The OPTIMOS-EVE phase A study Science Team studied a number of key science cases covering the three main science themes motivating the development of the E-ELT (Science Case for the E-ELT, Ed. I. Hook): (i) Planets and Stars; (ii) Stars and Galaxies; (iii) Galaxies and Cosmology. The list of the members of the OPTIMOS-EVE Science Team can be found in Appendix B.
With the advent of the E-ELT it will, for the first time, be possible to detect extra-solar planets in external galaxies and in remote, dense regions of the Galaxy, and to derive their orbits. The detailed spectroscopic study of the resolved stellar populations in nearby galaxies beyond the Local Group will become feasible. Also, with the E-ELT a major fraction of the Universe becomes accessible for spectroscopic observations, uncovering the early history of the Universe, and its gaseous and galaxy content up to the formation epoch of the first stars and galaxies. The Science Team explored 5 key science cases from which the scientific and technical requirements for OPTIMOS-EVE have been derived (cf. RD2, RD3):
1.Planets in the Galactic bulge and stellar clusters, and in external dwarf galaxies.
2.Resolved stellar populations in nearby galaxies.
3.Tracking the first galaxies and cosmic re-ionization from redshift 5 to 13.
4.Mapping the ionized gas motions at large scales in distant galactic haloes.
5.3D reconstruction of the Intergalactic Medium.
Spectroscopy over the wavelength region available from the ground, from the UV to the non-thermal infrared, has been, and will remain a key technique to investigate virtually all types of astrophysical targets. At z=0 most of the spectral lines, fundamental for deriving astrophysical information, are found in the UV-optical range. OPTIMOS-EVE is the only instrument under study for the E-ELT, which offers low-, medium-, and high-resolution spectroscopy (R ~ 5,000 - 30,000) from the ultraviolet to the near-infrared, for multi-object studies of sources nearby and at cosmological distances. Because of this combination, most of the science that will be explored by OPTIMOS-EVE cannot be addressed neither any other instrument concept under study for the E-ELT, nor by JWST instruments.
The OPTIMOS-EVE targets result from imaging observations obtained with other instruments, e.g. the JWST (possibly also LSST & EUCLID), GAIA and ALMA, and many OPTIMOS-EVE studies will highly benefit from combined observations with JWST and ALMA (see section 5.3).
Spectroscopic observations of astronomical objects are, in their broadest definition, the foundation on which we build or discard astrophysical models of the Universe and its constituents. This is true even in the case of the largest known `object', the cosmic microwave background. The breathtaking progress in astrophysics and cosmology within the last century can be traced back directly to the advances in spectroscopic techniques and instrumentation throughout the electromagnetic spectrum. OPTIMOS-EVE spectroscopy, with a large range in spectral resolution and different apertures on the sky, will be a crucial tool to warrant groundbreaking discoveries, including unexpected ones, in an era (2018+) for which new astronomical challenges and opportunities will occur that are hard to predict.
Figure 1. It shows an overview of the OPTIMOS-EVE Science Cases. Other contributing Institutes are OAB and OAT-INAF and NBI/DTU-Space.
Key science cases
Planets in the Galactic Bulge and in external dwarf galaxies. In spite of the fact that over 400 extra-solar planets are known, these are mostly hosted around stars in the solar vicinity. The few distant planets known do not have orbits, masses etc. We expect, on theoretical grounds, that the environment plays a significant role in the process of planet formation. Therefore, it is important to detect and characterize planets in environments different from the solar vicinity, like the Galactic Bulge and Local Group dwarf spheroidals. With its capability of obtaining a radial velocity precision of 10 m/s for giant stars down to the 20th magnitude OPTIMOS-EVE will make such a study possible. Its multiplex capabilities will allow to monitor a significant number of stars in each observed field.
Resolved stellar populations in nearby galaxies. With VLT we are beginning to study in detail the stellar populations of the Local Group galaxies. However, many galaxy types are not present in the Local Group. In order to make real progress in our understanding of galaxy formation and evolution we need to study in detail all the different types of galaxies which can be found in the groups of Sculptor and Cen A. The high efficiency low-resolution mode of OPTIMOS-EVE and its high multiplex grant this possibility.
