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CSIRO PUBLISHING

www.publish.csiro.au/journals/pasa

Publications of the Astronomical Society of Australia, 2011, 28, 215­248

EMU: Evolutionary Map of the Universe
Ray P. NorrisA,AM, A. M. HopkinsB,AJ, J. AfonsoC, S. BrownA, J. J. CondonD, L. DunneE, I. FeainA, R. HollowA, M. JarvisF,AL, M. Johnston-HollittG, E. LencA, E. MiddelbergH, P. Padovani I, I. Prandoni J, L. RudnickK, N. SeymourL, G. UmanaM, H. AndernachN, . D. M. AlexanderU, P N. AppletonO, D. BaconP, J. BanfieldA, W. BeckerQ,M.J.I.BrownR, S P. Ciliegi , C. JacksonA, S. EalesT, A. C. EdgeU, B. M. GaenslerV,AJ, G. Giovannini J, . C. A. HalesA,V, P HancockV,AJ, M. T. HuynhW, E. IbarX, R. J. IvisonX,Y, R. KennicuttZ, D opez-SanchezB,AK, Amy E. Kimball , A. M. KoekemoerAA, B. S. Koribalski A, A. R. L M. Y. MaoA,B,AB, T. MurphyV,AJ, H. MessiasC, K. A. PimbbletR, A. RaccanelliP, K. E. RandallA,V, T. H. ReiprichAD, I. G. RoseboomAE, H. RottgeringAF, D. J. SaikiaAG, R. G. SharpAH, O. B. SleeA, Ian SmailU, M. A. ThompsonF, J. S. UrquhartA, J. V. WallAI, and G.-B. ZhaoP
A B

CSIRO Astronomy & Space Science, Epping, NSW 1710, Australia Australian Astronomical Observatory, Epping, NSW 1710, Australia C Centro de Astronomia e Astrofisica da Universidade de Lisboa, Observatrio Astron ´ o omico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal D National Radio Astronomy Observatory, Charlottesville, VA 22903, USA E School of Physics & Astronomy, University of Nottingham, Nottingham, NG7 2RD, UK F Centre for Astrophysics Research, University of Hertfordshire, Hatfield, Hertfordshire, AL10 9AB, UK G School of Chemical & Physical Sciences, Victoria University of Wellington, Wellington 6140, New Zealand H Astronomisches Institut, Ruhr-Universitat Bochum, 44801 Bochum, Germany ¨ I European Southern Observatory, D-85748 Garching bei Munchen, Germany ¨ J INAF-IRA, 40129 Bologna, Italy K Department of Astronomy, University of Minnesota, Minneapolis, MN 55455, USA L University College London, Mullard Space Science Laboratory, Dorking, Surrey, RH5 6NT, UK M INAF-Catania Astrophysical Observatory, 95123 Catania, Italy N Depto. de Astronom´a, Universidad de Guanajuato, Guanajuato, C.P. 36000, GTO, Mexico i O NASA Herschel Science Center, Caltech, Pasadena, CA 91125, USA P Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth, PO1 3FX, UK Q Max-Planck Institut fur extraterr. Physik, 85741 Garching, Germany ¨ R School of Physics, Monash University, Clayton, VIC 3800, Australia S INAF ­ OABO, 40127 Bologna, Italy T School of Physics and Astronomy, Cardiff University, Cardiff, CF24 3AA, UK U Department of Physics, Durham University, Durham, DH1 3LE, UK V Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia W International Centre for Radio Astronomy Research, University of WA, Crawley, WA 6009, Australia X UK Astronomy Technology Centre, Royal Observatory, Edinburgh EH9 3HJ, UK Y Institute for Astronomy, University of Edinburgh, Edinburgh, EH9 3HJ, UK Z Institute of Astronomy, University of Cambridge, Cambridge, CB3 0HA, UK AA Space Telescope Science Institute, Baltimore MD 21218, USA AB School of Mathematics & Physics, University of Tasmania, Hobart, TAS 7001, Australia AD Argelander Institute for Astronomy, Bonn University, 53121 Bonn, Germany AE Dept of Physics and Astronomy, University of Sussex, Falmer, East Sussex, BN1 9RH, UK AF Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands AG National Centre for Radio Astrophysics, TIFR, Pune 411 007, India AH Mount Stromlo Observatory, Canberra, ACT 2611, Australia AI Dept. of Physics and Astronomy, University of British Columbia, Vancouver, B.C. V6T 1Z1, Canada AJ ARC Centre of Excellence for All-sky Astrophysics, University of Sydney, Sydney, NSW 2006, Australia AK Department of Physics and Astronomy, Macquarie University, North Ryde, NSW 2109, Australia AL Physics Department, University of the Western Cape, Cape Town 7535, South Africa AM Corresponding author. Email: ray.norris@csiro.au

