Chris Evans, Simon Morris, Jean-Gabriel Cuby, Matt Lehnert, Mark Swinbank and Mathieu Puech describe an instrument that could bring distant galaxies and stellar populations within reach of the European Extremely Large Telescope.
EAGLE is an instrument concept for a multi-object, near-infrared spectrograph for the European Extremely Large Telescope (E-ELT). EAGLE will employ wide-field adaptive optics (AO) to deliver excellent image quality which, when combined with the light grasp of the E-ELT, will be a unique and efficient facility for spatially resolved, spectroscopic surveys of high-redshift galaxies and resolved stellar populations.
The current generation of 8–10 m optical and infrared telescopes will continue to yield high-impact science beyond the next decade, but in many fields we know already that we need data beyond the current limits of sensitivity and angular resolution. Continuing our quest to understand the universe in which we find ourselves, the European astronomical community has compiled a detailed science case for a larger ground-based telescope: the E-ELT. The European Southern Observatory (ESO) is near the end of an intensive design phase of a 42 m diameter E-ELT (figure 1), with a full construction proposal to be completed by the end of 2010. An overview of the E-ELT project was given in A&G by Evans (2008). One of the key aspects of the telescope is a sophisticated five-mirror design to deliver a comparatively wide field-of-view (for a 42 m diameter telescope) with the potential for images limited in resolution only by diffraction.
One of the dominant scientific motivations for the E-ELT is the study of galaxy evolution. This encompasses a huge range of phenomena and scales — from studies of resolved stars in nearby galaxies, right out to our attempts to understand the properties of the most distant galaxies near the dawn of time. From consideration of these cases, and the broader astrophysical landscape, a set of science requirements were compiled for a near-infrared spectrograph with multiple, deployable integral field units (IFUs). Detailed science simulations were undertaken to quantify the requirements, employing the tools developed by Puech (2008) to generate simulated IFU datacubes for each of the principal cases (see, for example, Puech 2010a,b; Evans 2010). These requirements (table 1) have been used to develop the EAGLE concept, which combines superb image quality (via AO correction) across a wide field-of-view (38.5 arcmin2).
In this article we highlight the three most prominent science cases for EAGLE, followed by a brief summary of the synergies with other future facilities and the EAGLE design. While the design is optimized for these three cases, EAGLE is envisaged as a “work-horse” instrument for the E-ELT, meaning that it is expected to operate for at least 10 years, and cover a broad range of astrophysical problems. With this in mind, the concept was also designed to provide flexibility in the long term, e.g. maximizing the patrol field accessible on the sky and including the provision for a spectral resolving power, R, of 10 000 in each of the wavelength bands.
An example of the performance estimates we have made is a signal-to-noise ratio of five per spectral resolution element, for a point-like (or slightly resolved) source with JAB or HAB = 27, in 30 hrs. This is approximately two magnitudes fainter than the spectroscopic sensitivity of the James Webb Space Telescope (JWST).
The physics of high-redshift galaxies
Observational cosmology has made rapid and substantial progress over the past decade. The results from W-MAP, in combination with other observations, have led to the adoption of a concordance cosmology with accurate determinations of many of the cosmological parameters. This model includes a fairly mature view of the formation of hierarchical structure solely under the influence of gravity, in which the large-scale structure arises from the growth of the fluctuations (via gravitational aggregation and collapse) in the mass density of the early universe.
We do not have such a clear view of the evolution of galaxies because the evolution of the baryonic component does not appear to simply follow the hierarchical merging of dark matter structures. The opposing forces of cooling, angular momentum exchange, and feedback from both star formation and active galactic nuclei (AGN) are all factors. Indeed, at least three channels are thought to drive the rate of conversion of gas into stars: major mergers, where galaxies of comparable masses collide; minor mergers, in which a smaller galaxy collides with a more massive system (in which individual collisions have a weaker effect on a given galaxy but are thought to occur more often over its lifetime); and direct accretion of cold gas from intergalactic filaments. To date, we do not understand which is the dominant channel, nor the relative contributions of each channel, as a function of galaxy mass and time.
Phenomenological (“semi-empirical”) models, which rely heavily on analytic prescriptions of the physical mechanisms, have been developed to describe the formation and evolution of galaxies. The coefficients used are determined from the available observations, with important parameters being metallicity, angular momentum, the stellar initial mass function, the spatial distribution of the gas, and the frequency of major/minor mergers. These models can be verified only by confrontation with a sufficiently large sample (numbering thousands of galaxies) and over a large enough volume to rule out field-to-field variations (“cosmic variance”) biasing the results.
