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The Murchison Widefield Array (MWA) is one of three Square Kilometre Array Precursor telescopes and is located at the Murchison Radio-astronomy Observatory in the Murchison Shire of the mid-west of Western Australia, a location chosen for its extremely low levels of radio frequency interference. The MWA operates at low radio frequencies, 80-300 MHz, with a processed bandwidth of 30.72 MHz for both linear polarisations, and consists of 128 aperture arrays (known as tiles) distributed over a ß3-km diameter area. Novel hybrid hardware/software correlation and a real-time imaging and calibration systems comprise the MWA signal processing backend. In this paper, the as-built MWA is described both at a system and sub-system level, the expected performance of the array is presented, and the science goals of the instrument are summarised.

We present initial results from a time-series BVI survey of two fields in NGC 4258 using the Advanced Camera for Surveys onboard the Hubble Space Telescope. This galaxy was selected because of its accurate maser-based distance, which is anticipated to have a total uncertainty of ∼ 3%. The goal of the HST observations is to provide an absolute calibration of the Cepheid Distance Scale and to measure its dependence on chemical abundance (the so-called "metallicity effect").We carried out observations of two fields at different galactocentric distances with a mean abundance difference of 0.5 dex. We discovered a total of 281 Cepheids with periods ranging from 4 to 45 days (the duration of our observing window). We determine a Cepheid distance modulus for NGC 4258 (relative to the LMC) of ∆µ 0 =10.88 ± 0.04 (random) ±0.05 (systematic) mag. Given the published maser distance to the galaxy, this implies µ 0 (LM C) = 18.41 ± 0.10 r ± 0.13 s mag or D(LM C) = 48.1 ± 2.3 r ± 2.9 s kpc. We measure a metallicity effect of γ = −0.29 ± 0.09 r ± 0.05 s mag dex −1 . We see no evidence for a variation in the slope of the Period-Luminosity relation as a function of abundance.We estimate a Hubble Constant of H 0 = 74 ± 3 r ± 6 s km s −1 Mpc −1 using a recent sample of 4 well-observed type Ia SNe and our new calibration of the Cepheid Distance Scale. It may soon be possible to measure the value of H 0 with a total uncertainty of 5%, with consequent improvement in the determination of the equation of state of dark energy.

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The accurate measurement of extragalactic distances is a central challenge of modern astronomy, being required for any realistic description of the age, geometry and fate of the Universe. The measurement of relative extragalactic distances has become fairly routine, but estimates of absolute distances are rare 1 . In the vicinity of the Sun, direct geometric techniques for obtaining absolute distances, such as orbital parallax, are feasible, but such techniques have hitherto been dif®cult to apply to other galaxies. As a result, uncertainties in the expansion rate and age of the Universe are dominated by uncertainties in the absolute calibration of the extragalactic distance ladder 2 . Here we report a geometric distance to the galaxy NGC4258, which we infer from the direct measurement of orbital motions in a disk of gas surrounding the nucleus of this galaxy. The distance so deter-minedÐ7:2 6 0:3 MpcÐis the most precise absolute extragalactic distance yet measured, and is likely to play an important role in future distance-scale calibrations.NGC4258 is one of 22 nearby active galactic nuclei (AGN) known to possess nuclear water masers (the microwave equivalent of lasers). The enormous surface brightnesses ( ) 10 12 K), relatively small sizes ( ( 10 14 cm) and narrow linewidths (a few km s -1 ) of these masers make them ideal probes of the structure and dynamics of the molecular gas in which they residue. Very-long-baseline interferometry (VLBI) observations of the NGC4258 maser have provided the ®rst direct images of an AGN accretion disk, revealing a thin, subparsec-scale, differentially rotating warped disk in the nucleus of this relatively weak Seyfert 2 AGN 3±6 . Two distinct populations of masers exist in NGC4258. The ®rst are the highvelocity masers. These masers amplify their own spontaneous emission and are offset 61,000 km s -1 and 4.7±8.0 mas (0.16± 0.28 pc for a distance of 7.2 Mpc) on either side of the disk centre. The keplerian rotation curve traced by these masers requires a central binding mass (M), presumably in the form of a supermassive black hole, of 3:9 6 0:1 3 10 7 D=7:2 Mpcsin i s =sin 82 2 2 Figure 1 The NGC4258 water maser. The upper panel shows the best-®tting warped-disk model superposed on actual maser positions as measured by the VLBA of the NRAO, with top as North. The ®lled square marks the centre of the disk, as determined from a global disk-®tting analysis 8 . The ®lled triangles show the positions of the high-velocity masers, so called because they occur at frequencies corresponding to Doppler shifts of ,61,000 km s -1 with respect to the galaxy systemic velocity of ,470 km s -1 . This is apparent in the VLBA total power spectrum (lower panel). The inset shows line-of-sight (LOS) velocity versus impact parameter for the best-®tting keplerian disk, with the maser data superposed. The high-velocity masers trace a keplerian curve to better than 1%. Monitoring of these features indicates that they drift by less than ,1 km s -1 yr -1 (refs 14±16) and requires that they lie within 5±1...