ñ Astronomical Society of Australia 2011

10.1071/AS11021

1323-3580/11/03215


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Received 2011 May 19, accepted 2011 June 15 Abstract: EMU is a wide-field radio continuum survey planned for the new Australian Square Kilometre Array Pathfinder (ASKAP) telescope. The primary goal of EMU is to make a deep (rms ,10 mJy/beam) radio continuum survey of the entire Southern sky at 1.3 GHz, extending as far North as ×308 declination, with a resolution of 10 arcsec. EMU is expected to detect and catalogue about 70 million galaxies, including typical star-forming galaxies up to z , 1, powerful starbursts to even greater redshifts, and active galactic nuclei to the edge of the visible Universe. It will undoubtedly discover new classes of object. This paper defines the science goals and parameters of the survey, and describes the development of techniques necessary to maximise the science return from EMU. Keywords: telescopes -- surveys -- stars: activity -- galaxies: evolution -- galaxies: formation -- cosmology: observations -- radio continuum: general

1 Introduction 1.1 Background Deep continuum surveys of the radio sky have a distinguished history both for discovering new classes of object and for providing radio counterparts to astronomical objects studied at other wavelengths. The earliest large surveys, such as the 3C catalogue (Edge et al. 1959) and the Molonglo Reference Catalogue (Large et al. 1981), gave us the first insight into the physics of radio galaxies and radio-loud quasars, but were insufficiently sensitive to detect any but the nearest radio-quiet or star-forming galaxies. Later radio surveys reached flux densities where normal star-forming galaxies were detected, but were still largely dominated by radio-loud active galactic nuclei (AGN). Only very long integrations in narrow deep fields made it possible to start probing star-forming galaxies beyond the local Universe. This paper describes a planned survey, EMU (Evolutionary Map of the Universe), which will reach a similar sensitivity (,10 mJy/beam) as those deep surveys, but over the entire visible sky. At that sensitivity, EMU will be able to trace the evolution of galaxies over most of the lifetime of the Universe. Fig. 1 shows the major 20-cm continuum radio surveys. The largest existing radio survey, shown in the top right, is the wide but shallow NRAO VLA Sky Survey (NVSS), whose release paper (Condon et al. 1998)is one of the most cited papers in astronomy. The most sensitive existing radio survey is the deep but narrow Lockman Hole observation (Owen & Morison 2008) in the lower left. All current surveys are bounded by a diagonal line that roughly marks the limit of available telescope time of current-generation radio telescopes. The region to the left of this line is currently unexplored, and this area of observational phase space presumably contains as many potential new discoveries as the region to the right. The Square Kilometre Array (SKA) is a proposed major internationally funded radio telescope (Dewdney et al. 2009)whose construction is expected to be completed in 2022. It will be many times more sensitive than any existing radio telescope, and will answer fundamental

questions about the Universe (Carilli & Rawlings 2004). It is likely to consist of between 1000 and 1500 15-meter dishes in a central area of diameter 5 km, surrounded by an equal number of dishes in a region stretching up to thousands of kilometres. The Australian SKA Pathfinder (ASKAP) is a new radio telescope being built both to test and develop aspects of potential SKA technology, and to develop

Figure 1 Comparison of EMU with existing deep 20 cm radio surveys. Horizontal axis is 5-s sensitivity, and vertical axis shows the sky coverage. The diagonal dashed line shows the approximate envelope of existing surveys, which is largely determined by the availability of telescope time. The squares in the top left represent the EMU survey, discussed in this paper, and the complementary WODAN (Rottgering et al. 2010b) survey which has been proposed for the upgraded Westerbork telescope to cover the sky north of ×308. Surveys represented by diagonal lines are those which range from a wide shallow area to a smaller deep area. The horizontal line for ATLAS extends in sensitivity from the intermediate published data releases (Norris et al. 2006; Middelberg et al. 2008a; Hales et al. 2011) to the final data release (Banfield et al. 2011).


