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

24 Citations (Scopus)

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

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confusion
sky
radio
flux density
sidelobes
flattening
deconvolution
field of view
imaging techniques
ionospherics
probe
low frequencies
calibration
bandwidth
probes

Keywords

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

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Franzen, T. M. O., Jackson, C. A., Offringa, A. R., Ekers, R. D., Wayth, R. B., Bernardi, G., ... 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

The 154 MHz radio sky observed by the Murchison Widefield Array : Noise, confusion, and first source count analyses. / Franzen, T. M O; Jackson, C. A.; Offringa, A. R.; Ekers, R. D.; Wayth, R. B.; Bernardi, G.; Bowman, Judd; 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.; Morgan, E.; Morgan, J.; Oberoi, D.; Ord, S. M.; Prabu, T.; Seymour, N.; Shankar, N. Udaya; Srivani, K. S.; Subrahmanyan, R.; Tingay, S. J.; Trott, C. M.; Webster, R. L.; Williams, A.; Williams, C. L.

In: Monthly Notices of the Royal Astronomical Society, Vol. 459, No. 3, 01.07.2016, p. 3314-3325.

Research output: Contribution to journalArticle

Franzen, TMO, Jackson, CA, Offringa, AR, Ekers, RD, Wayth, RB, Bernardi, G, Bowman, J, Briggs, F, Cappallo, RJ, Deshpande, AA, Gaensler, BM, Greenhill, LJ, Hazelton, BJ, Johnston-Hollitt, M, Kaplan, DL, Lonsdale, CJ, McWhirter, SR, Mitchell, DA, Morales, MF, Morgan, E, Morgan, J, Oberoi, D, Ord, SM, Prabu, T, Seymour, N, Shankar, NU, Srivani, KS, Subrahmanyan, R, Tingay, SJ, Trott, CM, Webster, RL, Williams, A & Williams, CL 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, vol. 459, no. 3, pp. 3314-3325. https://doi.org/10.1093/mnras/stw823
Franzen, T. M O ; Jackson, C. A. ; Offringa, A. R. ; Ekers, R. D. ; Wayth, R. B. ; Bernardi, G. ; Bowman, Judd ; 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. ; Morgan, E. ; Morgan, J. ; Oberoi, D. ; Ord, S. M. ; Prabu, T. ; Seymour, N. ; Shankar, N. Udaya ; Srivani, K. S. ; Subrahmanyan, R. ; Tingay, S. J. ; Trott, C. M. ; Webster, R. L. ; Williams, A. ; Williams, C. L. / The 154 MHz radio sky observed by the Murchison Widefield Array : Noise, confusion, and first source count analyses. In: Monthly Notices of the Royal Astronomical Society. 2016 ; Vol. 459, No. 3. pp. 3314-3325.
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T1 - The 154 MHz radio sky observed by the Murchison Widefield Array

T2 - Noise, confusion, and first source count analyses

AU - Franzen, T. M O

AU - Jackson, C. A.

AU - Offringa, A. R.

AU - Ekers, R. D.

AU - Wayth, R. B.

AU - Bernardi, G.

AU - Bowman, Judd

AU - Briggs, F.

AU - Cappallo, R. J.

AU - Deshpande, A. A.

AU - Gaensler, B. M.

AU - Greenhill, L. J.

AU - Hazelton, B. J.

AU - Johnston-Hollitt, M.

AU - Kaplan, D. L.

AU - Lonsdale, C. J.

AU - McWhirter, S. R.

AU - Mitchell, D. A.

AU - Morales, M. F.

AU - Morgan, E.

AU - Morgan, J.

AU - Oberoi, D.

AU - Ord, S. M.

AU - Prabu, T.

AU - Seymour, N.

AU - Shankar, N. Udaya

AU - Srivani, K. S.

AU - Subrahmanyan, R.

AU - Tingay, S. J.

AU - Trott, C. M.

AU - Webster, R. L.

AU - Williams, A.

AU - Williams, C. L.

PY - 2016/7/1

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N2 - 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.

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KW - Methods: data analysis

KW - Radio continuum: galaxies

KW - Surveys

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