The Angular Size Distribution of μJy Radio Sources

Abstract

We made two new sensitive (rms noise μ Jy beam −1 ) high-resolution ( θ  = 3.″0 and θ  = 0.″66 FWHM) S-band (2 <  ν  < 4 GHz) images covering a single JVLA primary beam (FWHM ≈ 14′) centered on J2000 , δ  = +59°01′ in the Lockman Hole. These images yielded a catalog of 792 radio sources, 97.7 ± 0.8% of which have infrared counterparts stronger than S  ≈ 2 μ Jy at λ  = 4.5 μ m. About 91% of the radio sources found in our previously published, comparably sensitive low-resolution ( θ  = 8″ FWHM) image covering the same area were also detected at 0.″66 resolution, so most radio sources with S (3 GHz) ≳ 5  μ Jy have angular structure ϕ  ≲ 0.″66. The ratios of peak brightness in the 0.″66 and 3″ images have a distribution indicating that most μ Jy radio sources are quite compact, with a median Gaussian angular diameter FWHM and an rms scatter σ ϕ  ≲ 0.″3 of individual sizes. Most of our μ Jy radio sources obey the tight far-infrared/radio correlation, indicating that they are powered by star formation. The median effective angular radius enclosing half the light emitted by an exponential disk is , so the median effective radius of star-forming galaxies at redshifts z  ∼ 1 is .