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SKA science. ASKAP is being built on the Australian candidate SKA site in Western Australia, at the Murchison Radioastronomy Observatory, with a planned completion date of late 2012. In addition to developing SKA science and technology, ASKAP is a major telescope in its own right, likely to generate significant new astronomical discoveries. 1.2 ASKAP ASKAP (Johnston et al. 2007, 2008; Deboer et al. 2009) will consist of 36 12-metre antennas spread over a region 6 km in diameter. Although the array of antennas is no larger than many existing radio telescopes, the feed array at the focus of each antenna is revolutionary, with a phased-array feed (PAF: Bunton & Hay 2010) of 96 dualpolarisation pixels, designed to work in a frequency band of 700­1800 MHz, with an instantaneous bandwidth of 300 MHz. This will replace the single-pixel feeds that are almost universal in current-generation synthesis radio telescopes. As a result, ASKAP will have a field of view up to 30 deg2 enabling it to survey the sky up to thirty times faster than existing synthesis arrays, and allowing surveys of a scope that cannot be contemplated with current-generation telescopes. To ensure good calibration, the antennas are a novel 3-axis design, with the feed and reflector rotating to mimic the effect of an equatorial mount, ensuring a constant position angle of the PAF and sidelobes on the sky. The pointing accuracy of each antenna is significantly better than 30 arcsec. The ASKAP array configuration (Gupta et al. 2008) balances the need for high sensitivity to extended structures (particularly for neutral hydrogen surveys) with the need for high resolution for continuum projects such as EMU. To achieve this, 30 antennas follow a roughly Gaussian distribution with a scale of ,700 m, corresponding to a point spread function of ,30 arcsec using natural weighting, with a further six antennas extending to a maximum baseline of 6 km, corresponding to a point spread function of ,10 arcsec using uniform weighting. The positions of the antennas are optimised for uv coverage (i.e. coverage in the Fourier plane) between declination þ508 and ×108, but give excellent uv coverage between declination þ908 and ×308. The PAF is still under development, but the performance of prototypes gives us confidence that the EMU survey is feasible as planned. The PAF will consist of 96 dual-polarisation receivers, each with a system temperature ,50 K, which are combined in a beam-former to form up to 36 beams. Each of these beams has the same primary beam response as a single-pixel feed (,1.28 fullwidth half-maximum at 1.4 GHz), distributed in a uniform grid across an envelope of 30 deg2. The optimum weighting and number of beams is still being studied, but the current expectation is that 36 beams will be used for EMU, with the sensitivity over the 30 deg2 field of view (FOV) expected to be uniform to ,20%. This will be improved to ,10% uniformity by dithering, with no

