The enrichment of the intergalactic medium (IGM) with heavy elements provides us with a record of past star formation and with an opportunity to study the interactions between galaxies and their environments. We discuss what future observations with extremely large telescopes could do for this field. We conclude that a further increase in the quality of the spectra of bright, $z\sim 3$ quasars will be useful, but may not lead to dramatic progress. In contrast, the ability to obtain high-quality spectra of $z>5$ quasars will be extremely exciting because they will allow us to extend studies of the distribution of metals to early times and to lower density contrasts, and because they may enable us to study the end of reionization. Spectacular progress in our understanding of the interactions between galaxies and the IGM can be made at $z\sim 3$, by obtaining accurate redshifts for large numbers of faint galaxies in the fields of bright quasars, and by obtaining low and intermediate resolution absorption spectra of some of the brighter galaxies.

Current models for dark energy include a cosmological constant and scalar field models such as quintessence and k–essence. By measuring the dark energy sound speed, $c_{\hbox{\scriptsize\it eff}}$, and its equation of state parameter, $w$, one can distinguish between these. Here we investigate the possibility of measuring $c_{\hbox{\scriptsize\it eff}}$ and $w$ using combined observations from the SKA and CMB experiments. We present theoretical predictions for the cross–power spectrum of ISW fluctuations in the CMB and the expected field of HI galaxies detectable with the SKA.

I present a brief summary of the parallel session on exoplanets and star formation, and how their study drives the science requirements for extremely large ground-based telescopes. I also offer a few thoughts on the development of these highly ambitious and highly expensive new astronomical facilities, as viewed from the perspective of someone also involved in the James Webb Space Telescope.

Future extremely large telescopes will be the engines of major progress in the fields of star and planet formation, brown dwarfs, and extrasolar planets. Their throughput will enable spectroscopic studies of the structure of brown dwarf atmospheres; reveal the composition and kinematics of protoplanetary disks; extend radial velocity searches for extrasolar planets to fainter stars and lower masses; and characterize the surfaces of the most distant Kuiper Belt objects. Their resolution will allow us to resolve and track the orbits of close binary young stars and brown dwarfs, establishing their dynamical masses and calibrating their evolutionary tracks; probe the inner region of young stellar object disks, resolving the jet collimation region in accreting systems and disk inner holes in more evolved ones; and move the horizon for stellar companion searches inward to 40 mas, enabling the direct detection of hot young planets orbiting 10 AU from young T Tauri stars. The high contrast imaging capability of future ELTs is uncertain, and depends on future developments in extreme adaptive optics. Wavefront control considerations suggest a fundamental atmospheric contrast limit of 10$^{-8}$ for companion searches to nearby solar-type stars, below which detections are unlikely to be possible. To reach this performance level, continuing investments will be needed in extreme adaptive optics work, in addition to careful attention to the specialized requirements of ultra-high contrast imaging in ELT design. Above the 10$^{-8}$ contrast limit, a 30m telescope has the potential to directly image about a dozen of the currently known radial velocity planets.

We review the need for technology developments to meet the science cases that have been assembled for optical and infrared telescopes from 20 to 100 metres. Novel technologies can make an impact on the scientific capabilities of these ELTs, make them more affordable and decrease the operating costs. We consider those technologies highlighted by design studies and technology roadmapping exercises on both sides of the Atlantic, with particular emphasis on instrumentation. We do not consider adaptive optics (AO), which is covered by Ellerbroek & Hubin (this volume). Finally, we recommend a joint technology development programme to enable a world-wide suite of ELTs to be built.

Simulations of galaxy formation require strong feedback to produce realistic galaxies. Observations of starburst galaxies both at low and high redshift often show signatures of strong galaxy-wide winds. Are such winds common, and are they the mechanism which pollutes the intergalactic medium with metals? Which are the sources of ionising photons at higher redshift? Detailed observations of galaxies and their surroundings and comparison with cosmological simulations to eliminate biases and test models are required to address these questions.

The next generation of ground-based telescopes will give access to resolved stellar populations in previously unexplored environments. To highlight the potential for stellar spectroscopy using an extremely large telescope (ELT), we present data from an ongoing project with the Very Large Telescope (VLT) to calibrate a novel method of distance determination. An ELT would extend the reach of this method and would allow quantitative studies of individual stars well beyond the Local Group, sampling a wide range of galaxy morphologies and metallicities.

Over the coming two decades astronomers will plan, construct and exploit extremely large telescopes operating at both optical (ELTs) and radio (SKA) wavelengths. In studies of galaxy evolution and cosmology the combination of the two types of facility are likely to be extremely powerful. In this paper I will review the key questions that the ELT/SKA combination seem set to solve, focussing on the unique advantages afforded by optical and radio observations against the backdrop of the other theoretical and observational information to be available by 2020.

The star formation, mass assembly and chemical enrichment histories of galaxies, and their present distributions of dark matter, remain encoded in their stellar populations. Distinguishing the actual distribution functions of stellar age, metallicity and kinematics at several locations in a range of galaxies, sampling across Hubble types and representative environments, is the information required for a robust description of galaxy histories. Achieving this requires large aperture, to provide the sensitivity to reach a range of environs and Hubble types beyond the Local Group, to provide high spatial resolution, since the fields are crowded, and preferably with optical performance since age-sensitivity is greatest near the main-sequence turn-off, and metallicity-sensitivity for these warm stars is greatest in the optical.

Metallicity may play an important role in the symbiotic phenomenon. Unfortunately, chemical abundances of symbiotic stars have been thus far poorly studied. Ongoing abundance analysis of a sample of over 30 symbiotic stars based on high-resolution, near-infrared spectra obtained with the Phoenix spectrometer on Gemini South telescope will allow for the first time to address properly the metallicity problem as well as provide important information about the past history of these binaries.

