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3.1. Radio/IR

The radio emission properties of optically obscured AGN are similar to those of unobscured, type I AGN. A flat-spectrum, compact radio core is present in local AGN, with brightness temperatures TB > 105 K. Recent VLA observations (Nagar et al. 2000) also revealed these radio cores in low luminosity (L < 1041 ergs s-1) AGN, showing that radio emission is an ubiquitous property of all AGN. However, the fraction of luminosity emitted in the radio band is in all cases negligible with respect to the bolometric luminosity.

In some cases, free-free absorption can alter the observed radio spectrum of an AGN covered by a compact layer of warm gas (Neufeld et al. 1994). At high radio luminosity, HI absorption is also common (Veron-Cetty et al. 2000). HI and/or free-free absorption is preferentially found in X-ray heavily absorbed (column density NH > 1024 cm-2), optically type II sources (Risaliti, Woltjer, & Salvati 2003b).