A key science driver for future ELTs is to chronicle the complete formation and evolutionary histories of a meaningful number of nearby galaxies through their resolved stars. The goal will be to measure the entire star formation and chemical enrichment histories of a sample of galaxies that includes all Hubble types and covers all of their components, demanding photometry of stars in regions with high surface brightness at distances of up to 20Mpc. We present simulations that compare the abilities of 20, 30, 50, and 100m telescopes to recover the correct stellar population mix represented in field star color-magnitude diagrams observed with $J$, $H$, and $K$ filters. As input, our simulations use scenes containing stars drawn from a mix of model isochrones with differing ages and metallicities, with surface densities set to match that found in the M31 bulge and at the effective radius of NGC 3379. We convolve these scenes with PSFs corresponding to the projected performance of MCAO systems containing two deformable mirrors, including the effect of realistic variations in the atmospheric turbulence profile over the span of the observations. These simulations provide a way to evaluate the scientific advances enabled by ELTs of differing apertures in the area of extragalactic stellar populations.

Late-type giants (i.e., stars on the red and asymptotic giant branches, RGB/AGB, respectively) are dominant contributors to the overall spectral appearance of intermediate age and old stellar populations, especially in the red/near-infrared part of the spectrum. Being intrinsically bright, they are well suited for probing distant/obscured populations, especially those that can not be studied with their fainter members, like main sequence turn-off stars or subgiants. Late-type giants and supergiants will be the only stellar types accessible in intermediate age and old populations beyond the distances of several Mpc with the future 30-50 m class extremely large telescopes (Olsen et al. 2003). Indeed, proper understanding of their observable properties by means of theoretical models is of key importance for studying the evolution of stellar populations and their host galaxies.

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.

Supernova remnants are the remains of the outer envelope of the supernova progenitor star. Neutron stars are one of the possible remnants of the region where the explosion detonated. Our knowledge, from galactic studies, of SNRs is well established as their morphology can be understood in terms of the initial blast and out-gassing during the last stages of the progenitor. However, observationally the link between neutron stars and SNRs is only poorly established. During a supernova there are a number of possibilities to produce a condensed remnant - no remnant; a neutron star (pulsar?); a magnetar; a black hole or something more exotic. We do not know what fraction of supernovae go down these possible paths. In the ELT era we will have the first real opportunity to sample the pulsar population in external galaxies and get a more comprehensive survey of optical emission from local pulsars. Such a survey would have significantly reduced biases compared to the current state of radio surveys particularly in the area of pulsar-SNR statistics. Furthermore, a 50m telescope will be able to survey galaxies out to at least 20 Mpc for young SNRs using H$\alpha$:[OIII] and H$\alpha$:[SII] ratios. Currently there are over 1500 radio pulsars detected, 14 of which have been observed at optical wavelengths. Although small in number the family of optical pulsars yield much useful information in bridging the gap between the long wavelength radio emission and the high energy gamma-ray emission from pulsars. Specifically optical techniques are currently the only way of detecting polarisation in the high-energy regime. The advent of ELTs will increase the detection rate of local galactic pulsars and provides the possibility of detecting a significant number of extragalactic optical pulsars. Phenomenologically, the Pacini scaling law predicts $\sim$150 galactic pulsars to have pulsed optical emission with an m$_{V} <$32. Using SKA, Crab-like giant radio pulses should be detectable out to 7 Mpc. In contrast Crab-like pulsars would have a normal peak pulsed m$_{V}\,\sim$31 at 10 Mpc making ELT optical observations more sensitive than radio observations and the best method for extragalactic pulsar discovery. To date only five AXPs have observed IR emission and two optical emission. ELTs will be able to sample the AXP population within the Galaxy as well as the local group again providing better statistics for the birth rate of AXPs compared to ‘normal’ pulsars. A combination of an AXP and ‘normal’ pulsar survey will make a significant contribution to the birthrate question - what fraction of supernovae produce pulsars compared to AXPs and other condensed objects?

Star formation in starbursts produces compact star clusters which extend in mass up to the super star clusters (SSCs) which resemble young globular clusters. Compact young massive star clusters (cYMCs) in turn cluster along with other young stars into starburst clumps. Using M82 as an example we briefly review how the presence of starburst clumps affects the evolution of the host galaxy. Extremely large telescopes (ELTs) will be essential for understanding how starburst clumps and their constituent star clusters evolve. In nearby systems their combination of sensitivity and angular resolution will allow us to explore the structures, kinematics, and abundances of cYMCs. For systems at cosmological distances the high surface brightnesses of the starburst clumps makes them prime gateways for exploring the early evolution of galaxies.

