The Square Kilometre Array

. The Square Kilometre Array (SKA) is a global project to design and build a new generation radio telescope at metre to centimetre wavelengths.


Introduction and SKA status
Over the last few decades, pulsar astronomy and Cosmic Microwave Background (CMB) observations have shown how radio astronomy can deliver profound results. Neutral Hydrogen (HI), pulsar and other observations with the Square Kilometre Array (SKA) have an exceptionally strong chance of continuing this story of scientific success.
The quest for a radio telescope capable of imaging neutral Hydrogen (HI) to the limits of the observable Universe has been pursued for two decades (Wilkinson 1991), and astronomers appear to be at, or very close to, the point of making statistical detections of HI at high redshift (Chang et al. 2010;Paciga et al. 2010). This quest has been organized via a global Square Kilometre Array (SKA) program for which a science case much broader than HI and pulsars has been identified, refined and published (Taylor & Braun 1999;Carilli & Rawlings 2004). To obtain both the temperature sensitivity needed for HI and to find and time pulsars requires a huge (kilometre scale) essentially-fullyfilled aperture, and thus requires that the SKA is built as a giant ground-based facility 1 .
1 For a filled-aperture telescope, the temperature, or surface brightness, sensitivity (measured in K) is independent of telescope size, explaining how fundamental breakthroughs in CMB astronomy have been made by modest-sized (few-mclass), millimetre-wavelength space telescopes like WMAP and Planck. The θ ∼ λ/D law of diffraction means that studying similar angular scale (arcmin to degree) temperature fluctuations in redshifted HI (at metre wavelengths) requires telescopes of high filling factor (µ f , the fraction of the physical area covered by the effective collecting area) and a minimum characteristic size of around a kilometre. Pulsar astronomy requires a similar centrally-concentrated collecting area to avoid computational-overload: for pulsar searching the field of view (FOV) of each independent receptor must be covered by the smallest number of beams formed by adding or correlating sig-At the time of the Crete meeting, the SKA effort was coordinated by the SKA Science and Engineering Committee, the Agencies SKA Group and an SKA Program Development Office funded by the international partners and an FP7 EC grant PrepSKA 2 . At the time of writing, the outputs of the policy work packages of PrepSKA -reports, including legal advice, concerning governance, procurement and funding -are currently being used to ensure that, before the end of PrepSKA in March 2012, full control of the SKA project will be passed to a new legal entity based at the SKA Project Office (SPO) to be housed in a new building at the University of Manchester's Jodrell Bank Observatory (JBO) 3 .
The history of European funding for the SKA reflects its inclusion in the roadmap of the European Strategy Forum for Research Infrastructures, and its ranking 4 alongside the E-ELT as equal top priority for future large-scale ground-based astronomy projects; additionally the US Astro2010 Decadal Survey noted that the SKA represents the 'long-term future for radio astronomy'. Elsewhere in the world, the SKA has excited considerable government-level interest, most notably in the two countries short-listed as potential hosts of the inner regions of the SKA: 'Pathway to SKA' is the top priority for radio astronomy in Australia, and significant funds have flowed into new initiatives such as the International Centre for Radio Astronomy Research (ICRAR 5 ), and the Australian SKA Pathfinder (ASKAP); the Heads of State of the African Union have acknowledged the importance of the SKA in the development of knowledge-based economies and South-African funds have flowed into an SKA-related human capital development programme and the South African SKA precursor MeerKAT. 6 The schedule for the SKA features the following key decision points and milestones: -April 2011. Having passed external international reviews -a System Level Conceptual Design Review (CoDR, Feb 2011) and a review of the pre-construction Project Execution Plan (the SKA PEP) -the SKA Founding Board was established to manage the transition from the current governance structure to a new SKA legal entity. -Apr 2018. SKA 2 construction begins.
The construction and operation of the SKA in two phases allows the science return to benefit optimally from the continued gains in ICT capability with time, as is loosely encapsulated by 'Moore's Law', and also allows the participation of the widest possible collaboration of international funders, taking into account the spread in their likely profiles of spend on SKA. The SKA PEP includes an Advanced Instrumentation Program (AIP) that allows for the further development and verification of Aperture Array (AA) and smart-dish-feed technologies.
