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

TL;DR

This paper presents deep 90cm radio continuum observations, analyzes spectral indices across 20cm and 90cm wavelengths, and discusses source populations, spectral properties, and source counts, revealing insights into AGN and galaxy contributions at faint flux levels.

Contribution

The study provides the deepest 90cm radio survey to date, combined with 20cm data, and analyzes spectral indices and source counts, highlighting the nature of faint radio sources.

Findings

Spectral index peaks near 0.7 for 90cm-selected sources.

No large population of very steep spectrum microJy sources detected.

Log N-log S counts flatten below 5 mJy, indicating a change in source population.

Abstract

We present one of the deepest radio continuum surveys to date at a wavelength ~1 meter, at 324.5 MHz. The data reduction and analysis are described and an electronic catalog of the sources detected above 5 sigma is presented. We also discuss the observed angular size distribution for the sample. Using our deeper 20cm survey of the same field, we calculate spectral indices for sources detected in both surveys. The spectral indices for 90cm-selected sources, defined as S ~nu^(-alpha}, shows a peak near 0.7 and only a few sources with very steep spectra. Thus no large population of very steep spectrum microJy sources seems to exist down to the limit of our survey. For 20cm-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 90cm, we use stacking to study the radio spectra. We find that the spectral indices of small (<3") 20cm-selected sources with S_20< 10 mJy have mean and median alpha(90,20)~0.3-0.5. This is flatter than the spectral indices of the stronger source population. We report log N-log S counts at 90cm 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 occurs at radio luminosities normally associated with AGN rather than with galaxies dominated by star-formation.