Angular Sizes of μJy Radio Sources

We made two new sensitive (rms noise sigma_n 1 microJy/beam) high resolution (theta = 3.0" and theta = 0.66" FWHM) S–band (2 < nu < 4 GHz) images covering a single JVLA primary beam (FWHM 14') centered on J2000 RA = 10 46, Dec = 59 01 in the Lockman Hole. These images yielded a catalog of 792 radio sources, 97.7 +/- 0.8 S 2 microJy at lambda = 4.5 micron. About 91 our previously published, comparably sensitive low resolution (theta = 8" FWHM) image covering the same area were also detected at 0.66" resolution, so most radio sources with S_3GHz > 5 microJy have angular structure phi < 0.66". The ratios of peak brightness in the 0.66" and 3" images have a distribution indicating that most microJy radio sources are quite compact, with a median Gaussian angular diameter = 0.3" +/- 0.1" FWHM and an rms scatter sigma_phi < 0.3" of individual sizes. Most of our microJy 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 /2.43 0.12", so the median effective radius of star-forming galaxies at redshifts z 1 is 1.0 kpc.