5). This
can be accomplished with a 100m-class telescope by mapping out the spatially resolved kinematics, star-formation,
and chemical abundances of galaxies as well as measuring the kinematics of their satellite objects (both their
internal kinematics and their velocity relative to the most massive component). In addition, kinematic
decomposition of galaxies into disk/bulge components would allow us to reliably trace the build up of both
components - and test the influences of environment on galaxy star formation and morphology. This is a unique
avenue to understand the growth and evolution of both the baryonic and dark matter components of high redshift
galaxies.
3.1. Our Current Understanding of Dark Matter and the Growth of Galaxies.
A local census of the distribution of the baryonic mass reveals – albeit with large uncertainties -- that of the baryons
presently locked in stars, a majority reside in spheroids (as opposed to disks8). When and how did all these baryons
come to reside in spheroids and disks? There are two competing explanations. The classical pictures are the
``monolithic collapse''9 versus the (hierarchical) merging model10. The hierarchical picture of galaxy formation and
evolution is, for many excellent reasons, widely favored11. It predicts that small galaxies formed first and that
massive galaxies grew at later times by the accretion and merging with smaller (proto-)galaxies. Monolithic
collapse, as the name suggests, is a simple process in that the gas collapses over a few crossing times and the starformation proceeds rapidly. When including realistic feedback mechanisms from the intense star-formation, such
collapse is the stretched out to about 1 Gyr. The debate is not between which model is correct since merging
obviously plays an important role in galaxy evolution. The real questions are “how did star formation proceed in
galaxies?”, “are there galaxies that formed anti-hierarchically?”, and “is our understanding of the hierarchical frame
work complete?”.
Unlike the complexity and the large number of (non-linear) physical processes that are involved in the growth of
the baryonic component of galaxies, the growth of the dark matter component should be through simple
gravitational, non-dissipative accretion and merging of individual ``clumps'' of dark matter. While it is true that the
processes involved for the dark matter can also take on a highly non-linear character and can interact with the
baryonic matter, these are driven by the large scale distribution of dark matter which is not uniform but is simply
set by the initial conditions of the early universe. However, all these effects are soluble using complex codes and
large computers with the only limitation that there is a size and mass scale beyond which the current generation of
super computers can not yet model. One of the best ways of addressing which hypothesis of galaxy formation and
evolution is correct is to not only study the baryonic but also study the growth of the dark matter component of
galaxies as a function of epoch.
3.2. The Challenge
This is indeed a challenging and ambitious goal and one that will demand us to be at the edge of what is
technologically feasible even with a large aperture 30-100m. Why is this? To map the dark matter halos of galaxies
from redshifts from when the universe was half to only twenty percent of its current age (z approximately 1 - 3)
requires us to obtain spectra of very faint and physically small galaxies. This is necessary because we wish to
probe the dynamics of galaxies in the halos and also obtain redshifts of possible background sources that have been
lensed by the gravitational potential of individual galaxies. Through such measurements, it is possible to estimate
statistically the halo masses of the galaxies. Coupled with spatially resolved measurements of the dynamics of the
parent (most massive) galaxy in the halo, it is possible then to construct a mass versus radius relation for galaxies as
a function of their estimated baryonic mass and angular momentum. Of course, since any one halo is likely to be
populated by several tens of galaxies, many of which may be too faint to obtain the required measurements, one
would have to attach many galaxies with similar total baryonic mass contents. Making such measurements would
then yield the total dark matter mass and the fraction contributed by the baryons ("the bias factor"). In addition, the
measurement of the dynamics of the objects in the halo would allow us to estimate the likely merging time scale
and thus the likely rate of growth of mass and angular momentum of the parent galaxy.
The galaxies in individual halos will be moving relative to the parent galaxy at velocities of several tens of km/s.
Low mass star-forming local galaxies that populate the halos at low redshift typically have emission line
luminosities (in H-alpha, [OIII]5007, [OII]3727, which are the strongest optical emission lines in low mass, low
metallicity galaxies that are likely to be the most appropriate targets in the halos) of 10 39-40 ergs/s/cm2. The typical
flux of halo galaxies at z=3 would then be about 10-19 to 10-20 ergs/s/cm2. Thus, even on an 100m, the integration
time is likely to be nights for each field. As described below in the section on design requirements, this project
would considerably benefit from a spectrograph equipped with several integral field units (IFUs).
