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.

This paper describes the on-telescope performance of the Wide Field Spectrograph (WiFeS). The design characteristics of this instrument, at the Research School of Astronomy and Astrophysics (RSAA) of the Australian National University (ANU) and mounted on the ANU 2.3 m telescope at the Siding Spring Observatory has been already described in an earlier paper (Dopita et al. in Astrophys. Space Sci. 310:255, 2007). Here we describe the throughput, resolution and stability of the instrument, and describe some minor issues which have been encountered. We also give a description of the data reduction pipeline, and show some preliminary results.

We measure the neutral atomic hydrogen (H I) gas content of field galaxies at intermediate redshifts of z ∼ 0.1 and z ∼ 0.2 using hydrogen 21-cm emission lines observed with the Westerbork Synthesis Radio Telescope (WSRT). In order to make high signal-to-noise ratio detections, an H I signal stacking technique is applied: H I emission spectra from multiple galaxies, optically selected by the CNOC2 redshift survey project, are co-added to measure the average H I mass of galaxies in the two redshift bins. We calculate the cosmic H I gas densities (Ω H I ) at the two redshift regimes and compare those with measurements at other redshifts to investigate the global evolution of the H I gas density over cosmic time. From a total of 59 galaxies at z ∼ 0.1 we find Ω H I = (0.33 ± 0.05)×10 −3 , and at z ∼ 0.2 we find Ω H I = (0.34 ± 0.09) × 10 −3 , based on 96 galaxies. These measurements help bridge the gap between high-z damped Lyman-α observations and blind 21-cm surveys at z = 0. We find that our measurements of Ω H I at z ∼ 0.1 and 0.2 are consistent with the H I gas density at z ∼ 0 and that all measurements of Ω H I from 21-cm emission observations at z 0.2 are in agreement with no evolution of the H I gas content in galaxies during the last 2.4 Gyr.

The Widefield ASKAP L-band Legacy All-sky Blind surveY (WALLABY) is a next-generation survey of neutral hydrogen (H I) in the Local Universe. It uses the widefield, high-resolution capability of the Australian Square Kilometer Array Pathfinder (ASKAP), a radio interferometer consisting of 36 × 12-m dishes equipped with Phased-Array Feeds (PAFs), located in an extremely radioquiet zone in Western Australia. WALLABY aims to survey three-quarters of the sky (−90 • < δ < +30 • ) to a redshift of z 0.26, and generate spectral line image cubes at ∼30 arcsec resolution and ∼1.6 mJy beam −1 per 4 km s −1 channel sensitivity. ASKAP's instantaneous field of view at 1.4 GHz, delivered by the PAF's 36 beams, is about 30 sq deg. At an integrated signal-to-noise ratio of five, WALLABY is expected to detect around half a million galaxies with a mean redshift of z ∼ 0.05 (∼200 Mpc). The scientific goals of WALLABY include: (a) a census of gas-rich galaxies in the vicinity of the Local Group; (b) a study of the H I properties of galaxies, groups and clusters, in particular the influence of the environment on galaxy evolution; and (c) the refinement of cosmological parameters using the spatial and redshift distribution of low-bias gas-rich galaxies. For context we provide an overview of recent and planned large-scale H I surveys. Combined with existing and new multi-wavelength sky surveys, WALLABY will enable an exciting new generation of panchromatic studies of the Local Universe. -First results from the WALLABY pilot survey are revealed, with initial data products publicly available in the CSIRO ASKAP Science Data Archive (CASDA).