High Sensitivity Array Observations of thez= 4.4 QSO BRI 1335-0417

Abstract

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 189 × 113 mas (1.25 × 0.75 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 255 × 138 mas (1.7 × 0.9 kpc), and the derived intrinsic brightness temperature is ∼3.5 × 104 K. No continuum emission is detected at the full VLBI resolution (32 × 7 mas, 211 × 46 pc), with a 4 σ point-source upper limit of 34 μJy beam-1, or an upper limit to the intrinsic brightness temperature of 5.6 × 105 K. The highest angular resolution with at least a 4.5 σ detection of the radio continuum emission is 53 × 27 mas (0.35 × 0.18 kpc). At this resolution the image shows a continuum feature in BRI 1335-0417 with a size of 64 × 35 mas (0.42 × 0.23 kpc) at FWHM and an intrinsic brightness temperature of ∼2 × 105 K. 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-1. Moreover, the absence of any compact high brightness temperature source suggests that there is no radio-loud AGN in this z = 4.4 QSO.