In brief summary (see also Garrett et al. 2010 andDewdney et al. 2010) SKA 1 will consist of -50 AA stations (operating ∼ 70 − 450 MHz) with a total physical collecting area ∼ 10 6 m 2 , 50 per cent within a core of diameter D core = 1 km, with AA stations reaching out to a radius r = 100 km. -250 15-m dishes (operating ∼ 0.3 − 10 GHz), 50 per cent within D core = 1 km and reaching r = 100 km, potentially (depending on the AIP) with smart feeds allowing field-of-view or bandwidth expansion.
We emphasise in Sec.2 that the science drivers of SKA 1 and SKA 2 remain those identified in Carilli & Rawlings (2004), but that this phased approach allows some of the key science results to emerge once SKA 1 begins science operation towards the end of this decade.
Although we caution that temperature (rather than flux) sensitivity, or mapping speed, or other metrics are sometimes the correct figure of merit for SKA science, SKA 2 is defined by a requirement that across the full radio 6 http://www.ska.ac.za/ frequency range from 70 MHz to 10 GHz (with goals to extend to lower and higher frequencies where technically feasible and affordable) it has an r.m.s flux sensitivity S rms (combining two polarizations for a point source) of: where δν is the bandwidth (in Hz) and δt is the exposure time (in s), and, by definition, f = 1 for an SKA 2 realization with sensitivity A eff /T sys ∼ 20, 000 m 2 K −1 (as assumed within Carilli & Rawlings 2004); the AA part of SKA 1 has f ∼ 0.1 (assuming, away from the Galactic Plane, T sys = 500 K) and filling factor µ f ≈ 0.8, and the dish part of SKA 1 has f ≈ 0.05. The working realization of SKA 2 (adapted from Schilizzi et al. 2007) has f ≈ [0.5(AA), 0.5(AA) + 0.5(dish), 0.5(dish)] at frequencies ν = [0.13, 0.47, 1.7] GHz and consists of the following: 250 low-frequency (70-450 MHz) AA stations (at least 125 in a D core = 5 km core); 250 mid-frequency (0.3-1.4 GHz) AA stations (at least 125 in a separate D core = 5 km core); and 2500 15-m dishes.

Science Drivers: SKA 1 and SKA 2
Although the SKA science case is exceptionally broad (Carilli & Rawlings 2004), the design considerations of the SKA are determined by science drivers that exploit the unique opportunities radio astronomy brings to mapping out the origins of structure, to studying the physical laws and contents of the Universe, and to the discovery of new phenomena. The '21-cm line' of neutral Hydrogen (HI) is a cornerstone of the SKA 1 science case as its observation at high redshift allows astronomers to bridge a gap in studies of the distant Universe. This gap lies between ∼ 0.4 Myr (z ∼ 1100) after the Big Bang -when observations of the CMB show that the seeds of all structure were still tiny (∼ 1 part in 10 5 ) perturbations in an otherwise smooth, featureless Universe -and the relatively recent Universe (∼ 0.7 − 13.7 Gyr after the Big Bang, or z ∼ 6 − 0) in which observations show the Universe to be rich in galaxies, stars and planets. This gap is called the 'Dark Ages' when the first stars, galaxies and black holes formed, and ends at z ∼ 10 with the 'Epoch of Reionization' (EoR); conditions in the EoR are probed indirectly by CMB polarization studies that deliver predictions regarding the evolving HI signal level. Directly mapping out the EoR is thus one of the two main science drivers for SKA 1 using the HI line redshifted to ∼ 100 − 200 MHz (HI at z ∼ 13 − 6; Carilli et al. 2004).
How HI evolves outside this 13 − 6 redshift range also constitutes basic information about the Universe. At z > 13, observations become difficult because of the sky temperature and foregrounds, but also because, at these early times, the predicted HI signal level is increasingly speculative. However, statistical information could plausibly be recoverable with SKA 1 out to z ∼ 19 assuming a 70 MHz lower-frequency cut-off (Garrett et al 2010).
