WEAVE is a wide-field survey facility for studying galaxy structure and cosmology.

There is now a broad consensus that the scientific exploitation of major space- and ground-based programmes in the coming decade requires new optical spectroscopic survey instruments. Our UK-led consortium has developed a conceptual design for a new prime focus facility at the WHT that can address the wide-ranging survey goals of Galactic and extra-Galactic astronomy using a single competitive and cost-effective instrument, which could begin operations in early 2017.

The principal science goals are:

(1) Milky Way archaeology: exploiting Gaia's scientific legacy.
The Milky Way (MW) is the only galaxy for which we can determine a precise chemo-dynamical formation and evolutionary history. The Gaia satellite will revolutionize the study of the MW and its companions, delivering photometry, 3-D positions, and proper motions for > 10⁹ stars brighter than V = 20 within five years of its launch in 2012. Gaia's Radial Velocity Spectrograph (RVS) will study stars with V < 17, yielding radial velocities, metallicities and stellar parameters, though with strongly decreasing accuracy at V > 11. To take full advantage of the substantial investment in Gaia requires a comprehensive MW spectroscopic census, combining the three components crucial for progress in near-field cosmology: kinematics, elemental abundances, and a view of all the stellar components of the MW. WEAVE will provide two of the key tools proposed to complete Gaia's MW census (Feltzing & Walton 2010); R=5000 spectroscopy for complete 6-D phase-space information for stars with V > 17 and R = 20000 spectroscopy of stars with V < 18 for chemical labelling.

(1.1) The WEAVE low-resolution survey of > 10⁶ stars with δv_r < 5 km/s in the range 17 < V < 20 will enable unique studies including:

1. Determination of fundamental Galactic parameters and the origin (scattering from spiral arms and/or the bulge) of thin disk substructures (with velocities of 10 - 30 km/s), requiring velocity accuracy of < 5 km/s for stars at V > 17 over 100 times the volume of the original Hipparcos survey of ~10⁴ stars, impossible with RVS;
2. A kinematic map of 10⁵ stars in the Galactic thick disk will determine the variation of orbital eccentricity with galactocentric radius, providing a strong diagnostic of its formation process;
3. CDM (cold dark matter) models predict a large number of completely dark, very cold bound substructures in the halos of galaxies, detectable only by perturbations of the dynamics of visible cold halo streamers. Complementing Gaia's proper motions and distances with radial velocities of stars at V > 17 will allow detection of these perturbations.

Northern Hemisphere coverage is crucial to target the outer MW disk. The WEAVE low-resolution survey will be unique; no other existing or proposed facility (e.g. LAMOST) will provide such a survey to similar depth or spectral resolution. The southern sky inaccessible to WEAVE will be covered by the proposed 4MOST survey (from 2018, if this is selected by ESO).

(1.2) The WEAVE high-resolution survey will provide abundances accurate to ~ 0.1 dex for a large number of elements probing most nucleosynthetic processes. This will allow us to distinguish between cosmological formation scenarios put forward to explain the assembly and evolution of the MW. These include addressing the formation of a thick stellar disk and its nature, as well as the study of halo streams to understand the fraction of the stellar halo originating in accreted dwarf galaxies. A survey targeting ~ 50000 halo giants in ~ 500 streams, over ~ 2500 deg² at V < 17 - 18, will be sufficient to characterize the 'building blocks' of the MW halo. Such a survey is necessary given the large number of streams, but the low density of halo stars. In contrast HERMES at the AAT will target only much brighter stars (V < 14) at somewhat higher spectral resolution but lower multiplex and hence will not be competitive in this science topic.

(2) Galaxy evolution and cosmology: exploiting SKA Pathfinders.
Using its integral field (IFU) capability, WEAVE will survey low redshift galaxies detected in neutral Hydrogen by APERTIF, providing samples of star forming galaxies with spatially resolved kinematics in HI, starlight and ionised gas. At higher redshifts WEAVE will provide spectroscopic redshifts for large samples of radio galaxies detected in LOFAR and APERTIF continuum surveys, which will determine the evolution of both nuclear activity and star formation from the epoch of reionisation to the present. WEAVE will also provide redshifts of far IR and sub-mm sources detected by Herschel and Scuba-II, using precise radio-determined positions. Galaxies detected in these surveys will mostly exhibit strong emission lines, requiring a 4-m instrument to provide an optimal detection strategy.

(2.1) The WEAVE-APERTIF survey will:

1. Target ~ 10⁴ galaxies over ~ 500 deg² out to z ~ 0.4 detected by APERTIF in the 21-cm HI line with WEAVE's IFUs, yielding spatially-resolved kinematics of ionized gas and stars in galaxies to be directly compared with their spatially-resolved HI kinematics.
2. Provide, via IFU spectroscopy at R ~ 10000 with WEAVE, maps of stellar velocity dispersions of hundreds of nearby galaxies, allowing the detailed study of their luminous and dark matter contents and providing a direct link from WEAVE's MW studies to other galaxies.

(2.2) The WEAVE galaxy evolution and cosmology survey will provide emission-line redshifts for all star-forming galaxies at z < 1.3 using Hα and [OII] emission and at z > 2 using Lyman&alpha emission for ~ 10⁷ objects selected from LOFAR and APERTIF continuum surveys over 10000 deg².

1. At these redshifts the SKA Pathfinders will only detect synchrotron continuum radiation, offering an obscuration-free way to measure star-formation rates. For many galaxies metallicities and stellar velocity dispersions will provide further important information on the chemistry and dynamical mass of systems and their evolution, and the accretion mechanism for radio-AGN (active galactic nuclei) can be determined, all crucial ingredients in galaxy formation and evolution models.
2. The Baryon Acoustic Oscillation (BAO) technique provides a standard ruler at different epochs that can provide measurements of the cosmic expansion (WEAVE constraints would be ~ 0.7% in H(z); ~ 0.4% in d_A). These surveys also provide measurements of the growth rate of cosmological structures through the imprint of infall velocities (redshift-space distortions [RSDs]) on the apparent clustering. RSD measurements from a survey of this scale set tight constraints (~ 0.3%) on the derivative of the growth rate dD / dlog a ∝ f(z) σ₈(z,mass) and provide a direct test of gravity models. The radio survey selection proposed for WEAVE will sample a larger range of redshifts and star-formation rates than is possible with the ongoing BOSS survey, and produce a survey of comparable impact to the proposed BigBOSS experiment, but at a substantially lower cost.