Integrated Radio Continuum Spectra of Galaxies
  We investigate the spectral shape of the total continuum radiation, between
74 MHz and 5 GHz (400 to 6 cm in wavelength), for a large sample of bright
galaxies. We take advantage of the overlapping survey coverage of the VLA
Low-Frequency Sky Survey, the Westerbork Northern Sky Survey, the NRAO VLA Sky
Survey, and the Green Bank 6 cm survey to achieve significantly better
resolution, sensitivity, and sample size compared to prior efforts of this
nature. For our sample of 250 bright galaxies we measure a mean spectral index,
${\alpha}$, of -0.69 between 1.4 and 4.85 GHz, -0.55 between 325 MHz and 1.4
GHz, and -0.45 between 74 and 325 MHz, which amounts to a detection of
curvature in the mean spectrum. The magnitude of this curvature is
approximately ${\Delta \alpha}$ = -0.2 per logarithmic frequency decade when
fit with a generalized function having constant curvature. No trend in low
frequency spectral flattening versus galaxy inclination is evident in our data,
suggesting that free-free absorption is not a satisfying explanation for the
observed curvature. The ratio of thermal to non-thermal emission is estimated
by two independent methods, (1) using the IRAS far-IR fluxes, and (2) with the
value of the total spectral index. Method (1) results in a distribution of 1.4
GHz thermal fractions of 9% ${\pm}$ 3%, which is consistent with previous
studies, while method (2) produces a mean 1.4 GHz thermal fraction of 51% with
dispersion 26%. The highly implausible values produced by method (2) indicate
that the sum of typical power-law thermal and non-thermal components is not a
viable model for the total spectral index between 325 and 1.4 GHz.