significant loss of sensitivity, so that the images from the 36 beams can be jointly imaged and deconvolved as a single image covering the FOV. Consequently, it is expected that the telescope will dwell on one position in the sky for 12 hours, reaching an rms sensitivity of ,10 mJy/beam over a ,30 deg2 FOV. The strategy for achieving this is still under development, and is discussed in y3.6. Although high spatial resolution is essential for EMU, the short spacings of ASKAP also deliver excellent sensitivity to low surface brightness emission, which is essential for a number of science drivers such as studies of radio emission from nearby clusters (y2.7). The ,10 mJy/ beam rms continuum sensitivity in 12 hours is approximately constant for beam sizes from 10 to 30 arcsec, then increases to ,20 mJy/beam for a 90 arcsec beam and ,40 mJy/beam for a 3 arcmin beam. Science data processing (Cornwell et al. 2011) will take place in an automated pipeline processor in real time. To keep up with the large data rate (,2.5 GB/s, or 100 PB/ year), all science data processing steps, from the output of the correlator to science-qualified images, spectra, and catalogues, are performed in automated pipelines running on a highly distributed parallel processing computer. These steps include flagging bad data, calibration, imaging, source-finding, and archiving. A typical ASKAP field will contain about 50 Jy of flux in compact or slightly resolved sources. ASKAP can observe the entire visible 20 cm continuum sky to an rms sensitivity of ,1 mJy/beam in one day, so that initial observations will produce a global sky model (an accurate description of all sources stronger than ,1 mJy) which significantly simplifies subsequent processing, as strong sources will be subtracted from the visibility data before processing. This sky model also means that antenna complex gains can be self-calibrated in one minute without any need to switch to calibrator sources. It is expected that individual receiver gains will be sufficiently stable that the dominant causes of antenna complex gain variation (i.e. ionosphere and troposphere) will be common to all pixels, so that a gain solution in one beam of an antenna can be transferred to other beams of that antenna. In continuum mode, ASKAP will observe a 300 MHz band, split into 1 MHz channels, with full Stokes parameters measured in each channel. The data will be processed in a multi-frequency synthesis mode, in which data from each channel are correctly gridded in the uv plane. As well as producing images and source catalogues, the processing pipeline will also measure spectral index, spectral curvature, and all polarisation products across the band. Completion of the Boolardy Engineering Test Array (BETA), which is a 6-antenna subset of ASKAP, is expected in late 2011. BETA will be equipped with 6 PAFs, and all the necessary beamformers, correlators, and processing hardware to produce images over the full 30 deg2 field. The primary goal of BETA is to enable engineering tests and commissioning activities while the


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remaining ASKAP hardware is being constructed. If engineering commissioning proceeds as expected, science observations on BETA will commence in 2012, on a small number of test fields on which good radio-astronomical and ancillary data already exist. These test BETA observations will be used to debug and fine-tune not only ASKAP, but also the processes for handling the data. The full ASKAP array is expected to be commissioned in early 2013, and the science surveys are expected to start in late 2013. There is no proprietary period on ASKAP data, with all data being placed in the public domain after quality control, so data are expected to start flowing to the astronomical community by the end of 2013. Expressions of interest for ASKAP survey projects were sought in November 2008, and full proposals were solicited in mid-2009 (Ball et al. 2009). Of the 38 initial expressions of interest, ten proposals were eventually selected, with two, EMU (Evolutionary Map of the Universe) and WALLABY (Wide-field ASKAP L-band Legacy All-sky Blink SurveY: Koribalski et al. 2011), being selected as highest priority. EMU is an all-sky continuum survey, while WALLABY is an all-sky survey for neutral hydrogen. ASKAP design is now being driven by the requirement to maximise the science return from these ten projects, with a particular focus on maximising the science from EMU and WALLABY. It is planned that EMU, WALLABY, and some other projects will observe commensally, i.e., they will agree on an observing schedule, and will observe the sky in both continuum and HI modes at the same time, splitting the two data streams into two separate processing pipelines. More information on all the ASKAP projects, including links to their individual websites, can be found at http://askap.org. 1.3 EMU The primary goal of EMU is to make a deep (10 mJy/beam rms) radio continuum survey of the entire Southern sky, extending as far North as ×308. EMU will cover roughly the same fraction (75%) of the sky as the benchmark NVSS survey (Condon et al. 1998), but will be 45 times more sensitive, and will have an angular resolution (10 arcsec) 4.5 times better. Because of the excellent short-spacing uv coverage of ASKAP, EMU will also have higher sensitivity to extended structures. The sky coverage of EMU is shown in Fig.2, and the EMU specifications are summarised in Table 1. Like most radio surveys, EMU will adopt a 5-s cutoff, leading to a source detection threshold of 50 mJy/beam. EMU is expected to generate a catalogue of about 70 million galaxies, and all radio data from the EMU survey will be placed in the public domain as soon as the data quality has been assured. Currently, only a total of about 5 deg2 of the sky has been surveyed at 20 cm to the planned 10 mJy/beam rms of EMU, in fields such as the Hubble, Chandra, COSMOS and Phoenix deep fields (Huynh et al. 2005; Miller et al.