Until recently, the study of high-redshift galaxies was generally limited to redshift determinations and analysis of the integrated spectrum from each system. The availability of IFUs on 8–10 m class telescopes has heralded a new era of galaxy studies, where we can derive spatially resolved kinematics and physical properties of selected distant galaxies up to z ∼ 3 (e.g. Förster Schreiber 2006, 2009; Lehnert 2009; Flores 2006). However, these are typically only the most massive and luminous systems. The E-ELT offers the potential for spatially resolved observations of an unbiased and unprecedented sample of high-redshift galaxies (e.g. figure 2). This will provide us with a clear view of their star-formation histories, gas distributions, extinction, metallicities, clustering and kinematics to chart the evolution of most types of galaxy over time.
The capabilities of EAGLE, combined with the sensitivity of the E-ELT, will bring such a survey of high-redshift galaxies within our reach. The fine spatial-sampling of EAGLE (75 milliarcsec) will allow us to study the physical processes taking place at sub-kpc scales, providing the perfect complement to studies of cold gas and dust with the Atacama Large Millimetre Array (ALMA) at approximately the same resolution. With greater spectroscopic sensitivity than the JWST, and with a high multiplex combined with spatially resolved mapping from the IFUs, EAGLE is uniquely placed to deliver a definitive view of galaxy evolution in only a few hundred hours — i.e. a well targeted ESO Large Programme.
Galaxies at the highest redshifts
In the standard cosmological model described above, dark matter halos form through the gravitational collapse of primordial perturbations in the initial dark-matter density distribution. Cooling of baryons is very efficient on these scales – the gas quickly loses pressure support and collapses to the centre of the halo where it cools further, eventually forming the first stars and galaxies. Gaining an inventory of the basic properties of the first galaxies between z = 7 and z ∼ 20 is one of the greatest challenges in modern astronomy. Armed with a view of the formation of the first stars, assembly of the first galaxies, and the growth of super-massive black holes through gas accretion, we can build a complete picture of the star formation and quasar activity now believed to be responsible for the reionization of the universe.
Observational constraints on the properties of galaxies at z > 6 are scarce, with only a handful of galaxies confirmed at this epoch. Due to their extremely faint magnitudes, recent efforts have focused on redshift confirmation, often using only the rest-frame Lyman-α line (λ = 1215 Å) combined with detection of a break in the stellar continuum at that wavelength (caused by absorption by neutral hydrogen in the inter-galactic medium, the Lyman break). This means that little is known about basic properties of the objects detected, such as their stellar and dynamical masses, their stellar populations or the chemical composition of their interstellar gas.
One of the principal science goals for the E-ELT is to probe the properties of these first galaxies. For instance, how do they compare with galaxies at lower redshift and models of galaxy formation? Observations at 3 < z < 5 indicate that most Lyman-α emitters are dust-free galaxies with little (or no)AGNs and with moderately high star-formation rates (e.g. Gawiser 2006). Galaxies at greater redshifts may well be very different entities as they are forming from pristine gas, with very low (effectively zero) metal abundances.
Observations of rest-frame Lyman-α will be crucial to investigate the most distant galaxies, but it can also be difficult to interpret owing to scattering and absorption by dust. A more detailed knowledge of the physical properties of such galaxies is likely to be gleaned from other rest-frame UV lines such as Si iv and C iv, which trace the young, massive star populations associated with intense starburst activity. The He iiλ = 1640 Å line will also be of interest to search for the signatures of the elusive Population III stars, thought to be formed in the earliest epochs of star formation.
The high-redshift objects recently discovered by the HST WFC3 (Oesch 2010, Bouwens 2010, McLure 2010; figure 3) are merely a taste of what awaits in the coming decade. Further deep imaging with the HST, with ground-based telescopes (e.g. VISTA, VLT) and, ultimately, with the JWST, will discover large samples of very distant Lyman-break and Lyman-α emitting galaxies. But discovery is merely the first step, understanding their physical properties is an entirely different matter. For instance, the objects recently discovered with WFC3 are beyond the spectroscopic sensitivity of the JWST, which will be limited to redshift confirmation of the small subset of galaxies with the very strongest emission lines.
