Unveiling hidden AGN by IR observations

Abstract

AGN can be dust-hidden by two ways and this talk considers how to unveil them in the infrared: firstly, the powerful 3CR radio galaxies, i.e., type-2 AGN which suffer from anisotropic obscuration of the nuclear region by the dust torus, and secondly, type-1 quasars, which seem to be isotropically reddened by dust, so that they escape detection via optical-UV colour criteria.

Using Spitzer MIR spectroscopy we show that the powerful 3CR radio galaxies have similar high excitation lines, for instance, [Ne V]λ14.3 μm, as quasars matched in radio power and redshift. This solves the long debate about the apparent difference of quasars and radio galaxies in favor of the orientation-dependent unified scheme.

Combining the ISOCAM Parallel Mode Survey at 6.7 μm (LW2 filter) with the Two Micron All Sky Survey we used moderate colour criteria H - K > 0.5 and K - LW2 > 2.7 to search for AGN independent of dust extinction. With a surface density of about 2 deg^-2 down to R < 18 mag the ISO-2MASS type-1 QSOs outnumber the 1.35 deg^-2 of the SDSS DR3 quasar survey by 50%. About one third of the ISO-2MASS QSOs show very red star-like optical colours.