The radio spectra of star-forming galaxies

Josh Marvil (NMT), Jean Eilek (NMT), Frazer Owen (NRAO)

Simple models of radio spectra for star forming galaxies are unsatisfactory. Calorimeter models of the radio-infrared correlation predict a spectral aging break below 1 GHz. Simple free-free absorption models predict steep low-frequency turndowns in the spectra of strongly star-forming galaxies (such as ULIRGs). To investigate this, we have combined previous data with 74 MHz data from the VLSS to derive broad-band spectra (30 MHz to 10 GHz) of a large set of both normal spirals and strongly starbursting galaxies. We find significant disagreement between these simple models and the observed spectra. The spectra of many normal spirals are consistent with a single power-law spectrum over the entire frequency range; the spectra if some strong starbursting galaxies do show low-frequency flattening, but not nearly enough to agree with the simple models. We will discuss how these spectra correlate with thesize and power of the galaxy. We will discuss possible new models of the observed spectra, which incorporate inhomogeneities in the galaxy's magnetic field and absorbing column.