Tracking the first galaxies and cosmic re-ionization from redshift 5 to 13. We know, from the fluctuations of the microwave background, that at z=10 the Universe was already largely re-ionized. Nevertheless, we know little of the objects that produced this re-ionization. The search for the “first lights”, the sources of the photons responsible for the re-ionization can be carried out with OPTIMOS-EVE, which, thanks to the IR arm, holds the promise of tracing these up to z=13.
Mapping the ionized gas motions at large scales in distant galactic haloes. The observations of the local Universe have shown that galaxies are surrounded by extended haloes of ionized gas. These haloes are the interfaces of the galaxies to the Intergalactic Medium and the way to enrich it of metals. The study of these haloes also allows understanding the history of galaxy-galaxy interactions, since these always leave recognizable signatures on the halo kinematics. Such studies can be conducted with an instrument like GIRAFFE at the VLT up to a redshift of z~0.5; OPTIMOS-EVE will make this possible up to a redshift of 3.5.
3D reconstruction of the Intergalactic Medium. We know that the space between galaxies and galaxy clusters is not empty, but is filled with a very low density warm medium. Such a medium shows up as Ly alpha absorption in the spectra of distant quasars. Although this affords a “cut through” of the structure of the IGM along the line of sight, nothing is known about its transverse structure. Cosmological simulations show that the IGM has a filamentary structure and filament crossings correspond to the locations of galaxy clusters. Some information on the transverse structure can be obtained in the case of pairs of gravitationally lensed quasars. OPTIMOS-EVE will provide sufficient resolution and sensitivity to use Lyman-break galaxies of 25th magnitude as background sources. These galaxies have a sufficient space density to allow a 3D reconstruction (tomography) of the IGM, a real 3D picture which may be compared to cosmological simulations.
Already during the first science operations of the E-ELT, OPTIMOS-EVE can address several science cases, which in a limited amount of observing time may provide groundbreaking results.
Masses of giant stars in Local Group galaxies. The high-resolution mode of OPTIMOS-EVE will allow the detection of oscillations in giant K stars. These oscillations will provide a direct measurement of the masses of these stars, thus providing a solid test-bench for stellar evolution theory. The observations can be completed in a few nights (less than a week).
Lithium in the metal-poor TO stars of Sagittarius. With a few nights of observation it will be possible to confirm whether the metal-poor population of Sgr displays a “Spite plateau” and, if so, whether it is at the same level as that observed in the Galaxy. This will be of paramount importance to the interpretation of the Spite plateau.
The first kinematics studies of the haloes of galaxies at redshift in the range 2 to 3.5 can be obtained within a small number of nights. Such observations will revolutionize our knowledge of the gas exchange between galaxies and their environments. Different galaxy formation scenarios depend on whether the gas far beyond the optical radius is rotating, infalling or has chaotic motions.
Gas motions in the surroundings of the most distant galaxies, to z=6.5 (visible arm) or to z=13 (NIR arm). One or several Lyman alpha blobs can be targeted by either the large IFU or by several medium IFUs in a few nights.
Immediate detection of several tens of the most distant galaxies (z > 10, NIR arm) and the first opportunity to estimate whether or not these galaxies can be responsible of the re-ionization (e.g. following up on JWST candidates).
From the five main science cases as presented in RD2, the lop-level requirements (TLR) for OPTIMOS-EVE were defined. These TLRs have been used in the basic design of the instrument as presented here. Of specific interest for the basic design of OPTIMOS-EVE are the TLRs on field size, spectral resolution, multiplicity, fibers vs. IFUs and spectral coverage. The full set of requirements is given in RD3. However, we give a brief outline here.