HI evolution between z ∼ 6 − 0 can also be followed by SKA 1 in the 0.2-1.4 GHz waveband using both dishes and, potentially, the AAs. At these epochs the IGM is reionized, so the HI is likely be associated with galaxies, allowing straightforward utilisation of the 'stacking technique', and incorporation of information from HI absorption to study the cosmic evolution of the ratio of atomic to molecular material involving sub-millimetre facilities like ALMA. If SKA 1 were able to operate to above ∼ 15 GHz 7 then it is likely to also become the premier instrument for studying molecular material, through redshifted CO(1-0), in the EoR (Heywood et al. 2011a 2011b), and via cross-correlation techniques (Rawlings 2011) this could contribute to the first detailed studies of the EoR. These would complement wider SKA studies of origins such as how galaxies evolved , where cosmic magnetism originated (Gaensler et al. 2004) and how planets formed in protoplanetary disks (Lazio et al. 2004).
The other cornerstone of the SKA 1 science case is pulsar astronomy 8 . Providing a facility capable of accurately timing pulsars -especially those in the Southern Hemisphere where the Galactic Centre is easily observable, and where the SKA is the only planned highsensitivity radio facility -is the second main science driver for SKA 1 . Through the discovery and follow-up timing of pulsars, the SKA is capable of making unique probes of fundamental physics (Kramer 2011;Karastergiou et al. 2011). The discovery of yet-more-exotic systems than the double-pulsar system (Lyne et al. 2004) -and specifically the discovery of any pulsar in the 'strong-field limit' around a supermassive black hole -would, with SKA follow-up timing, push tests of Einstein's General Relativity into regimes where uncertain (e.g. Quantum Gravity) or completely new physics may become evident (Kramer et al. 2004). The SKA can expect to be centrally involved in the first detections of gravitational waves, an exceptionally promising target being the cosmic background due to the merging of supermassive black holes (see also Martinez-Sansigre & Rawlings 2011), and ultimately to be able to pinpoint gravitational-wave sources (Sesana & Vecchio 2010).

7
The SKA design process currently has 10 GHz as the required upper frequency limit. However, we note that SKA dishes must also be capable of high-dynamic-range imaging, and thus have a target r.m.s surface accuracy of 0.5 mm (Garrett et al. 2010). The Ruze-equation (Ruze 1952) for dishes indicates that their efficiency at 15 GHz will be 95 per cent of their efficiency value below 10 GHz.
8 The technical demands placed on SKA1 for pulsar astronomy -most critically high sensitivity and time resolution, and requirement for correcting for effects such as dispersion and intrinsic time-varying properties of pulsars -have strong overlap with the requirements of finding and studying other fast transient radio sources and hence extreme physics (Karastergiou et al. 2011). This means that the discovery of new transient phenomena, and the role of serendipity in such discoveries, can largely be pursued using an SKA designed around the requirements of pulsar astronomy (Cordes 2011;Kramer 2011).
The SKA 2 contribution to the study of fundamental laws can also utilise HI redshift surveys, containing a billion or so individually detected galaxies (Abdalla et al. 2010) or statistical detection, via HI, of large-scale structure (Chang et al. 2010) -to study key topics in modern physics. These include the nature of dark energy or post-Einstein gravity, neutrino mass, and the processes driving cosmic inflation .
It is a happy coincidence that HI and pulsars, as tools for unique studies the Universe, require that the SKA project constructs about one million square metres of collecting area, operating as an interferometer at frequencies from ∼70 MHz (for HI work) to at least ∼10 GHz (for pulsar timing work). There must be two detector technologies: AAs at the lower frequencies; and dishes, potentially with smart (providing increased instantaneous bandwidth or field of view) feeds, at the higher frequencies. Each detector technology needs one, and in the case of AAs probably two, compact (5-km diameter) cores containing about 50 per cent of the overall collecting area. This provides the raw temperature and flux sensitivity that is sufficient and necessary for the SKA to be able to: image HI structures in the EoR ; study the evolution of HI across cosmic time; probe large-scale structure traced by HI in galaxies with sufficient accuracy to constrain w to less than 1 per cent accuracy (Abdalla et al. 2010) and measure neutrino masses (Abdalla & Rawlings 2007); to find all the pulsars in our galaxy (Kramer et al. 2004); to time the most exotic pulsar systems accurately enough to make fundamental new test of GR (Kramer et al. 2004); and to construct a Pulsar Timing Array (PTA) 9 .
