Radio continuum imaging of far-infrared-luminous QSOs at z > 6

We present sensitive imaging at 1.4 GHz of the two highest redshift far-infrared (FIR) luminous QSOs, SDSS J114816.65+525150.2 (z = 6.42) and SDSS J104845.05+463718.3 (z = 6.2). Radio continuum emission is detected from J1148+5251 with S_1.4 = 55 ± 12 μJy, while J1048+4637 is marginally detected with S_1.4 = 26 ± 12 μJy. Comparison of the radio and FIR luminosities shows that both sources follow the radio-FIR correlation for star-forming galaxies, with implied (massive) star formation rates ∼10³ M_⊙ yr^-1, although we cannot rule out as much as 50% of the FIR luminosity being powered by the active galactic nucleus. Five bright (>22 mJy) radio sources are detected within 8′ of J1148+5251. This is a factor of 30 more than expected for a random field. Two sources have SDSS redshifts, including a z = 1.633 radio-loud quasar and a z = 0.05 radio galaxy. However, we do not find evidence for a galaxy cluster in the SDSS data, at least out to z = 0.2. Considering the faint SDSS magnitudes of the remaining radio sources, we conclude that the overdensity of radio sources could either be a statistical fluke or a very large scale structure (>8 Mpc comoving) at z ≥ 1. We also consider the possibility of gravitational lensing by the closest (in angle) bright galaxy in the SDSS data at z = 0.05 and conclude that the galaxy provides negligible magnification.