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3rd of December 2015
The radio view of the Seyfert galaxy PKS 2004-447
by Phil Edwards (CASS)
The Fermi gamma-ray satellite has identified over 1500 gamma-ray sources with other galaxies. The majority are classified as flat-spectrum radio quasars, or BL Lacerate objects: just a handful have been identified with narrow-line Seyfert galaxies. The brightest of these at radio wavelengths is PKS 2004-447, and Schulz et al. have made the first detailed study of the radio morphology and long-term radio spectral evolution this galaxy, which are essential to understand the diversity of the radio properties of this class of object. The image above shows the central few hundred light-years of the galaxy, made using the technique of Very Long Baseline Interferometry (VLBI) as part of the TANAMI project.

TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry) uses radio telescopes in Australia, Antarctica, Chile, New Zealand, and South Africa to monitor the jets of radio-loud active galaxies in the southern hemisphere. In addition, flux density measurements at multiple radio frequencies were made over four years with the Australia Telescope Compact Array (ATCA).

These observations reveal an extended one-sided jet with a dominant compact VLBI core, and a persistent steep radio spectrum with moderate flux-density variability. These features set PKS 2004-447 apart from the other narrow-line Seyferts detected at gamma-ray energies. It exhibits blazar-like features, such as a flat featureless X-ray spectrum and a core dominated, one-sided parsec-scale jet with indications for relativistic beaming. However, the data also reveal properties atypical for blazars, such as a radio spectrum and large-scale size consistent with Compact-Steep-Spectrum (CSS) objects, which are usually associated with young radio sources. These characteristics are extremely rare among gamma-ray loud AGN.

Reference: "The Gamma-Ray Emitting Radio-Loud Narrow-Line Seyfert 1 Galaxy PKS 2004-447 II. The Radio View" Schulz et al. (2015, A&A, in press

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