4. THE FIRST OBJECTS AND THE RE-IONISATION STRUCTURE OF THE UNIVERSE
A key goal of astrophysics is to understand how and when the first objects in the universe formed, the nature of these
objects and how they contributed to ionizing and enriching the gas that pervades the Universe.
The combination of results from WMAP with observations of the highest redshift quasars known today have raised the
most tantalizing question of the reionization history of the universe during the period from the very early Universe
(possibly as early as redshift 17) to somewhat later times at redshift about 6. Possibilities include two reionization
epochs, due to the first generation of massive stars and then later the first quasars and galaxies, or a slower, highly
inhomogeneous reionization period. This can be probed by the discovery of the first objects and the effects of their
mechanical feedback on either their close environment or the intergalactic medium (IGM). This metal pollution has
several consequences for structure formation in the IGM as well as for the second generation of massive stars. A major
challenge is the faintness of these first objects which is a very strong driver for the next generation of ELTs with very
high sensitivity in the near infra-red.
The first ``fairly bright'' objects are not only markers of the beginning of the reionization epoch, but are also crucial for
probing the inhomogeneous structure and metal enrichment of the IGM from metal absorption lines in their spectra due
to intervening ionized structures of the IGM. The short-lived gamma-ray bursts (GRBs) are an obvious population that
can be detected up to z~15-20. Explosions of population III stars (events fainter than GRBs) can be used to probe the
IGM at z ~<12, although this population is rapidly disappearing with time for regions with metal enrichment higher than
1/10000 of the solar value. Although the epoch of quasar formation is a fully open question, the SLOAN quasars at
redshifts around 6 are powered by supermassive black holes, thus intermediate mass black holes (corresponding to
quasars of intermediate luminosity) must exist at earlier epochs (up to at least redshifts of about 10). These rare objects
will be detected by dedicated missions/telescopes in the case of GRBs and the supernovae resulting from the explosions
of population III stars. The highest redshift quasars will be discovered during the next decade by the JWST space
mission. As shown below, probing the physics of the IGM at redshifts from 10 to 20 requires intermediate/high
resolution spectroscopy in the near IR, which can only be carried out with telescopes of the 60-100m class due to the
predicted low fluxes of these first "background" objects.
4.1. Very high redshift galaxies
The first galaxies compete with the first quasars for the reionization of the IGM. Although less luminous than quasars,
they are far more numerous and can be directly investigated with ELTs. Candidate star-forming galaxies out to redshift
about 6 have already been discovered and a few have been confirmed spectroscopically. These galaxies are high-redshift
analogues of Lyman Break Galaxies at redshift of 3 and are resolved on 0.1-0.2 arcsec scales. The objects detected
thusfar typically have AB magnitudes of i=25.5 and z'=25.5 at a surface density of 500 and 160 per square degree, per
unit redshift at z=5.5 and z=6.0 respectively. Identical objects at z=9 and 16 would have magnitudes of JAB=27 and
KAB=28 respectively. Such very high-redshift objects would be detectable with JWST by broad-band photometric
Lyman-Break techniques. However, only a 100m-class ELT can provide key diagnostics of both the inter-stellar
medium and stellar populations in these galaxies by intermediate resolution spectroscopy in the near IR (to z~ 15-17).
Such objects are expected to exist out to z>10 for two main reasons. Firstly, analysis of the Cosmic Microwave
Background results from Wilkinson-MAP indicates that the reionization of hydrogen in the universe was underway at
z>10, this reionization being caused by ultra-violet emission from the first objects. Secondly, the amount of time
between z=10 and z=6.5 is so short in cosmological terms, there is simply too little time to go from a universe
containing no galaxies to the universe we see at z=6.5.
