The GOODS-N Jansky VLA 10 GHz Pilot Survey: Sizes of Star-forming μjY Radio Sources

Our sensitive (σ_n 572 nJy beam^-1), high-resolution (FWHMθ1/2 = 0. ″22 ≈pc at z ≳ ), 10 GHz image covering a single Karl G. Jansky Very Large Array (VLA) primary beam (FWHM⊙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 (⊙_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 lowresolution (⊙1/2 = 1.″0) image with ≈10× better brightness sensitivity, in order to detect extended sources and measure matched-resolution spectral indices α^10GHz _{1.4 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 ^{10 GHz} _{1.4 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^{10 GHz} _{1.4 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 αNT ≈ -0.85, the median thermal fraction of sources selected at median rest-frame frequency ≈20 GHz is ≳ 48%.