The spectral energy distribution of powerful starburst galaxies - I. Modelling the radio continuum

T. J. Galvin, N. Seymour, J. Marvil, M. D. Filipović, N. F.H. Tothill, R. M. McDermid, N. Hurley-Walker, P. J. Hancock, J. R. Callingham, R. H. Cook, R. P. Norris, M. E. Bell, K. S. Dwarakanath, B. 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
DOIs
Publication statusPublished - 1 Feb 2018

Bibliographical note

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 474, Issue 1, 11 February 2018, Pages 779–799, https://doi.org/10.1093/mnras/stx2613. Copyright 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

Keywords

  • Galaxies: starburst
  • Radio continuum: galaxies

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