The Spectral Energy Distribution of Powerful Starburst Galaxies I : Modelling the Radio Continuum
Author
Galvin, T J
Seymour, N.
Marvil, J.
Filipovic, M D
Tothill, N.F.H.
McDermid, Richard M.
Hurley-Walker, N.
Hancock, P. J.
Callingham, J. R.
Cook, R H
Norris, Ray P.
Bell, M E
Dwarakanath, K. S.
For, B. -Q.
Gaensler, B. M.
Hindson, L
Johnston-Hollitt, M.
Kapińska, A. D.
Lenc, E.
McKinley, B.
Morgan, J
Offringa, A. R.
Procopio, P.
Staveley-Smith, L.
Wayth, R. B.
Wu, C.
Zheng, Q.
Attention
2299/20580
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.