The spectral resolution and high sensitivity of EAGLE on the E-ELT will allow us to probe the warm and hot ionized gas in the interstellar medium (ISM) of these extremely distant galaxies via studies of the rest-frame UV emission and absorption lines. Indeed, only EAGLE will have the combination of capabilities — i.e. the required sensitivity, spectral coverage, wide field and multiplex — to allow us to reproduce at z ∼ 7–10, what we now take almost take for granted at z ∼ 3–5 with 8–10 m class telescopes. In a few hundred hours of observations, EAGLE will study the spatially resolved properties of ∼100 Lyman-break galaxies in the early universe in exquisite detail, providing the first understanding of the dynamics and characteristics of the ISM in the first galaxies. This programme is at the forefront of astrophysics, building on the science drivers of the JWST and ALMA, with EAGLE a unique and essential facility.
Resolved stellar populations in the local volume
Recent discoveries of disrupted satellite galaxies and extended outer-galaxy halos have shown that our evolutionary picture of the Milky Way and other Local Group galaxies is far from complete. Resolved stellar populations provide us with a valuable fossil record of the past star formation and interaction histories of these galaxies. For example, there is evidence for the accretion of numerous low-mass satellites in the assembly of the present-day Milky Way (e.g. Belokurov 2006). What remains unclear is whether the Milky Way and our near neighbours (e.g. M31 and M33) are typical examples of galaxy evolution. For instance, do we see a similar hierarchical build-up at work in galaxies with very different morphological types? Only by looking beyond the Local Group, at Mpc distances, can we investigate the histories of a much broader range of galaxies, including other spiral-dominated groups such as the Sculptor and M83 Groups, starbursts such as M82, the nearest ellipticals (Cen A and NGC 3379), and very metal-poor irregulars.
Photometric methods are immensely powerful when applied to extragalactic stellar populations, but only armed with chemical abundances and stellar kinematics can we break the age—metallicity degeneracy, while also disentangling the populations associated with different structures — in other words, we need high-quality spectroscopy. Over the past decade the calcium triplet (CaT, spanning 0.85–0.87 λm) has become an increasingly popular diagnostic of stellar metallicities and radial velocities in nearby galaxies, providing new views of their star-formation histories and substructure (e.g. Tolstoy 2004). However, 8–10 m class telescopes are already at their limits in pursuit of CaT spectra of the evolved populations in galaxies beyond ∼300 kpc, e.g. Keck-DEIMOS observations in M31 struggled to yield useful signal-to-noise below the tip of the red giant branch at I > 21.5 (Chapman 2006).
The high contrast and sensitivity of wide-field AO on the E-ELT will bring spectroscopic studies of individual evolved stars at Mpc distances within our grasp for the first time. This will unlock a wealth of new and exciting targets in which we can study galaxy evolution directly. This “stellar archaeology” is almost certain to play a leading role in constraining our theoretical understanding of galaxy formation and evolution in the future — EAGLE provides the means by which we can efficiently compile large samples of stars in each galaxy. Work is underway to better characterize stellar diagnostics in the near-infrared (to take advantage of the more effective AO correction), but the CaT is likely to remain the principal diagnostic in this area for the foreseeable future, leading to the inclusion of the IZ band in the EAGLE concept.
With its high multiplex and large field-of-view, EAGLE will allow us to study the stellar populations in the extended halos of external galaxies, while also observing towards the core or bulge regions of the galaxy at the same time — a capability unmatched by any other of the proposed E-ELT instruments, particularly those that will rely on seeing-limited or ground-layer AO performance. The area covered by individual IFUs enhances the multiplex by allowing us to study multiple stars within each channel, as well as the fainter unresolved population. This “effective multiplex” is a huge advantage for an AO-corrected spectrograph. A large sample (>>1000) of faint stars (down to I ∼ 24.5) will be observed in galaxies beyond the Local Group in 200–300 hours (e.g. NGC 300 in the Sculptor Group, figure 4). Remembering this is absorption line spectroscopy at R ∼ 10 000, this is well beyond the capability of current instrumentation and any other proposed E-ELT instrument.
EAGLE will have strong synergies with both the JWST and ALMA. Others facilities, such as the ESO VLTs and VISTA, will also provide support via pre-imaging and target selection.
The JWST has superb imaging sensitivity and so offers a vast discovery space, but is limited in terms of its spectroscopic sensitivity due to its (relatively) small collecting area — EAGLE will provide the capability to obtain spectroscopy of the bulk of the targets discovered and or identified with the JWST. Note that the pixel scales between JWST-NIRCam and EAGLE are well matched (32 cf. 37.5 milliarcsec), delivering spectroscopic follow-up at comparable angular resolution. The JWST deep fields will also cover areas that are perfectly matched to the EAGLE patrol field. In essence, the JWST will provide initial images and spectral energy distributions, and EAGLE will reveal the astrophysics behind these discoveries.
