The GOODS-N Jansky VLA 10 GHz Pilot Survey: Sizes of Star-Forming $\mu$Jy Radio Sources

Abstract

(Abridged) Our sensitive (${\sigma }_{\mathrm{n}}=572{\mathrm{nJy}\mathrm{beam}}^{-1}$), high-resolution (FWHM ${\theta }_{1/2}=220\mathrm{mas}\approx 2\mathrm{kpc}\mathrm{at}z\gtrsim 1$) 10GHz image covering a single Karl G.~Jansky Very Large Array (VLA) primary beam (FWHM ${\Theta }_{1/2}=4.25^{\prime }$) in the GOODS-N field contains 32 sources with $S_{\mathrm{p}}\gtrsim 2\mu {\mathrm{Jy}\mathrm{beam}}^{-1}$ 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 $⟨{\theta }_{\mathrm{M}}⟩=167\pm 32\mathrm{mas}\approx 1.2\pm 0.28\mathrm{kpc}$ with rms scatter 91 was $\approx$ 0.79 kpc. In units of the effective radius $r_{\mathrm{e}}$ that encloses half their flux, these radio sizes are $⟨r_{\mathrm{e}}⟩=69\pm 13\mathrm{mas}\approx \pm 114\mathrm{pc}$ and have rms scatter $38\mathrm{mas}\approx 324\mathrm{pc}$. 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 $\approx 10\times$ better brightness sensitivity to detect extended sources and measure matched-resolution spectral indices ${\alpha }_{1.4}^{10}$. It contains 6 new sources with $S_{\mathrm{p}}\gtrsim 3.9\mu {\mathrm{Jy}\mathrm{beam}}^{-1}$ and OIR counterparts. The median redshift of all 38 sources is $1.24\pm 0.15$. The 19 sources with 1.4GHz counterparts have median spectral index $-0.74\pm 0.10$ with rms scatter $0.35$. Including upper limits on $\alpha$ for sources not detected at 1.4GHz 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 20GHz is $\gtrsim$48%.

18 pages, 13 figures, accepted for publication in the Astrophysical Journal

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