Thus, through both HI and pulsar astronomy, the SKA promises to open up areas of 'post-Einstein science', and through probing regimes too extreme in size or energy scale to ever study on Earth, complements other fundamental science experiments such as those undertaken by the Large Hadron Collider at CERN. With this ambitious vision, the SKA will deliver far more than the sum of two or more distinct parts: e.g. a low-frequency ( < ∼ 1 GHz) part for HI and a high-frequency part for pulsar work. The added value of a single system will already be evident in SKA 1 : HI evolution studies above 450 MHz (z < ∼ 2.1) will use dishes not AAs, whilst surveys for Southern pulsars are likely to be performed by AAs as well as dishes [Smits et al. (2009b) assuming, following Garrett et al (2010), an upper frequency of 450 MHz for the AA part of SKA 1 ].
The distribution of collecting area outside the SKA cores will be designed to provide the resolution needed to ensure the main science aims are not compromised by lack of spatial resolution or astrometric inaccuracy. This 9 Measurement of gravitational waves requires pulsar timing in both the Northern and Southern hemispheres to look for the large-scale angular coherence imprinted on pulsar timing residuals by low-frequency gravitational waves impinging on the Milky Way (Kramer et al. 2004). SKA2 will provide worldleading pulsar-timing capability: enhancements to the existing Northern PTA, e.g. with FAST (Smits et al. 2009a), providing complementary capabilities in the North. requires a range of longer baselines outside those available in the compact core. For the HI work, it will be necessary to separate HI in in the intergalactic medium from HI in galaxies so as to observe the transition from distributed to confined-to-galaxy HI as Universal re-ionization progresses, requiring ∼ 100-km (∼ 1 arcsec resolution for AAs; ∼ 0.1 arcsec resolution for dishes) baselines. For the pulsar work, astrometry is key to extracting the tightest constraints on GR from the timing follow-up, and so some very long (> 1000-km) baselines will be needed.
The SKA will naturally become a flexible facility: capable of routinely imaging at 'HST-like' (∼ 0.1 arcsec) resolution at higher frequencies; capable of non-confusionlimited resolution for surveys at low frequency where, for example, continuum surveys can detect all the quasars, radio-quiet or not, across most of the observable Universe (Jarvis & Rawlings 2004); and capable of participating in VLBI for astrometry and imaging with superb angular resolution. The SKA will naturally take its place alongside other great upcoming observatories (e.g. ALMA, JWST and ELTs) and the other future premier astronomy survey instruments (e.g. Euclid, LSST).

Towards SKA: SKA 0
There are a large number of technology demonstration programs and telescopes at various stages of completion that lie outside the direct PrepSKA 'Description of Work' but which have been identified as pathfinder activity for the SKA. In the case of telescopes under construction on the potential host sites, these are termed precursor facilities. It is a convenient simplification to refer to these collectively as SKA 0 since they are all doing SKA-relevant work under independent funding and direction.
-ASKAP: Australian dish precursor consisting of 36 12-m dishes (f ∼ 0.005) . Key technical pathfinding: inexpensive high-performance dishes; phased array feeds (PAFs) and associated digital back-end and data transport solutions. Key science pathfinding: large skyarea HI and continuum surveys. -MeerKAT: South-African dish precursor consisting of 64 13.5-m dishes (f ∼ 0.01). Key technical pathfinding: inexpensive high-performance dishes; single-pixel feeds and associated digital back-end and data transport solutions. Key science pathfinding: deep HI and continuum surveys; pulsar timing. -WSRT/APERTIF: PAF system (1-1.75 GHz) to be installed on 12 of the 14 WSRT antennae (f ∼ 0.007) to increase its survey speed by a factor 20. Key technical pathfinding: PAFs and digital backends. Key science pathfinding: wide-field Northern HI and continuum surveys.
