2005AJ....129.1809M

We present sensitive phase-referenced VLBI observations of the radio continuum emission from the z=4.7 double source BRI 1202-0725. The observations were carried out at 1425 MHz using the Very Long Baseline Array, the phased Very Large Array, and the Green Bank Telescope. Our sensitive VLBI images of BRI 1202-0725 at 0".25x0".14 resolution show a continuum structure in each of its two components. Fitting Gaussian models to these continuum structures yields total flux densities of 315±38 and 250±39 µJy for the northern and the southern components, respectively. The estimated intrinsic brightness temperatures of these continuum structures are ∼2x10⁴ K. Neither component is detected at the full VLBI resolution (29x7 mas²), with a 4 σ point-source upper limit of 40 µJy/beam, or an upper limit to the intrinsic brightness temperature of 6.7x10⁵ K. The highest angular resolution with at least a 4 σ detection is ∼85 mas. At this resolution, the images reveal a single continuum feature in the northern component of BRI 1202-0725 and two continuum features in the southern component, separated by 320 mas. This is similar to the structures seen in the high-resolution images of the CO emission. The extent of the observed continuum sources at 1.4 GHz and the derived brightness temperatures are consistent with nuclear starbursts. Moreover, the absence of any compact high brightness temperature source suggests that there is no radio-loud active galactic nucleus in BRI 1202-0725.