The Spectral Energy Distribution of Powerful Starburst Galaxies I: Modelling the Radio Continuum

T J Galvin, N. Seymour, J. Marvil, M D Filipovic, N.F.H. Tothill, Richard M. McDermid, N. Hurley-Walker, P. J. Hancock, J. R. Callingham, R H Cook, Ray P. Norris, M E Bell, K. S. Dwarakanath, B. -Q. For, B. M. Gaensler, L Hindson, M. Johnston-Hollitt, A. D. Kapińska, E. Lenc, B. McKinleyJ Morgan, A. R. Offringa, P. Procopio, L. Staveley-Smith, R. B. Wayth, C. Wu, Q. Zheng

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    Abstract

    We have acquired radio-continuum data between 70MHz and 48 GHz for a sample of 19 southern starburst galaxies at moderate redshifts (0.067 < z < 0.227) with the aim of separating synchrotron and free-free emission components. Using a Bayesian framework, we find the radio continuum is rarely characterized well by a single power law, instead often exhibiting lowfrequency turnovers below 500 MHz, steepening at mid to high frequencies, and a flattening at high frequencies where free-free emission begins to dominate over the synchrotron emission. These higher order curvature components may be attributed to free-free absorption across multiple regions of star formation with varying optical depths. The decomposed synchrotron and free-free emission components in our sample of galaxies form strong correlations with the total-infrared bolometric luminosities. Finally, we find that without accounting for free-free absorption with turnovers between 90 and 500MHz the radio continuum at low frequency (v < 200 MHz) could be overestimated by upwards of a factor of 12 if a simple power-law extrapolation is used from higher frequencies. The mean synchrotron spectral index of our sample is constrained to be α = -1.06, which is steeper than the canonical value of -0.8 for normal galaxies. We suggest this may be caused by an intrinsically steeper cosmic ray distribution.

    Original languageEnglish
    Pages (from-to)779–799
    Number of pages21
    JournalMonthly Notices of the Royal Astronomical Society
    Volume474
    Issue number1
    Early online date9 Nov 2017
    DOIs
    Publication statusPublished - 1 Feb 2018

    Keywords

    • Galaxies: starburst
    • Radio continuum: galaxies

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