ALMA will have a significant headstart in time over the E-ELT but EAGLE, with its high multiplex, can quickly catch up. Together they will reveal the star-formation rates and kinematics of distant galaxies at comparable resolutions. ALMA will probe the molecular gas, dust and the ionized species of atomic and simple molecules, while EAGLE will probe the warm and hot ionized medium in the ISM and the stellar continuum — i.e. providing complementary observations of different gas phases, in both nearby and distant galaxies. This synergy is neatly illustrated by recent observations of individual star-forming clumps on 100 pc scales at z = 2.3 (Swinbank 2010) — this is currently only possible in lensed systems, but near-infrared observations on the E-ELT will provide unique views of star-formation in distant galaxies. The synergy of the E-ELT with ALMA is strong, but only if equipped with EAGLE.
Further into the future, one can foresee valuable combined observing programmes using the SKA to measure H i 21 cm gas properties of galaxies with extant E-ELT and ALMA data, and also combinations of the above with the International X-ray Observatory.
EAGLE basic design
EAGLE is located at the Gravity Invariant Focal Station (GIFS) of the E-ELT, in which the instrument focal plane is parallel (rather than perpendicular) to the ground, avoiding the time-varying flexure which conventional Nasmyth instruments have to factor into their design. EAGLE sits on a mechanical derotator so that the whole instrument rotates to follow the sky during the course of an observation. A computer-aided design rendering of the mechnical design is shown in figure 6.
A pick-and-place robot is used to deploy 20 mirrors in the 38.5 arcmin2 focal plane, each of which relay the light to the IFUs (figure 7). To correct for the different path length to the IFU, which depends on the position of the mirror in the focal plane, a “trombone” system is used in the relay optics so that the light from each field travels the same distance. Each IFU has an individual field-of-view of 1.65 × 1.65″, which will deliver useful sky pixels around observations of a typical high-redshift galaxy. Pairs of IFUs are combined to illuminate 10 identical spectrographs, in which one of the IZ, YK, H or K bands can be observed at a spectral resolving power, R, of 4000. A high-resolution mode of R = 10 000 is provided (primarily) for observations of resolved stellar populations. There is a high level of duplication and redundancy in the instrument, providing robustness against single-point failures and easing maintenance.
EAGLE adaptive optics
EAGLE provides image sampling of 75 milliarcsec, which is a robust trade-off between AO performance, sensitivity, scientific requirement and cost. For many of the prominent E-ELT science cases there is no real advantage to be gained from finer spatial sampling, and a considerable penalty in surface brightness sensitivity, meaning that either much longer exposure times or much brighter (and rarer) targets need to be used. Even with this proposed sampling, in order to get a large fraction of the light from a point source into a spatial pixel, a very capable wide-field AO system is needed.
The EAGLE concept employs Multi-Object Adaptive Optics (MOAO), just one of the many flavours of wide-field AO. It is conceptually simple. Using laser and natural guide stars, the atmospheric turbulence in a roughly cylindrical volume (42 m in diameter) through the atmosphere above the telescope is reconstructed using tomographic mapping (figure 5). The corrections required for each target field within the focal plane are calculated using this information and are introduced in each IFU channel using local deformable mirrors. MOAO has been demonstrated in the lab and there are several programmes underway to further characterize it on sky. The most advanced of these is the CANARY experiment, the EAGLE MOAO pathfinder, at the William Herschel Telescope in La Palma. Further details on the AO study are provided by Rousset (2010).
In summary, EAGLE is a general facility-class instrument, which will serve a broad community of users. It uses the E-ELT at its best, over a wide field-of-view and with AO-corrected images, efficiently tackling some of the most prominent science cases advanced for building the 42 m E-ELT. •
We would like to thank all of those involved in the EAGLE consortium. The EAGLE consortium is a 50:50 partnership between the UK and France, including the Laboratoire Astrophysique Marseille (LAM), two groups at the Observatoire de Paris (GEPI and LESIA), the University of Durham Centre for Advanced Instrumentation (CfAI), and the UK Astronomy Technology Centre (UK ATC).
The E-ELT Science Case Summary, An Expanded View of the Universe — Science with the E-ELT, is available from http://www.eso.org/sci/facilities/eelt/science.