We present a Hubble Space Telescope image of the FR II radio galaxy 3C 401, obtained at 1.6µm with the NICMOS camera in which we identify the infrared counterpart of the brightest region of the radio jet. The jet has a complex radio structure and brightens where bending occurs, most likely as a result of relativistic beaming. We analyze archival data in the radio, optical and X-ray bands and we derive its spectral energy distribution. Differently from all of the previously known optical extragalactic jets, the jet in 3C 401 is not detected in the X-rays even in a long 48ksec X-ray Chandra exposure and the infrared emission dominates the overall SED. We propose that the dominant radiation mechanism of this jet is synchrotron. The low X-ray emission is then caused by two different effects: i) the lack of any strong external photon field and ii) the shape of the electron distribution. This affects the location of the synchrotron peak in the SED, resulting in a sharp cut-off at energies lower than the X-rays. Thus 3C 401 shows a new type of jet which has intermediate spectral properties between those of FR I, which are dominated by synchrotron emission up to X-ray energies and FR II/QSO, which show a strong high-energy emission due to inverse-Compton scattering of external photons. This might be a clue for the presence of a continuous “sequence” in the properties of large scale jets, in analogy with the “blazar sequence” already proposed for sub-pc scale jets.

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Also archived in: arXiv:astro-ph/0505034 v2 16 May 2005 This paper is based on observations obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS 5-26555.ISSN:1538-4357 Note: imported from RIT’s Digital Media Library running on DSpace to RIT Scholar Works in February 2014.

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Department, Program, or Center

School of Physics and Astronomy (COS)


RIT – Main Campus