2007AJ....134..694M

We present sensitive phase-referenced VLBI results on the radio continuum emission from the z=4.4 QSO BRI 1335-0417. The observations were carried out at 1.4 GHz using the High Sensitivity Array. Our sensitive VLBI image at 189x113 mas (1.25x0.75 kpc) resolution shows continuum emission in BRI 1335-0417 with a total flux density of 208±46 µJy, consistent with the flux density measured with the VLA. The size of the source at FWHM is 255x138 mas (1.7x0.9 kpc), and the derived intrinsic brightness temperature is ∼3.5x10⁴ K. No continuum emission is detected at the full VLBI resolution (32x7 mas, 211x46 pc), with a 4 σ point-source upper limit of 34 µJy/beam, or an upper limit to the intrinsic brightness temperature of 5.6x10⁵K. The highest angular resolution with at least a 4.5 σ detection of the radio continuum emission is 53x27 mas (0.35x0.18 kpc). At this resolution the image shows a continuum feature in BRI 1335-0417 with a size of 64x35 mas (0.42x0.23 kpc) at FWHM and an intrinsic brightness temperature of ∼2x10⁵K. The extent of the observed continuum sources at 1.4 GHz and the derived brightness temperatures show that the radio emission (and thus, presumably, the far-IR emission) in BRI 1335-0417 is powered by a major starburst with a massive star formation rate of order a few thousand M_☉/yr. Moreover, the absence of any compact high brightness temperature source suggests that there is no radio-loud AGN in this z=4.4 QSO.