Figure 2 A representation of the EMU sky coverage in Galactic coordinates overlaid on 23 GHz WMAP data (Gold et al. 2011). The dark area in the top left is the part of the sky not covered by EMU.

Table 1. Instantaneous FOV Area of survey Synthesised beamwidth Frequency range RMS sensitivity Total integration time Number of sources
1

EMU Specifications 30 deg2 Entire sky south of ×308 dec. 10 arcsec FWHM 1130­1430 MHz 10 mJy/beam ,1.5 years1 ,70 million

The primary specification is the sensitivity, rather than the integration time. If for any reason ASKAP is less sensitive than expected, EMU will increase the integration time rather than lose sensitivity. Conversely, an increase in sensitivity of ASKAP may reduce the total integration time.

2008; Schinnerer et al. 2007; Hopkins et al. 2003; Biggs & Ivison 2006; Morrison et al. 2010), with a further 7 deg2 expected in the immediate future as part of the ATLAS survey (Norris et al. 2006; Middelberg et al. 2008a; Hales et al. 2011; Banfield et al. 2011). Surveys at this depth extend beyond the traditional domains of radio astronomy, where sources are predominantly radio-loud galaxies and quasars, into the regime of star-forming galaxies. At this depth, even the most common active galactic nuclei (AGN) are radio-quiet AGNs, which make up most of the X-ray extragalactic sources. As a result, the role of radio astronomy is changing. Whereas most traditional radio-astronomical surveys had most impact on the niche area of radio-loud AGNs, current radio-astronomical surveys are dominated by the same galaxies as are studied by optical and IR surveys, making radio-astronomical surveys such as EMU an increasingly important component of multi-wavelength studies of galactic evolution. Because only a small area of sky has been surveyed to the depth of EMU, it is difficult to estimate precisely how many galaxies it will detect. Most surveys to this sensitivity cover only a small area of sky, so that source counts at this level are significantly affected by sample variance, completeness, and bias issues. Our estimate for the number per deg2 above a flux density of 50 mJy/beam is based on an extrapolation from source counts at higher flux densities (2263 sources/deg2; Jackson 2005), the compilation shown in Fig. 3 (2278 sources/deg2), and


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Figure 3 Distribution of differential radio source counts at 1.4 GHz, based on and updated from the distribution shown in Hopkins et al. (2003). The solid curve is the polynomial fit from Hopkins et al. (2003), the dashed curve is an updated polynomial fit and is the one used to estimate the EMU source numbers. The horizontal dot-dashed line represents a non-evolving population in a Euclidean universe. The shaded region shows the prediction based on fluctuations due to weak confusing sources (a `P(D) analysis') from Condon (1974); Mitchell & Condon (1985).

Figure 4 Differential fraction of star-forming galaxies as a function of 1.4 GHz flux density, from a selection of recent deep surveys. Shaded boxes, and the two lines for Padovani et al., show the range of uncertainty in the survey results. Arrows indicate constraints from other surveys. These results show that the fraction of star-forming galaxies increases rapidly below 1 mJy and, at the 50 mJy survey limit of EMU, about 75% of sources will be star-forming galaxies.