Galaxies analogous to those already discovered at 5<z<6.5 will certainly be detectable by ELTs, out to z>10 for
apertures of 100m assuming no dramatic evolution in their number density. These objects are often spatially resolved on
spatial scales of 0.1-0.2 arcsec. For observing these objects to z=8-9, a 100m telescope will be slightly more effective
than the planned JWST, assuming it is launched. However, large-aperture ground based telescopes are particularly
effective when studying spatially-unresolved objects, as discussed below.
4.2. Very high redshift GRBs, Supernovae and QSOs
The extremely fine diffraction limit of large apertures concentrates the light from unresolved objects such that there is
little or no effective background contamination from sky emission. Shortward of 3 microns, an ELT will be far superior
to any planned orbiting observatory at studying these objects, being able to do so in a more flexible way given the
instrumentation possibilities of a ground-based ELT.
The brightest high-redshift populations that can be best investigated with an ELT are Gamma-ray bursts (GRBs),
population III SNe and their massive star progenitors, and QSOs. They allow the identification of the early sites of star
and black hole formation and are also ideal candidates to probe the high redshift interstellar (ISM) and intergalactic
(IGM) medium. All such studies are key to understanding how the Universe was reionized and how the first stars
galaxies and AGN formed and evolved.
For ground-based surveys, the strong HI absorption by the IGM implies observations in the near IR: J to K bands for
z~9, H and K bands for z~12 and K band for z~16. Spectroscopic observations in the near IR of the first objects can be
performed with ELTs at different resolution and signal to noise for the 3 populations of object. In Table 1 we compare
theoretical signal-to-noise estimates for various types of object observed with a 100m and 30m ground-based telescope,
derived using the ESO on-line ELT exposure time calculator. Cosmological parameters of ΩM=0.3, ΩΛ=0.7 and H0=70
km s-1 Mpc-1 have been assumed.
4.2.1. Gamma Ray Bursts (GRBs)
The more luminous GRB afterglows at z=10 should have fluxes at l~2µm of 30µJy and 1.5 µJy at 1 and 10 days after
the burst respectively, while mean expected fluxes are 1.5 and 0.04 µJy at 1 and 10 days after the burst12. For GRBs at
z=10, spectroscopic observations of similar quality can be obtained with 30 and 100m telescopes although at different
times after the burst. As shown in Table 1, for a spectral resolution R=104 a 30m telescope could not observe the bulk of
the GRB population at 10 days after the burst (but could observe very bright GRBs and/or GRBs within ~1 day of the
burst).
4.2.2. Population III SNe
Massive stars (140-260 MSUN) should explode as very bright supernovae and could then be detectable from the ground
out to z~16 for about one month after the explosion, and at even higher redshift thus longer observed duration from
space. Theoretical models4 give peak AB magnitudes of 25.5 at z=20 for λobs in the range 1.5-5µm. From this we derive
fluxes of 650, 440 and 300 nJy at λobs =1.2µm (z~9), 1.6µm (z~12) and 2.1µm (z~16), respectively. For a broad range of
rest-frame UV frequencies, the monochromatic luminosities of population III SNe remain about constant for a
substantial fraction of the time. Thus different signal-to-noise will be obtained with 30 and 100m telescopes for a similar
spectral resolution. High spectral resolution data (R~104) are needed to derive the physical properties of the ISM and
IGM at z>10. As shown in Table 1, such studies can only be conducted with ELTs of very large size (70-100m).
4.2.3. QSOs :
The bright 6.0<z<6.5 QSOs discovered in the Sloan Digital Sky Survey (SDSS13) have magnitudes z’AB~20 or νrest x
Lνrest ~ 5x1046 erg s-1 at λrest= 1300Å. Assuming that they radiate at the Eddington limit, their black-hole mass is equal to
M/MSUN=Led/(1.3x1038erg s-1) ~ 4x108. There are very few such objects: currently 6 in 2870 deg2. Fainter QSOs, up to
z~5, have been searched for in the COMBO-17 survey14 and the luminosity function of z~5 QSOs has roughly the same
luminosity dependence (L-1.5) as that found at z~3. For a population one hundred times fainter than the z~6 SDSS QSOs,
the expected fluxes are 200, 140 and 95 nJy at z~9 (1.2µm), z~12 (1.6µm) and z~16 (2.1µm) respectively. It should
noted that QSOs with rest-frame monochromatic luminosities equal to those of z’AB~24 QSOs at z~6 would have
apparent fluxes similar to those of population III SNe at the same redshift.
