2017ApJ...839...35M

Our sensitive (σ_n~572 nJy beam^–1), high-resolution (FWHM θ_1/2=0′′ / . 22~2 kpc at z 1), 10 GHz image covering a single Karl G. Jansky Very Large Array (VLA) primary beam (FWHM Θ_1/2~4 ′ / . 25) in the GOODS-N field contains 32 sources with S_p 2 µJy beam^–1 and optical and/or near-infrared (OIR) counterparts. Most are about as large as the star-forming regions that power them. Their median FWHM major axis is <θ_M>=167±32 mas~1.2±0.28 kpc, with rms scatter ~91 mas~0.79 kpc. In units of the effective radius r_e that encloses half their flux, these radio sizes are <r_e>~69±13 mas~509±114 pc, with rms scatter ~38 mas~324 pc. These sizes are smaller than those measured at lower radio frequencies, but agree with dust emission sizes measured at mm/sub-mm wavelengths and extinction-corrected Hα sizes. We made a low-resolution (θ_1/2=1′′ / . 0) image with ~10×better brightness sensitivity, in order to detect extended sources and measure matched-resolution spectral indices α_1.4 GHz^10 GHz. It contains six new sources with S_p 3.9 µJy beam^–1 and OIR counterparts. The median redshift of all 38 sources is <z>=1.24±0.15. The 19 sources with 1.4 GHz counterparts have a median spectral index of <α_1.4 GHz^10 GHz>=-0.74±0.10, with rms scatter ~0.35. Including upper limits on α for sources not detected at 1.4 GHz flattens the median to <α_1.4 GHz^10 GHz>-0.61, suggesting that the µJy radio sources at higher redshifts-and hence those selected at higher rest-frame frequencies-may have flatter spectra. If the non-thermal spectral index is α_NT~-0.85, the median thermal fraction of sources selected at median rest-frame frequency ~20 GHz is >=48%.