Top of the wish list of any astronomer who wants to understand galaxy formation and evolution is to resolve the stellar populations of a sample of giant elliptical galaxies: to take spectra of the stars and make Colour-Magnitude Diagrams going down to the oldest main sequence turn-offs. It is only by measuring the relative numbers of stars on Main Sequence Turnoffs at ages ranging back to the time of the earliest star formation in the Universe that we can obtain unambiguous star formation histories. Understanding star formation histories of individual galaxies underpins all our theories of galaxy formation and evolution. To date we only have detailed star formation histories for the nearest objects in the Local Group, namely galaxies within 700kpc of our own. This means predominantly small diffuse dwarf galaxies in a poor group environment. To sample the full range of galaxy types and to consider galaxies in a high density environment (where much mass in the Universe resides) we need to be able to resolve stars at the distance of the Virgo ($\sim$17 Mpc) or Fornax ($\sim$18 Mpc) clusters. This ambitious goal requires an Extremely Large Telescope (ELT), with a diameter of 50–150m, operating in the optical/near-IR at its diffraction limit.

Spectacular progress in the observational study of rapidly oscillating Ap (roAp) stars has been achieved by considering high time- and spectral-resolution spectroscopy in addition to the classical high-speed photometric measurements. Such observations led to the discovery of a multitude of unexpected phenomena, generally pointing to an extreme vertical chemical non-uniformity of the atmospheres of magnetic CP stars. A detailed analysis of spectroscopic pulsational behaviour allows us to establish a relationship between pulsations and the vertical stratification of the chemical elements. This has become possible with the use of a Very Large Telescope on relatively bright stars. Using Extremely Large Telescopes promises farther heights for asteroseismology.

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.

We compare [$\alpha$/Fe], metallicity, and age distributions of globular clusters in elliptical, lenticular, and spiral galaxies, which we derive from Lick line index measurements. We find a large number of globular clusters in elliptical galaxies that reach significantly higher [$\alpha$/Fe] values ($>0.5$ dex) than clusters in lenticular and spiral galaxies. Most of these highly $\alpha$-enriched globular clusters are old ($t > 8$ Gyr) and exhibit relatively high metallicities up to solar values. Given the lower [$\alpha$/Fe] ratios of the diffuse stellar population in early-type galaxies, our results suggest that the extremely $\alpha$-enhanced globular clusters are members of the the very first generation of star clusters formed, and that their formation epochs likely predate the formation of the majority of stars in giant early-type galaxies.

We discuss the opportunities to use the future ELTs to study in detail the properties and the evolution peculiarities of individual massive stars with a metal content typical of galaxies in the Universe during the first 0.5–1Gyr, corresponding to the earliest known newly formed galaxies at redshifts of $z$ = 6–10. This is possible in principle due to the existence in the local Universe of starbursting galaxies with metallicities of $\sim$1/30 Z$_{\odot}$. The nearest such galaxies are DDO 68 at $\sim$6.5 Mpc with 12+log(O/H) = 7.21 and I Zw 18 at 15 Mpc with 12+log(O/H) = 7.17. For the youngest star clusters (with ages of T $<$ 4–5Myr) in these most metal-poor galaxies, stars with masses up to 40–60 M$_{\odot}$ should be present on both the Main Sequence and the later evolution stages, including the WR stage. They are expected to have apparent magnitudes as bright as $V\,{=}\,21^m$ for DDO 68 and $V\,{=}\,23^m$ for I Zw 18. Good S/N-ratio spectroscopy with telescopes like OWL or JWST, allowing near-milli-arcsecond angular resolution, will provide unique information to check the most up-to-date models of massive star evolution in the very low metallicity regime, and thus, establish a firm basis to model the effects of star formation in ‘primordial’ galaxies, at the epoch of galaxy formation. Such observations will also provide an independent channel to probe the primordial Helium abundance.

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 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.

We will review the metallicity of the CIV and OVI phases of the IGM in the redshift range $z\sim 0.1$-5 together with their contributions to the cosmic baryon density. The current studies are limited to regions with overdensities of 3-10; the heavy element enrichment in regions with gas densities close to the mean cosmic density will be probed with future ELTs. We will also investigate the possibilities of detecting metals at $z\sim 10$ and underline the requirements on ELTs due to the anticipated faintness of the early generations of galaxies, QSOs and transient objects.