We present the first VLBI maps of H 2 O maser emission (1.3 cm) in the nucleus of the Circinus galaxy, constructed from data obtained with the Australia Telescope Long Baseline Array. The maser emission traces a warped, edge-on accretion disk between radii of 0:11 AE 0:02 and $0.40 pc, as well as a wide-angle outflow that extends up to $1 pc from the estimated disk center. The disk rotation is close to Keplerian (v / r À0:5 ), the maximum detected rotation speed is 260 km s À1 , and the inferred central mass is ð1:7 AE 0:3Þ Â 10 6 M . The outflowing masers are irregularly distributed above and below the disk, with relative outflow velocities up to $AE160 km s À1 , projected along the line of sight. The flow probably originates closer than 0.1 pc to the central engine, possibly in an inward extension of the accretion disk, although there is only weak evidence of rotation in the outward-moving material. We observe that the warp of the disk appears to collimate the outflow and to fix the extent of the ionization cone observed on larger angular scales. This study provides the first direct evidence (i.e., through imaging) of dusty, high-density, molecular material in a nuclear outflow less than 1 pc from the central engine of a Seyfert galaxy, as well as the first graphic evidence that warped accretion disks can channel outflows and illumination patterns in active galactic nuclei. We speculate that the same arrangement, which in some ways obviates the need for a geometrically thick, dusty torus, may apply to other type 2 active galactic nuclei.

Astronomical widefield imaging of interferometric radio data is computationally expensive, especially for the large data volumes created by modern non-coplanar many-element arrays. We present a new widefield interferometric imager that uses the w-stacking algorithm and can make use of the w-snapshot algorithm. The performance dependencies of CASA's wprojection and our new imager are analysed and analytical functions are derived that describe the required computing cost for both imagers. On data from the Murchison Widefield Array, we find our new method to be an order of magnitude faster than w-projection, as well as being capable of full-sky imaging at full resolution and with correct polarisation correction. We predict the computing costs for several other arrays and estimate that our imager is a factor of 2-12 faster, depending on the array configuration. We estimate the computing cost for imaging the low-frequency Square-Kilometre Array observations to be 60 PetaFLOPS with current techniques. We find that combining w-stacking with the w-snapshot algorithm does not significantly improve computing requirements over pure w-stacking. The source code of our new imager is publicly released.

Using the Murchison Widefield Array (MWA), the low-frequency Square Kilometre Array (SKA1 LOW) precursor located in Western Australia, we have completed the GaLactic and Extragalactic All-sky MWA (GLEAM) survey, and present the resulting extragalactic catalogue, utilising the first year of observations. The catalogue covers 24, 831 square degrees, over declinations south of +30 • and Galactic latitudes outside 10 • of the Galactic plane, excluding some areas such as the Magellanic Clouds. It contains 307,455 radio sources with 20 separate flux density measurements across 72-231 MHz, selected from a time-and frequency-integrated image centred at 200 MHz, with a resolution of ≈ 2 . Over the catalogued region, we estimate that the catalogue is 90 % complete at 170 mJy, and 50 % complete at 55 mJy, and large areas are complete at even lower flux density levels. Its reliability is 99.97 % above the detection threshold of 5σ, which itself is typically 50 mJy. These observations constitute the widest fractional bandwidth and largest sky area survey at radio frequencies to date, and calibrate the low frequency flux density scale of the southern sky to better than 10 %. This paper presents details of the flagging, imaging, mosaicking, and source extraction/characterisation, as well as estimates of the completeness and reliability. All source measurements and images are available online . This is the first in a series of publications describing the GLEAM survey results.