Black-hole masses, thus luminosities, of high-redshift QSOs are limited by the time available for mass accretion onto
black-hole seeds. Even if the latter are massive (~103MSUN) population III stars, black-hole masses at z~10-15 may not
exceed 105-106 MSUN15. Consequently, The UV luminosities of high redshift QSOs may be at most of ~1x1044erg s-1
which gives expected fluxes of 42 and 29 nJy at z~9 (1.2µm), and z~12 (1.6µm) respectively.
QSOs at high redshift may be ten times less luminous than population III SNe. If so, they would be too faint to be
observed with 30m telescopes at a spectral resolution of R=2x103 and JWST will be more efficient at lower spectral
resolution (R~300). A spectral resolution of R=2x103 remains of interest to explore e.g. the metal-enrichment of the
IGM at early times from the study of the CIV forest. This could be done with 100m telescopes. The minimum flux
limits for a S/N=20 in 50 hr for spectroscopy at R=2x103 with 100m telescopes are shown in the lower three rows of
Table 1. For Eddington rest-frame luminosities, these corresponds to black-hole masses of (1.5, 5 and 13) x105MSUN at
z~9, 12 and 16, respectively. To get such high masses at z~16 would imply either seed black-holes at z~25-30 with
masses larger than 103MSUN or efficient merging of black-holes in dense stellar clusters at early times.
Object
Gamma Ray Burst (GRB) at z=10, observed with R=104
• Very bright GRB 10 days after burst (similar to bulk
of GRB population at 1 day after burst)
• bulk of GRB population at +10 days
• very bright GRB 1 day after burst
Population III Supernova
• at z~9
o observed with R=104
o observed with R=2x103
• at z~12
o observed with R=104
o observed with R=2x103
• at z~16
o observed with R=104
o observed with R=2x103
High redshift Quasar
• at z~9 observed with R=2x103
• at z~12 observed with R=2x103
• at z~16 observed with R=2x103
∆t
(hr)
100m
∆t(hr)
fνobs
λobs
(µJy)
(µm)
1.5
2
K=23.6
40
1.8
15
15
0.04
30
2
2
K=27.4
K=20.3
15
90
X
40
X
2.7
0.65
1.24
J=24.4
40
1.7
40
75
0.44
1.60
H=24.8
40
4
40
X
8
X
0.30
2.1
K=25.2
40
14
40
X
50
X
20
70
X
X
X
X
X
X
0.008
0.019
0.033
1.24
1.60
2.1
mAB
J=29.1
H=28.2
K=27.6
S/N
20
20
20
50
50
50
S/N
30m
Table 1: Estimated signal to noise ratios and exposure times needed to observe various types of very high redshift object (GRBs,
Population III supernovae and quasars) with a 100m and a 30m ground-based telescope. Entries marked by “X” are infeasible
observations with a 30m telescope. Credit: based on observability calculations by Jacqueline Bergeron and Malcolm Bremer.
5. DESIGN REQUIREMENTS
The most demanding requirements on the telescope design are set by the goal of detecting and characterising extra-solar
planets. These are challenging observations even for a 100m-class telescope because of the extreme contrast between
the bright central star and the faint planets near to it. As described in section 1, if the Sun-Earth system were observed at
a distance of 30 parsecs, the "Earth" would appear at a projected separation of only 33 milli-arcseconds from its parent
star, but the star would be 10 orders of magnitude brighter. To make these observations feasible requires very tight
control of scattered light within the telescope and instrument system. Careful study of the telescope design (for example
alignment of mirror segments) and performance of adaptive optics systems and wavefront control in general are needed.
However, initial studies of the expected point spread function show that exo-planet detectability limits are very
promising. Apart from the sheer collecting area of an ELT (which is needed to study such faint planets), the main
advantage of larger telescope size is that the diffraction limit is smaller, giving cleaner separation between the star and
its faint companion planets. This is one of the main drivers towards the largest telescope size possible.
