A deep, high-resolution survey of the low-frequency radio sky

Abstract

We report on the first wide-field, very long baseline interferometry (VLBI) survey at 90 cm. The survey area consists of two overlapping 28 deg2 fields centered on the quasar J0226+3421 and the gravitational lens B0218+357. A total of 618 sources were targeted in these fields, based on identifications from Westerbork Northern Sky Survey (WENSS) data. Of these sources, 272 had flux densities that, if unresolved, would fall above the sensitivity limit of the VLBI observations. A total of 27 sources were detected as far as 2 from the phase center. The results of the survey suggest that at least 10%of moderately faint (S ≈ 100mJy) sources found at 90 cm contain compact components smaller than ≈ 0.1" ≈ 0.3" and stronger than 10% of their total flux densities. A ≈ 90 mJy sourcewas detected in the VLBI data that was not seen in the WENSS and NRAO VLA Sky Survey (NVSS) data and may be a transient or highly variable source that has been serendipitously detected. This survey is the first systematic (and nonbiased), deep, high-resolution survey of the low-frequency radio sky. It is also the widest field of view VLBI survey with a single pointing to date, exceeding the total survey area of previous higher frequency surveys by 2 orders ofmagnitude. These initial results suggest that new low-frequency telescopes, such as LOFAR, should detect many compact radio sources and that plans to extend these arrays to baselines of several thousand kilometers are warranted.