the COSMOS survey (2261 sources/deg2; Scoville et al. 2007; Schinnerer et al. 2007). These three figures are in good agreement and predict a total of ,70 million sources in EMU, which is therefore the number adopted throughout this paper. Estimating the fraction of these radio sources which are AGN is difficult. Below 1 mJy, star-forming galaxies start to become a major component of the 1.4 GHz source counts, dominating below ,0.15 mJy (Seymour et al. 2008; Ibar et al. 2009), but, even at these levels, there is still a significant proportion of low-luminosity AGNs (Jarvis et al. 2004; Afonso et al. 2005, 2006; Norris et al. 2006; Simpson et al. 2006; Smolcic et al. 2008; Seymour et al. 2008; Mignano et al. 2008; Padovani et al. 2009). Seymour et al. (2008) have presented the most comprehensive attempt so far to divide radio sources into AGN and SF galaxies, and their result, together with other recent estimates, is shown in Fig. 4. From these we estimate that about 75% of EMU sources will be starforming galaxies. To estimate the redshift distribution of AGN and SF galaxies, we use the SKADS simulation (Wilman et al. 2008, 2010), shown in Fig. 5. About 50 million of the EMU sources are expected to be star-forming galaxies (see y2.1) at redshifts up to z , 3, with a mean redshift of z , 1.08. The remainder are AGNs with a mean z , 1.88, and extend up to z , 6. However, if any FRII (Fanaroff & Riley 1974) galaxies exist beyond that redshift (e.g. L , 3.3á1025 WHzþ1 at z ¼ 10), EMU will detect them. Confusion of radio sources, discussed more thoroughly in y3.6.1, is well-understood at this level, since previous surveys have already imaged small areas of sky to this depth and beyond.

Figure 5 Expected redshift distribution of EMU sources, based on the SKADS simulations (Wilman et al. 2008, 2010). The five lines show the distributions for star-forming galaxies (SFG), starburst galaxies(SB), radio-quiet quasars (RQQ), and radio-loud galaxies of Fanaroff­Riley types I and II (FRI & FR2; Fanaroff & Riley 1974). Vertical scale shows the total number of sources expected to be detected by EMU.

EMU differs from many previous surveys in that a goal of the project is to cross-identify the detected radio sources with major surveys at other wavelengths, and produce public-domain VO-accessible catalogues as `value-added' data products. This is facilitated by the growth in the number of large southern hemisphere telescopes and associated planned major surveys spanning all wavelengths, discussed below in y3.9. 1.4 Science Broadly, the key science goals for EMU are: To trace the evolution of star-forming galaxies from z ¼ 2 to the present day, using a wavelength unbiased by dust or molecular emission,


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To trace the evolution of massive black holes throughout the history of the Universe, and understand their relationship to star formation, To use the distribution of radio sources to explore the large-scale structure and cosmological parameters of the Universe, and to test fundamental physics, To determine how radio sources populate dark matter haloes, as a step towards understanding the underlying astrophysics of clusters and haloes, To create the most sensitive wide-field atlas of Galactic continuum emission yet made in the Southern Hemisphere, addressing areas such as star formation, supernovae, and Galactic structure, To explore an uncharted region of observational parameter space, with a high likelihood of finding new classes of object. Table 1 gives an overview of EMU specifications. In addition to the well-defined scientific goals outlined above, and the obvious legacy value, the large EMU dataset will include extremely rare objects, which is only made possible by covering large areas. A challenge for EMU will be the lack of spectroscopic redshifts, since no existing or planned redshift survey can cover more than a tiny fraction of EMU's 70 million sources. As discussed in y3.11, ,30% of EMU sources will have multi-wavelength optical/IR photometric data at the time of data release, increasing to ,70% in 2020. We expect these to provide accurate photometric redshifts for the majority of star-forming galaxies in EMU, and a minority of AGN (for which photometric redshifts tend to be unreliable). In addition, many of the EMU sources will have `statistical redshifts', which are valuable for some statistical tests. For example, most polarised sources are AGNs (mean z , 1.88), while most unpolarised sources are star-forming galaxies (mean z , 1.08). More precise statistical redshifts can be derived where optical/IR photometry is available, as discussed in y3.11. A further goal of EMU is to test and develop strategies for the SKA. Many aspects of ASKAP, such as the automated observing, calibration, and data reduction processes, and the phased-array feeds, are potential technologies for the SKA, and it will be important to test whether these approaches deliver the planned results. 1.5 Relationship to Other Surveys The following radio surveys are particularly complementary to the scientific goals of EMU. The WODAN survey (Rottgering et al. 2010b) has been proposed for the Westerbork telescope which is currently being upgraded with a phased array feed (Oosterloo et al. 2009). WODAN will cover the northern 25% of the sky (i.e. North of declination ×308) that is inaccessible to ASKAP, to an rms sensitivity of 10 mJy/beam and a spatial resolution of 15 arcsec. Together, EMU and WODAN will provide full-sky 1.3 GHz imaging at ,10­15 arcsec resolution to an rms noise level of