Field-of-view: the required field of view size is set by the surface density of targets at a given magnitude in the key science cases. The most stringent of these is the case of high redshift galaxies (z>5), and the 3D reconstruction of the intergalactic medium. For the high-redshift galaxies a sufficiently high surface density per pointing is needed to make the observations efficient for these long integrations. Given their rarity a field of view as large as possible is required. To properly reconstruct the 3D reconstruction of the intergalactic medium one has to probe scales ranging from 2 arcminutes to a degree. A larger field-of-view therefore directly sets the efficiency with which this reconstruction can be done.
Spectral resolution: A minimum spectral resolution is set to R=5000-10000 (both in VIS as well as in NIR) by the resolved stellar populations studies. Below this number no meaningful analysis of the stellar spectra can be obtained in terms of metallicity, rotation, binary velocity fields, and wind features in massive stars. In the NIR a firm lower limit of R > 4500 is set by the density of the OH lines. X-Shooter NIR observations show that at this resolution the OH lines are well resolved and access is gained to the very dark interline continuum background. NIR observations at resolutions less than 4500 are useless in natural seeing limited observations on the E-ELT. Spectral resolutions of 10000-20000 are set by the chemical abundance studies in the resolved stellar populations case and 30000 is set by the extragalactic planets case.
Multiplicity: A minimum of 200 fully deployable mono-object fibers is required for efficient use of the required telescope time in the high-redshift case, as well as the 3D reconstruction of the IGM case. Sky aperture is set by the natural seeing condition and has been set at 0.9". The Galactic halo case, the resolved stellar populations case and the 3D mapping case all strongly benefit from medium sized
IFUs (few square arcseconds) as well as at least one large IFU (< 100 square arcseconds). A typical example is a moderate redshift merger system surrounded by a large number of globular clusters and/or tidal tails. The medium sized IFUs (>20) should be fully deployable where as the (>=1) large IFU can be fixed at the field center.
Spectral coverage: Simultaneous VIS & NIR observations are crucial for all science cases. The extrasolar planet case will benefit from a maximum number of VIS spectral lines, and the NIR lines will be required to distinguish planets from stellar oscillations. Access to a minimum number of metal lines in all other cases requires an instantaneous wavelength coverage of at least lambda/10 in the VIS and
lambda/20 in the NIR. The full wavelength range between 0.37 - 1.6 micron should be available. Extended blue coverage down to 0.31 micron would strongly boost the resolved stellar population and IGM at intermediate redshift science cases.
Throughput: As in any instrument throughput is always to be maximized. Target depths are I=30 at R = 5000 for high redshift galaxies, z=3 Ly break galaxies at I=25 for R = 5000-1000 and I=23.5 for stars at R = 20000-30000.
In summary, the top-level requirements from RD3 are:
Number of targets: >200 VIS and NIR single objects, several IFUs
Patrol field of view: >7 arcmin diameter (unvignetted)
Apertures on sky: The apertures on the sky of OPTIMOS-EVE are designed in such a
way that the instrument can work in seeing-limited mode, from UV to
NIR, or in GLAO assisted mode (see RD13). Higher order corrections
are not necessary and are not matched to the apertures.
OPTIMOS-EVE in the context of other facilities
There are several strong synergies between OPTIMOS-EVE and future facilities such as the JWST, ALMA and the ESA GAIA mission. In this section we highlight some of these, certainly many more can be identified and will emerge as science progresses. One of the main goals of OPTIMOS-EVE is to study the stellar populations in galaxies in nearby groups, such as Cen A and Sculptor. This study needs complementary photometry and astrometry for target selection and fibre positioning. Although for some galaxies such photometry already exists from HST (e.g. Rejkuba et al. 2005), these reach at most the horizontal branch of the populations. The deeper and sharper images that will be provided by the JWST (and EUCLID) will certainly provide a much better census of the stellar populations and allow a clearer selection of the targets for spectroscopy with OPTIMOS-EVE.