The study of resolved stellar populations in galaxies far beyond our Local Group also derives the telescope requirements
to very large apertures and exquisite image quality. To study a representative section of the Universe requires observing
individual stars in the nearest large galaxy clusters that contain large elliptical galaxies - i.e. the Virgo or Fornax galaxy
clusters at distances of 16 or 20 Mega-parsecs respectively. The challenge here is twofold. Firstly individual stars at
these distances appear very faint (about V=35 magnitudes). Secondly the stars must be individually resolved from each
other in order to determine their ages and chemical composition. As in the case of detecting faint planets, both these
challenges are addressed in parallel with an extremely large telescope - the diameter of the telescope allows the image of
each star to be sufficiently sharp that they can be separated even in crowded regions. Producing smaller images also
reduces the contamination from the sky background, and, in combination with the larger collecting area, allows
observations of very faint point sources. However, to observe sufficiently faint stars to detect the main-sequence turnoff for stars of all ages in the Virgo cluster still requires a telescope of order 100m in size. A particular challenge here
will be to provide AO-corrected images at optical wavelengths, which are needed for diagniostics of age and metallicity.
However the field of view required is quite modest: taking the canonical value of one star per twenty resolution
elements (to avoid too much confusion) and an image size of 2 milliarcsec, then even a 10 x 10 arcsec field of view will
provide more than a million stars to work with.
By contrast, the study of distant supernovae at cosmological distances requires a larger field of view because of the
relative scarcity of these sources. As described in Section 2.1, a 100m with a 2arcmin x 2 arcmin field of view could
find 6-8 supernovae per field per year out to redshift about 10 - therefore 50 fields are required for a sample of about
400 supernovae, and this would take about 130 nights. A smaller field area would force the survey to take proportionally
longer - but any field size smaller than about 1 arcmin x 1 arcmin would lose the multiplex advantage. Again, some
level of AO correction over this field is needed in order to make the observations sufficiently sensitive to point sources
(a 50% strehl was assumed in the example above).
An even larger field of view is needed to study extended dark matter haloes and the motion of galaxies within them. To
encompass projected clustering scales (4 to 9 Mpc) at high redshift requires a field of view of about 10 x 10 arcmin. In
this case the focal plane would not need to be fully instrumented or AO corrected, but deployable integral field units
(IFUs) could pick off the crucial parts of the image such as the principal galaxies in the halo and its satellites. Large
IFUs (up to few sq. arcsec.) would observe the spatially resolved dynamics of the large galaxies, while small IFUs (few
tenths of sq. arcsec.) would be devoted to the spectroscopy of their (generally unresolved) satellites. AO correction
could be concentrated at the key regions of the image. Development of such innovative instrumentation is essential to
the success of an extremely large telescope because the physical dimensions involved and the sampling that is needed.
To obtain exquisitely detailed velocity fields of large galaxies down to few hundreds of parsecs would require spatial
sampling of 0.05 arcsec FWHM, and spectral resolution of R=10000+/-5000. Since the typical flux of the halo galaxies
are expected to be about 10-19 to 10-20 ergs/s/cm2, the integration time is likely to be nights for each field, even on a
100m telescope.
Finally, study of the ionisation structure of the universe at very high redshift (observed in absorption against the most
distant observable sources) also requires spectral resolution of about R=10,000. Bright background sources will be rare,
but with an ELT the more numerous fainter sources can be used. As mentioned above, large telescopes working close to
the diffraction limit are particularly effective for observing faint unresolved sources. Thus very high redshift (z~10 to
16) Gamma Ray Bursts (GRBs), Population III supernovae and Quasars are all ideal targets. Distant GRBs are
potentially the brightest background targets, provided the telescope can be scheduled to allow for rapid response preferably observing within hours of the burst. For population III supernovae and the faint QSO population, only
telescopes of very large size (70-100m) can obtain spectra of sufficient resolution for studies of the physical properties
of the IGM and the "forest" of metal lines at redshifts above 10.
ACKNOWLEDGEMENTS
The European ELT science working group is supported by the Optical Infrared Coordination Network (OPTICON).
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