The future planning working group of the optical and infrared astronomy community of Japan completed its two-year study to yield a concrete plan for ground based telescopes and space missions for the coming decades. The R&D issues towards the realization of extremely large telescopes reported in this paper include a novel optical design, development of new ceramic mirrors, high precision grinding approach to reduce the involved cost and time for fabricating aspheric segmented mirrors, and conceptual studies of instruments.

The present paper is essentially a slightly updated version of the paper presented in the workshop “Instrumentation for Extremely Large Telescopes” held at Ringberg Castle, Bavaria, 25-29 July 2005.

We report an on-going blank-field multi-wavelength deep and wide survey of the Subaru/XMM-Newton Deep Survey Field (SXDF). The SXDF has been the focus of a wide range of multi-wavelength observing programs spanning the X-ray to the radio. These observations cover a large enough area (the initial optical imaging covers $\sim$1.3 deg$^{2}$) and depth ($B=28.2$) that they are not affected by large-scale structures (which exist on tens of Mpc scales) and allow us to study the distribution and evolution of high-$z$ galaxies and AGNs, and thus constrain theories for their formation. Our early results include: i) an indication of the primeval Large Scale Structure (LSS) at z $\sim$ 5.7, ii) an indication for the down-sizing of galaxy formation at z $\sim$ 1, iii) identifications of passively evolving systems, and evidence for early formation and the passive evolution of present-day early-type galaxies, and iv) discovery of a large number of optically obscured QSOs. As for the next step forward - we express our hope to use the next generation optical/IR Extremely Large Telescopes (ELTs) to obtain larger and deeper spectroscopy samples of the high-$z$ objects.

Neutron stars, containing typically 1.4 solar masses within a diameter of about 15km, are among the smallest astronomical sources and the densest known form of directly observable matter in the Universe. Many aspects of the neutron star remain poorly understood. Most theoretical models for neutron stars cannot, so far, account for many of the observations, which have been largely made at radio wavelengths. This paper discusses the importance of multiwavelength studies, using large telescopes, to better understand the properties and behaviour of these objects.

A Mid-IR instrumentation study for OWL has been performed by the Max-Planck-Institut für Astronomie in Heidelberg (Germany), and a Dutch consortium led by the Leiden Observatory (The Netherlands). MIR imaging and spectroscopic observational capabilities are compared to contemporary IR to sub-millimeter facilities, especially concentrating on the MIR-capabilities of JWST(MIRI). Our best effort calculation of the sensitivity for both MIR imager and spectrograph indicate a huge discovery potential in numerous areas from our planetary system to the high redshift Universe. Here we focus on the field of exo-planets and nearby star formation. Starting with the science cases, top level requirements are deduced and summarized including MIR instrumental constrains for the telescope itself.

The extreme contrast in mass and luminosity between the extra-solar planets and their host stars make detailed studies of these planets very challenging. In particular, direct observations of extra-solar planets is still beyond the capabilities of the currently available instrumentation, save for perhaps a few extreme cases of very young and massive planets at large distances from the central star. While progress in instrumentation might allow significant progress in detection capabilities either with the 8 and 10-m ground-based telescopes (Planet Finder instruments on the VLT and Gemini) or with the next generation space telescope (JWST), imaging of extra-solar planets over a wide range of parameters, and possibly down to terrestrial planets, will require extremely large ground-based telescopes like OWL or dedicated space instrumentation (TPF or Darwin for instance). We outline here the scientific objectives of EPICS, the OWL Earth-like Planet Imager and Spectrograph, summarize the corresponding high level requirements, present the foreseen observing modes and give a first estimate of its performance.

In this contribution, we show how a future ELT ($>$25 m diameter) helps to understand the formation and early dynamical evolution of massive stars embedded in dust-enshrouded very compact HII regions. We describe how to exploit the ELT's near- and mid-IR enhanced sensitivity and high angular resolution to peer through huge amounts of dust extinction, taking direct nearly diffraction-limited images and doing IFU spectroscopy. Together with ALMA, an ELT will be a powerful observing platform to reveal one of the most hidden secrets of stellar astrophysics: the origin of massive stars.

The 8-instrument suite studied by the ESO community in the frame of the conceptual study of the 100 m OWL telescope is briefly presented. Potential capability for unique science and the main technical challenges are identified.

Brown dwarfs and very low-mass stars are likely to harbour planetary systems with rocky planets. We discuss the possibility of detecting them using accurate radial velocity measurements with a cross-dispersed high-resolution spectrograph coupled to a ground-based extremely large telescope.

Modelling and simulation, based on observational data, were used to examine the potential of ELT photometry for studies of the evolution of distant galaxies. An open cluster, a globular cluster and two mixed field populations were employed. Colour-magnitude and metallicity diagrams were examined. For younger populations, excellent turn-off-point age data and abundance data can be obtained even beyond 20Mpc. Higher population age weakens data if not improved with longer exposures. Still, the great potential of ELT photometry for studies of the evolution of galaxies is confirmed. Comments are given on adaptive optics and photometry.

A description of the Herschel Space Observatory and the Atacama Large Millimeter Array is presented. Their scientific potential and possible synergistic effects with ELTs is discussed. Herschel is a space-based far-infrared and submillimeter telescope to be launched in 2008 and will offer 3 years of routine science operations. ALMA will become fully operational around 2012 and will co-exist with ELTs. The synergy of being able to observe the same objects with similar angular resolution and sensitivity at long and short wavelengths will contribute to our understanding of astrophysical objects and processes in general.