The Deep SWIRE Field. II. 90 cm Continuum Observations and 20 cm-90 cm Spectra

We present one of the deepest radio continuum surveys to date at a wavelength gsim1 m. The observations were taken with the VLA at 324.5 MHz covering a region of the SWIRE Spitzer Legacy survey, centered at 10^h46^m00^s, 59°01'00'' (J2000). The data reduction and analysis are described and an electronic catalog of the sources detected above 5σ is presented. We also discuss the observed angular size distribution for the sample. Using our deeper 20 cm survey of the same field, we calculate spectral indices for sources detected in both surveys. The spectral indices for 90 cm selected sources, defined as S vprop ν^-α, shows a peak near 0.7 and only a few sources with very steep spectra, i.e., α⁹⁰ ₂₀ > >1. Thus no large population of very steep spectrum μJy sources seems to exist down to the limit of our survey. For 20 cm selected sources, we find similar mean spectral indices for sources with S ₂₀ > 1 mJy. For weaker sources, below the detection limit for individual sources at 90 cm, we use stacking to study the radio spectra. We find that the spectral indices of small (<3'') 20 cm selected sources with S ₂₀ < 10 mJy have mean and median α⁹⁰ ₂₀ ~ 0.3-0.5. This is flatter than the spectral indices of the stronger source population. At the low end of the 20 cm survey, the spectral indices appear to be steepening again. We report log N-log S counts at 90 cm which show a flattening below 5 mJy. Given the median redshift of the population, z ~ 1, the spectral flattening and the flattening of the log N-log S counts occur at radio luminosities normally associated with active galactic nuclei rather than with galaxies dominated by star formation.