The VLA-COSMOS Survey: V. 324 MHz continuum observations

TL;DR

This study presents 90 cm radio observations of the COSMOS field, cataloging sources, analyzing their spectral properties, and finding that most are AGN with spectral indices consistent across redshifts, supporting their use as tracers of intermediate-redshift galaxies.

Contribution

First detailed 90 cm continuum survey of the COSMOS field, providing source catalog, spectral index analysis, and insights into the nature and redshift distribution of radio sources.

Findings

Catalog contains 182 sources with 30 multi-component sources.

Spectral index average of -0.70, with few ultra-steep-spectrum sources.

Sources with spectral index steeper than -1 are at z>1.

Abstract

We present 90 cm VLA imaging of the COSMOS field, comprising a circular area of 3.14 square degrees at 8.0"x6.0" angular resolution with an average rms of 0.5 mJy/beam. The extracted catalog contains 182 sources (down to 5.5sigma), 30 of which are multi-component sources. Using Monte Carlo artificial source simulations we derive the completeness of the catalog, and we show that our 90 cm source counts agree very well with those from previous studies. Using X-ray, NUV-NIR and radio COSMOS data to investigate the population mix of our 90 cm radio sample, we find that our sample is dominated by active galactic nuclei (AGN). The average 90-20 cm spectral index (S_nu~nu**alpha, where S_nu is the flux density at frequency nu, and alpha the spectral index) of our 90 cm selected sources is -0.70, with an interquartile range of -0.90 to -0.53. Only a few ultra-steep-spectrum sources are present in our sample, consistent with results in the literature for similar fields. Our data do not show clear steepening of the spectral index with redshift. Nevertheless, our sample suggests that sources with spectral indices steeper than -1 all lie at z>1, in agreement with the idea that ultra-steep-spectrum radio sources may trace intermediate-redshift galaxies (z>1).