-MWA: USA and Australian AA precursor (80-300 MHz with one antenna type) to consist of 512 tiles (each with 16 antennae, so f ∼ 0.001 and filling factor µ f ∼ 0.01 within 1-km) on candidate Australian SKA site. Key technical pathfinding: small amounts of signal aggregation prior to correlation-rich processing. Key science pathfinding: EoR experiment.

Concluding Remarks
In this paper we have motivated the specifications for the SKA, and its phased realization as Phase-1 SKA (SKA 1 ) and Phase-2 SKA (SKA 2 ), by the transformational science we know it can deliver. The difference between this coherent approach and the diverse range of pathfinding activity summarised in Sec. 3 is not accidental, indeed we believe it is a critical step towards making the SKA happen now rather than at an indeterminate point in the future. The recent formation of the SKA Founding Board signals the seriousness of this endeavour. SKA 1 will allow HI studies to map out the Epoch of Re-ionization (EoR) and open up the Dark Ages of the Universe; it will also use pulsars to make unique tests of GR, including an extremely good chance of being critical to the first detections of gravitational waves using PTAs. As a bonus there will be a wealth of other astronomy possible with SKA 1 including all-Southernsky arcsec-resolution radio continuum surveys identifying counterparts to the next-generation of photometric (e.g. LSST) and spectroscopic surveys planned in other astronomical wavebands, and sub-arcsec-resolution imaging ca-pabilities needed to complement other great observatories like ALMA, ELTs and the JWST. We illustrate these synergies with just one example: ALMA and SKA are 'natural integral-field-unit (IFU) spectrometers' providing data cubes showing the location and kinematics of molecular, atomic and ionized material in distant galaxies: early-light E-ELT IFU instruments like HARMONI  will image individual HII regions and deliver kinematics from stellar and gas tracers -all these facilities will be needed to get a comprehensive picture of galaxy formation and evolution.
The plan for SKA 1 ensures breakthrough science early in the project, but the truly transformational science return will need the full capabilities of SKA 2 . SKA 2 will probe the Dark Ages from z = 6 to perhaps z ∼ 20 − 30. It will use HI to make the 'billion galaxy' surveys needed to address key questions such as neutrino mass and subper-cent accuracy on the dark energy w parameter. It will make deep polarisation-sensitive 'all-southern-sky' surveys that will probe the origin of magnetic field in the Universe, determining its role in the formation of structure from the cosmic web, through galaxies, to stars. For pulsar surveys, SKA 2 will deliver the full census of ∼ 20, 000 normal and ∼ 1000 millisecond pulsars in our Galaxy, with a concomitant increase in chances of finding the rare 'holy grail' systems that allow precision tests of GR and measurement of the equation of state of nuclear matter at supranuclear densities. Pulsar timing will be revolutionised by SKA 2 capabilities allowing fundamental studies of the graviton and the ability to pinpoint gravitational wave sources. SKA 2 will also be a critical instrument for understanding planet formation and astrobiology.
There is a strong expectation that, as the SKA is realised in phases, that the community using it will expand. Space scientists will use the SKA to track spacecraft and near-earth objects and to monitor space weather, embedding SKA in Solar-System exploration, and space safety and security systems (Butler et al. 2004;Jones 2004). The SKA's visibility as a research infrastructure for fundamental physics will develop not only through its key science topics but in other ways such as the plans to monitor radio flashes from the moon that are expected to result from interactions between ultra-high-energy neutrinos and the Moon's regolith (Falcke et al. 2004).
The societal impact of SKA research will be maximised by fostering a close relationship between SKA and 'Citizen Science': by expanding on initiatives such as 'SETI@home' 10 and 'GalaxyZoo' 11 , the SKA can expect 10's of millions of individuals in global society to get truly involved by donating their CPU cycles and brain power to SKA data re-processing and science.
With the SKA, as with other research infrastructures of the future, there is an expectation that its design, construction and development will be implemented by multisectoral consortia of industry and academic institutions carrying out packages of work under the direction and authority of the SPO. By optimising the involvement of industry and academia across the World, this should help ensure that the SKA has the widest possible impact in the form of technology spin-off and human capital development. As well as revealing profound truths about the Universe, we hope the SKA project will generate new ways of doing ICT, using green energy wherever feasible.