Radio spectra of several ultraluminous IRAS galaxies

A sample of IRAS galaxies with infrared luminosities as high as 1011.8 to 1012.8 L⊙ were observed using the Australia Telescope Compact Array (ATCA) at 8.6, 4.8, 2.4 and 1.4 GHz. 4 galaxies in the sample are found to have steep spectra at high frequency and their flux densities decrease at low frequency, showing a peak-like spectral pattern with the peak flux at several GHz. Two possible mechanisms, synchrotron self-absorption and free-free absorption, for the turning-over in the low-frequency side are discussed. From the analysis of the spectra of the IRAS sources, 00199-7426, 02587-6336, 07020-5423 and 21547-5823, we conclude that free-free absorption may be the cause for the low-frequency turnover. By model fitting to the observations several physical parameters of the sources are obtained.