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 \(\alpha^{1.53}_{4.67} < -\)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 \(23\rlap{.}{''}1\) 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 ~\(0\rlap{.}{''}17\). It is thus likely situated at lower redshift than the quasar.