High Sensitivity Array Observations of the $z=4.4$ 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 (HSA). Our sensitive VLBI image at $189\times 113$ mas ($1.25\times 0.75$ kpc) resolution shows continuum emission in BRI 1335--0417 with a total flux density of $208\pm 46\mu$Jy, consistent with the flux density measured with the VLA. The size of the source at FWHM is $255\times 138$ mas ($1.7\times 0.9$ kpc) and the derived intrinsic brightness temperature is $\sim 3.5\times 10^4$ K. No continuum emission is detected at the full VLBI resolution ($32\times 7$ mas, $211\times 46$ pc), with a 4$\sigma$ point source upper limit of 34 $\mu$Jy beam${}^{-1}$, or an upper limit to the intrinsic brightness temperature of $5.6\times 10^5$ K. The highest angular resolution with at least a 4.5$\sigma$ detection of the radio continuum emission is $53\times 27$ mas ($0.35\times 0.18$ kpc). At this resolution, the image shows a continuum feature in BRI 1335--0417 with a size of $64\times 35$ mas ($0.42\times 0.23$ kpc) at FWHM, and intrinsic brightness temperature of $\sim 2\times 10^5$ 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-infrared emission) in BRI 1335--0417 is powered by a major starburst, with a massive star formation rate of order a few thousand M_{\odot} {\rm yr}^{-1}$.Moreover,theabsenceofanycompacthigh-brightnesstemperaturesourcesuggeststhatthereisnoradio-loudAGNinthis$z=4.4$ QSO.

13 pages, 3 figures, AJ accepted

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