3.3 μm PAH observations of the central kiloparsecs of Centaurus A^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
e-mail: ltacconi@eso.org
² Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße, 85748 Garching bei München, Germany
e-mail: sturm@mpe.mpg.de

Received: 29 October 2012
Accepted: 4 February 2013

Abstract

Aims. The aim of this work is to further investigate the nature of polycyclic aromatic hydrocarbon (PAH) excitation and emission especially in the context of tracing star formation in a variety of extragalactic environments. Here we turn our attention to the energetic environment of the closest active galactic nucleus (AGN) in our sample, Centaurus A.

Methods. Using ISAAC on the ESO VLT UT1 (Antu) we have made high spatial resolution 3.3 μm imaging observations of the central kiloparsec of CenA. These observations have been compared with star formation tracers in the near- and mid-infrared, as well as with mid-infrared tracers of nuclear activity.

Results. The nucleus is not devoid of PAH emission, implying that the PAH particles are not destroyed in the nucleus as might be expected for such a harsh environment. However, we see the feature to continuum ratio decrease towards the AGN. As well, the 3.3  μm PAH feature emission generally traces the sites of star formation in Cen A, but in detail there are spatial offsets, consistent with an earlier study of the starburst galaxies NGC 253 and NGC 1808. However, the feature-to-continuum ratio does not drop at the positions of star formation as was previously seen in that earlier study. The cause for this difference remains uncertain. Finally, our data reveal possible evidence for a nearly face-on, circular or spiral, dust structure surrounding the nucleus.