Extragalactic Peaked-Spectrum Radio Sources at Low Frequencies

J. R. Callingham, R. D. Ekers, B. M. Gaensler, J. L. B. Line, N. Hurley-Walker, E. M. Sadler, S. J. Tingay, P. J. Hancock, M. E. Bell, K. S. Dwarakanath, B. -Q. For, T. M. O. Franzen, Luke Hindson, M. Johnston-Hollitt, A. D. Kapinska, E. Lenc, B. McKinley, J. Morgan, A. R. Offringa, P. ProcopioL. Staveley-Smith, R. B. Wayth, C. Wu, Q. Zheng

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    Abstract

    We present a sample of 1,483 sources that display spectral peaks between 72 MHz and 1.4 GHz, selected from the GaLactic and Extragalactic All-sky Murchison Widefield Array (GLEAM) survey. The GLEAM survey is the widest fractional bandwidth all-sky survey to date, ideal for identifying peaked-spectrum sources at low radio frequencies. Our peaked-spectrum sources are the low frequency analogues of gigahertz-peaked spectrum (GPS) and compact-steep spectrum (CSS) sources, which have been hypothesized to be the precursors to massive radio galaxies. Our sample more than doubles the number of known peaked-spectrum candidates, and 95% of our sample have a newly characterized spectral peak. We highlight that some GPS sources peaking above 5 GHz have had multiple epochs of nuclear activity, and demonstrate the possibility of identifying high redshift ($z > 2$) galaxies via steep optically thin spectral indices and low observed peak frequencies. The distribution of the optically thick spectral indices of our sample is consistent with past GPS/CSS samples but with a large dispersion, suggesting that the spectral peak is a product of an inhomogeneous environment that is individualistic. We find no dependence of observed peak frequency with redshift, consistent with the peaked-spectrum sample comprising both local CSS sources and high-redshift GPS sources. The 5 GHz luminosity distribution lacks the brightest GPS and CSS sources of previous samples, implying that a convolution of source evolution and redshift influences the type of peaked-spectrum sources identified below 1 GHz. Finally, we discuss sources with optically thick spectral indices that exceed the synchrotron self-absorption limit.
    Original languageEnglish
    Number of pages28
    JournalThe Astrophysical Journal
    Volume836
    Issue number2
    Early online date17 Feb 2017
    DOIs
    Publication statusPublished - 20 Feb 2017

    Keywords

    • galaxies: active
    • radiation mechanisms: general
    • radio continuum: general

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