The high accuracy proper motions provided by GAIA will allow to study the environment of Galactic star clusters (both open and globular) and to detect their tidal debris, if any. The majority of the stars in this tidal debris will be dwarfs, and therefore faint (V > 16). OPTIMOS-EVE will obtain spectroscopy of this tidal debris and establish if there are notable differences compared to the cluster stars. At the same time it will be possible to use OPTIMOS-EVE for radial velocity monitoring to see if there are planets in the tidal debris and link this to an analogous campaign in the cluster itself. This will be a way to probe planet formation and properties in different environments.
With its large IFU (13.5×7.8 arcsec2), OPTIMOS-EVE will be able to detect the ionized gaseous phase up to z=3.5 ([OII]3727) and down to 10-19 erg s−1 cm−2 , in regions of 100 kpc surrounding massive galaxies (see Sect. 7 of RD2, science case number 4). There is a very strong synergy with JWST/NIRCAM on this science case. Indeed, understanding the nature of the motions in the halo will require a comparison between kinematics with deep and high spatial resolution imaging, and only JWST/NIRCAM will be able to offer such data above z~2. Spatially resolved spectroscopy alone will allow us to make a significant leap forward in our understanding of the structure of galaxy inner halos, which are currently poorly constrained. But combined with high resolution imaging, it will become possible to construct numerical models of the different processes that occur within the halo, namely in- and out-flow motions, the presence of satellites or of debris of former mergers, or in one sentence, the full balance of gas redistribution between the IGM and the galaxy. Going one step further, it will even become possible to constrain the shape of the underlying dark-matter gravitational potential using the deduced orbital motions of satellites over a wide spectrum of halo mass. A follow-up of several tens of galaxies from z=0.5 to z=3.5 may be an ultimate test of the hierarchical scenario including the evolution of both the dark and the baryonic matter component.
Beyond z=3.5, the Ly-alpha emission line will be available for spectroscopy with OPTIMOS-EVE. This line will allow us to sample the extension of the ionized gas in distant galaxies (Ly-alpha emitters, see Sect. 6 of the science case document, science case number 3) and of the inner halo (see RD2, science case number 4), up to z=13. However, Ly-alpha is generally subject to many scatterings, which result in a more diffuse emission (see Kunth et al. 2003), so that it cannot be used as a reliable kinematics tracer. Above z=3.5, there is therefore a strong synergy with JWST/MIRI which will be able to follow up Ly-alpha emitters (LAE) in Balmer emission lines using its IFU mode and get their kinematics at a resolution of 0.18 to 0.29 arcsec with a FOV of 3.6 × 3.6 to 7.6 × 7.6 arcsec2, respectively. The combination of Ly-alpha and Balmer spectroscopy will allow us to better constrain the escape fraction of photons out of these regions, which is currently a major uncertainty in numerical models.
The nature of Lyman-alpha blobs (LAB) remains even more puzzling. These regions extend up to 30-50 kpc and more (i.e., up to 9 arcsec at z > 2). Recent studies claim that such blobs are possibly direct evidence for the cold accretion mode in distant massive halos (Goerdt et al. 2009; Dijkstra & Loeb 2009), which are expected to feed distant galaxy disks in fresh gas (Dekel et al. 2009; Kereš et al. 2009). Again, there is a strong synergy between OPTIMOS-EVE, which will be ideal for detecting and characterizing gas motions in LABs on large spatial scales using its large and medium IFUs, and both JWST/NIRCAM and JWST/MIRI. The former will provide high-resolution images in the Ly-alpha emission line, which will be essential to probe the filamentary structure predicted by numerical models. The latter will provide a kinematical follow-up in Balmer emission lines and help constrain the photon escape fraction in these regions, as in LAEs. We note a further synergy with ALMA, which should be able to directly detect the underlying molecular gas (using CO lines) within cold gas filaments. The combination of all these data will be simply invaluable for testing and constraining state-of-the-art cosmological simulations at depths that remain currently inaccessible at such very high redshifts.
The OPTical Multi Object Spectrograph 'Extreme Visual Explorer' for the European - Extremely Large Telescope - Consortium partners: GEPI, NOVA, RAL, NBI, INAF, AIP, LSW, IAG, LNA, ON
