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 .
An investigation into relationships between spectral index , infrared-derived quantities and additional source properties reveals that galaxies with high radio luminosity in our sample are found to have , on average , a flatter radio spectral index , and early types tend to have excess radio emission when compared to the radio-infrared ratio of later types .
Early types also have radio emission which is more compact than later type galaxies , as compared to the optical size of the galaxy .
Despite these differences , no relation between spectral index and galaxy type is detected .