The Nature of the Compact/Symmetric Near-IR Continuum Source in 4C 40.36

Abstract

Using NICMOS on HST, we have imaged the emission-line nebulae and the line-free continuum in 4C 40.36, a ultra-steep spectrum FR II radio galaxy at z = 2.269. The line-free continuum was found to be extremely compact and symmetric while the emission-line nebulae seen in Hɑ+[N II] show very clumpy structures spreading almost linearly over 16 kpc. However, this linear structure is clearly misaligned from the radio axis. The SED of the line-free continuum is very flat (ƒv ∝ v^(-0·5)), suggesting that if the continuum emission is produced by a single source, it is likely to be a young bursting stellar population or scattered AGN light. However, because of the lack of a line-free optical image with a comparable spatial resolution, we cannot exclude the possibility that the observed SED is a composite of a young blue population and an old red population.