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

T. M O Franzen, C. A. Jackson, A. R. Offringa, R. D. Ekers, R. B. Wayth, G. Bernardi, Judd Bowman, F. Briggs, R. J. Cappallo, A. A. Deshpande, B. M. Gaensler, L. J. Greenhill, B. J. Hazelton, M. Johnston-Hollitt, D. L. Kaplan, C. J. Lonsdale, S. R. McWhirter, D. A. Mitchell, M. F. Morales, E. MorganJ. Morgan, D. Oberoi, S. M. Ord, T. Prabu, N. Seymour, N. Udaya Shankar, K. S. Srivani, R. Subrahmanyan, S. J. Tingay, C. M. Trott, R. L. Webster, A. Williams, C. L. Williams

Research output: Contribution to journalArticle

26 Scopus citations

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.

Original languageEnglish (US)
Pages (from-to)3314-3325
Number of pages12
JournalMonthly Notices of the Royal Astronomical Society
Volume459
Issue number3
DOIs
StatePublished - Jul 1 2016

Keywords

  • Catalogues
  • Galaxies: active
  • Methods: data analysis
  • Radio continuum: galaxies
  • Surveys

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Franzen, T. M. O., Jackson, C. A., Offringa, A. R., Ekers, R. D., Wayth, R. B., Bernardi, G., Bowman, J., Briggs, F., Cappallo, R. J., Deshpande, A. A., Gaensler, B. M., Greenhill, L. J., Hazelton, B. J., Johnston-Hollitt, M., Kaplan, D. L., Lonsdale, C. J., McWhirter, S. R., Mitchell, D. A., Morales, M. F., ... Williams, C. L. (2016). The 154 MHz radio sky observed by the Murchison Widefield Array: Noise, confusion, and first source count analyses. Monthly Notices of the Royal Astronomical Society, 459(3), 3314-3325. https://doi.org/10.1093/mnras/stw823