The GOODS-N Jansky VLA 10 GHz Pilot Survey: Sizes of Star-Forming
  $\mu$Jy Radio Sources
  (Abridged) Our sensitive ($\sigma_{\rm n} = 572\,{\rm nJy\,beam}^{-1}$),
high-resolution (FWHM $\theta_{1/2} = 220\,{\rm mas}
\approx2\mathrm{\,kpc~at~}z\gtrsim1$) 10$\,$GHz image covering a single Karl
G.~Jansky Very Large Array (VLA) primary beam (FWHM $\Theta_{1/2} = 4.25'$) in
the GOODS-N field contains 32 sources with $S_{\rm p}\gtrsim2\,\mu{\rm
Jy~beam}^{-1}$ 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 $\langle\theta_{\rm M} \rangle=167\pm32\,{\rm mas} \approx
1.2\pm0.28\,{\rm kpc}$ with rms scatter 91 was $\approx$ 0.79 kpc. In units of
the effective radius $r_{\rm e}$ that encloses half their flux, these radio
sizes are $\langle r_{\rm e}\rangle = 69\pm13{\rm mas} \approx
\pm114\mathrm{\,pc}$ and have rms scatter
$38\mathrm{\,mas}\approx324\mathrm{\,pc}$. 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 H$\alpha$ sizes. We
made a low-resolution ($\theta_{1/2}=1.0"$) image with $\approx10\times$ better
brightness sensitivity to detect extended sources and measure
matched-resolution spectral indices $\alpha_{1.4}^{10}$. It contains 6 new
sources with $S_{\rm p}\gtrsim3.9\,\mu{\rm Jy~beam}^{-1}$ and OIR counterparts.
The median redshift of all 38 sources is $1.24\pm0.15$. The 19 sources with
1.4$\,$GHz counterparts have median spectral index $-0.74\pm0.10$ with rms
scatter $0.35$. Including upper limits on $\alpha$ for sources not detected at
1.4$\,$GHz flattens the median to $\gtrsim-0.61$, suggesting that the $\mu$Jy
radio sources at higher redshifts, and hence selected at higher rest-frame
frequencies, may have flatter spectra. If the non-thermal spectral index is
-0.85, the median thermal fraction at rest-frame frequency 20$\,$GHz is
$\gtrsim$48%.
