The 154 MHz radio sky observed by the Murchison Widefield Array: noise, confusion, and first source count analyses

Journal
MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY
Date Issued
2016
Author(s)
Franzen, T. M. O.
•
Jackson, C. A.
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Offringa, A. R.
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Ekers, R. D.
•
Wayth, R. B.
•
•
Bowman, J. D.
•
Briggs, F.
•
Cappallo, R. J.
•
Deshpande, A. A.
•
Gaensler, B. M.
•
Greenhill, L. J.
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Hazelton, B. J.
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Johnston-Hollitt, M.
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Kaplan, D. L.
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Lonsdale, C. J.
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McWhirter, S. R.
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Mitchell, D. A.
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Morales, M. F.
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Morgan, E.
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Morgan, J.
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Oberoi, D.
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Ord, S. M.
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Prabu, T.
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Seymour, N.
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Shankar, N. Udaya
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Srivani, K. S.
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Subrahmanyan, R.
•
Tingay, S. J.
•
Trott, C. M.
•
Webster, R. L.
•
Williams, A.
•
Williams, C. L.
DOI
10.1093/mnras/stw823
Abstract
We analyse a 154 MHz image made from a 12 h observation with the Murchison Widefield Array (MWA) to determine the noise contribution and behaviour of the source counts down to 30 mJy. The MWA image has a bandwidth of 30.72 MHz, a field-of-view within the half-power contour of the primary beam of 570 deg2, a resolution of 2.3 arcmin and contains 13 458 sources above 5σ. The rms noise in the centre of the image is 4-5 mJy beam-1. The MWA counts are in excellent agreement with counts from other instruments and are the most precise ever derived in the flux density range 30-200 mJy due to the sky area covered. Using the deepest available source count data, we find that the MWA image is affected by sidelobe confusion noise at the ≈3.5 mJy beam-1 level, due to incompletely peeled and out-of-image sources, and classical confusion becomes apparent at ≈1.7 mJy beam-1. This work highlights that (I) further improvements in ionospheric calibration and deconvolution imaging techniques would be required to probe to the classical confusion limit and (II) the shape of low-frequency source counts, including any flattening towards lower flux densities, must be determined from deeper ≈150 MHz surveys as it cannot be directly inferred from higher frequency data.
Volume
459
Issue
3
Start page
3314
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