Absorption lines of molecular hydrogen provide a most sensitive and powerful tool to understand the early evolution of galaxies and the intergalactic medium (IGM). In addition these lines are useful to investigate the space and time variations of the ratio of electron to proton mass. In this presentation I review the status of the search for H$_2$ at high $z$. High resolution spectrograph on ELTs will allow us to detect H$_2$ in the Lyman Break Galaxies (LBGs) and CO and HD in DLAs that already show H$_2$. This will allow us to investigate the astrochemistry at early epochs and study the physics of interstellar medium (ISM) at high redshifts.

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.

Dwarfs of the spectral types late-M, L and T span mass regimes from very-low-mass stars through brown dwarfs down to young planetary objects. They all show massive molecular line-blanketing and the condensation of refractory species with decreasing $T_{\mathrm{eff}}$, leading to changes in chemical equilibrium composition and absorption due to dust grains. The spectral evolution from late-M through L to mid- and late-T classes is now understood as chiefly due to increasing amounts of condensates in the visible photosphere up to mid-L types, and the settling of dust clouds into deeper regions at the transition from L to T, resulting in a depletion of condensable elements in the upper atmosphere. The ensuing photospheric cooling also drives a change in carbon chemistry leading to the hallmark methane absorption features of T dwarfs.

Recent observations of brown dwarfs in the L-T spectral sequence and model atmosphere calculations have shown that these changes in spectral features reveal differences in the efficiency of cloud removal that seem to be triggered by an additional parameter besides effective temperature. We present models describing the settling of particle clouds as an equilibrium process between condensation, gravitational sedimentation and convective and turbulent mixing, based on 3D-hydrodynamical simulations for the description of the velocity field. These calculations predict a strong dependence of the settling on gravity, and can therefore explain observed differences between the spectral energy distributions of brown dwarfs of equal luminosity as an effect of different mass. Based on these calculations we would expect even stronger deviations in the colours of young brown dwarfs of very low mass from those commonly observed in the field. Such predictions seem to be in aggreement with the first observations of candidate planetary-mass objects, and would imply that extrasolar planets at very young ages have spectral energy distributions significantly different from previous models. Our models also predict characteristic time scales for the cloud formation processes that may be compared to observed variability in brown dwarfs.

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.

A forthcoming step in the study of extrasolar planetary systems is the direct detection and characterization of Earth-like planets. An asset of the ELTs in that context is their very high angular resolution and their collecting area. The luminosity ratio between a terrestrial planet and its star ($10^{-10}$) is such an ambitious goal that a thorough study needs to be carried out. We started with a simple analysis of the fundamental limitations for the detection of extraterrestrial planets with ELTs. Here, we considered an extreme adaptive optics device upstream of a perfect coronagraph. Even with high Strehl ratios, the coronagraphic halo level is only $10^{-6}$ to $10^{-7}$ at typical exo-Earth angular distances. A calibration device is therefore mandatory to reach the contrast between a terrestrial planet and its star in the near infra-red. We considered a simple but realistic model taking into account dynamic aberrations left uncorrected by the adaptive optics system, static aberrations of optical system and differential static aberrations due to the calibration channel itself. Numerical simulations prove that, after the calibration, the limitations are set by the static aberrations which cannot be neglected anymore.

A software for simulations of observations with an Integral Field Spectrograph attached to an extreme adaptive optics system is presented here. This code, written in IDL, has been conceived within the CHEOPS project, a second generation “Planet Finder” instrument for ESO's VLT, and it has been successfully extended to the case of various ELTs in order to test the capabilities in detecting planets of an instrument based on Integral Field Spectroscopy (IFS). We present here, beside a general description of the procedure adopted to simulate IFS observations, a set of significant simulations of detections using OWL for different kinds of planets, in order to understand which objects are the best targets for an IFS-Planet Finder mounted on a 100m-class telescope.

White dwarfs are excellent targets for direct imaging searches for extra-solar planets, since they are up to $10^4$ times fainter than their main sequence progenitors, providing a huge gain in the contrast problem. In addition, the orbits of planetary companions that lie beyond the maximum extent of the Red Giant envelope are expected to widen considerably, improving resolution and further encouraging direct detection. We discuss current searches for planetary companions to white dwarfs, including our own “DODO” programme. At the time of writing, no planetary companion to a white dwarf has been detected. The most sensitive searches have been capable of detecting companions ${\gtrsim}5$M$_{\rm Jup}$, and their non-detection is consistent with the conclusions of McCarthy & Zuckerman (2004), that no more than 3% of stars harbour 5–10M$_{\rm Jup}$ planets at orbits between 75–300AU. Extremely Large Telescopes are required to enable deeper searches sensitive to lower mass planets, and to provide larger target samples including more distant and older white dwarfs. ELTs will also enable spectroscopic follow-up for any resolved planets, and follow-up of any planetary companions discovered astrometrically by GAIA and SIM.