10 mJy/beam, providing an unprecedented sensitive all-sky radio survey as a legacy for astronomers at all wavelengths. The WODAN survey will overlap with EMU by a few degrees of declination to provide a comparison and cross-validation, to ensure consistent calibration, and to check on completeness and potential sources of bias between the surveys. The LOFAR continuum survey (Rottgering et al. 2010a) will cover the northern half of the sky (i.e. North of declination 08) with the new LOFAR telescope operating at low frequencies (15­200 MHz). LOFAR will be especially complementary to WODAN and EMU in surveying the sky at high sensitivity and resolution but at a much lower frequency. The MIGHTEE survey (van der Heyden & Jarvis 2010) on the Meerkat telescope (Jonas 2009) will probe to much fainter flux densities (0.1­1 mJy rms) over smaller areas (,35 deg2) at higher angular resolution, providing the completeness as a function of flux density for the EMU and WODAN Surveys. The higher sensitivity and resolution will enable exploration of the AGN and star-forming galaxy populations to higher redshifts and lower luminosities. POSSUM (Gaensler et al. 2010) is an all-sky ASKAP survey of linear polarisation. It is expected that POSSUM will be commensal with EMU, and that the two surveys will overlap considerably in their analysis pipelines and source catalogues. POSSUM will provide a catalogue of polarised fluxes and Faraday rotation measures for approximately 3 million compact extragalactic sources. These data will be used to determine the large-scale magnetic field geometry of the Milky Way, to study the turbulent properties of the interstellar medium, and to constrain the evolution of intergalactic magnetic fields as a function of cosmic time. POSSUM will also be a valuable counterpart to EMU, in that it will provide polarisation properties or upper limits to polarisation for all sources detected by EMU. FLASH (Ball et al. 2009) is an ASKAP survey whose goal is to detect extragalactic neutral hydrogen absorption. To do so it will observe at frequencies outside the 1130­1430 MHz band of EMU, thus yielding valuable spectral index information for those sources common to both surveys. DINGO (Ball et al. 2009)is an ASKAP survey whose goal is to detect faint extragalactic neutral hydrogen emission, and to do so it will spend many days on one ASKAP pointing. As a byproduct, it will thus provide sensitive continuum images over smaller areas (several tens of deg2), allowing EMU to explore fainter flux densities in an optimal tiered survey structure, and also to quantify the effects of confusion at this level. However, the continuum images from DINGO will be severely confusion-limited at flux densities below a few mJy/beam. It may be possible to transcend this limit by subtracting known sources from the image, such as those star-forming galaxies which are seen in infrared images and whose radio flux can be predicted


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using the IR­radio correlation. However, this challenge is currently external to the core EMU project. VAST (Chatterjee et al. 2010) is an ASKAP survey that will observe partly commensally with EMU, with the goal of detecting transients and variable sources. EMU has no planned transient capability, since all information on variability of EMU sources will be available from VAST. This separation enables each of EMU and VAST to focus on its specific science goals, although significant coordination between the projects will clearly be essential. WALLABY (Koribalski et al. 2011) is an HI survey which will deliver high-sensitivity spectral line data over the same area of sky as EMU, and will observe commensally with EMU. Observations will give a velocity coverage of þ2,000 to ×77,000 km sþ1 (z ¼ 0 ­ 0.26) and velocity resolution of 4 km sþ1. The angular resolution for WALLABY will be 30 arcsec, a factor of three lower than EMU, as computing resources to make the large spectral line data cubes are restricted to baselines shorter than ,2 km. Nearly all the ,5 á 105 sources detected by WALLABY will also be detected by EMU, and WALLABY will provide an HI redshift for each of these, adding significantly to the redshift information for low-redshift EMU sources. This paper defines the EMU survey, setting out its science goals in y 2, and identifying the challenges to achieve these goals. y 3 describes how these challenges are being addressed in the EMU Design Study, and y 4 describes the survey operational plan, primary data products, and the data r