The B3VLA Sample at Low Frequencies: Results from a Survey at 74 MHz

The low-frequency (< 150 MHz) region is among the most poorly explored of the entire radio spectrum despite the many unique astrophysical questions that can be addressed with observations in these bands. In the framework of our on-going study of the radio continuum spectra of the B3VLA survey we have used observations of Tschager et al. [1] obtained with the VLA in A-array at 74MHz to extend our database towards lower frequencies – with a resolution of about a factor of 3 higher than the upcoming VLA Low-frequency Sky Survey (VLSS, [2]). For about a third of the sample (~ 360 radio sources) we have now 6 or more measurements in the range between 74MHz and 10.5 GHz. This unique frequency coverage allows consistency checks of the new 74MHz flux densities and provides the comparison data to test the influence of various observational effects at such low frequencies. We have performed a spectral analysis to determine particular features like low-frequency turn-overs caused by synchrotron self-absorption or free-free absorption. Some radio sources show extended and complex morphologies not seen at higher frequencies, indicating the presence of diffuse structures with very steep spectra. Our project is an example of a typical application of the future LOFAR telescope in the field of source evolution.