Resolving the Radio Emission from the Quasar P172+18 at $z=6.82$

Abstract

We present high angular resolution imaging of the quasar PSO J172.3556+18.7734 at $z=6.82$ with the Very Long Baseline Array (VLBA). This source currently holds the record of being the highest redshift radio-loud quasar. These observations reveal a dominant radio source with a flux density of $398.4\pm 61.4\mu$Jy at 1.53 GHz, a deconvolved size of $9.9\times 3.5$ mas ($52.5\times 18.6$ pc), and an intrinsic brightness temperature of ($4.7\pm 0.7)\times 10^7$ K. A weak unresolved radio extension from the main source is also detected at $\sim 3.1\sigma$ level. The total flux density recovered with the VLBA at 1.53 GHz is consistent with that measured with the Very Large Array (VLA) at a similar frequency. The quasar is not detected at 4.67 GHz with the VLBA, suggesting a steep spectral index with a limit of ${\alpha }_{4.67}^{1.53}<-$1.55. The quasar is also not detected with the VLBA at 7.67 GHz. The overall characteristics of the quasar suggest that it is a very young radio source similar to lower redshift Gigahertz Peaked Spectrum radio sources, with an estimated kinematic age of $\sim 10^3$ years. The VLA observations of this quasar revealed a second radio source in the field $23\text{.}{}^{\prime \prime }1$ away. This radio source, which does not have an optical or IR counterpart, is not detected with the VLBA at any of the observed frequencies. Its non-detection at the lowest observed VLBA frequency suggests that it is resolved out, implying a size larger than ~$0\text{.}{}^{\prime \prime }17$. It is thus likely situated at lower redshift than the quasar.

5 pages, 5 figures, accepted for publication in the Astronomical Journal

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