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

April 2017 • 2017ApJ...839...35M

Authors • Murphy, Eric J. • Momjian, Emmanuel • Condon, James J. • Chary, Ranga-Ram • Dickinson, Mark • Inami, Hanae • Taylor, Andrew R. • Weiner, Benjamin J.

Abstract • Our sensitive ({σ }_{{n}}≈ 572 {nJy} {{beam}}^-1), high-resolution (FWHM {θ }_1/2=0\buildrel{\prime\prime}\over{.} 22≈ 2 {kpc} {at} z≳ 1), 10 GHz image covering a single Karl G. Jansky Very Large Array (VLA) primary beam (FWHM {{{\Theta }}}_1/2≈ 4\buildrel{ \prime}\over{.} 25) in the GOODS-N field contains 32 sources with {S}_{{p}}≳ 2 μ {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 < {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\buildrel{\prime\prime}\over{.} 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 μ {Jy} {{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%.

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Ranga-Ram Chary

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