N ‐Band Imaging of Seyfert Nuclei and the Mid‐Infrared–X‐Ray Correlation

Abstract

We present new mid-infrared (N-band) images of a sample of eight nearbySeyfert galaxies. In all of our targets, we detect a central unresolvedsource, which in some cases has been identified for the first time. Inparticular, we have detected the mid-infrared emission from the activenucleus of NGC 4945, which previously remained undetected at anywavelength but hard X-rays. We also detect circumnuclear extendedemission in the Circinus galaxy along its major axis and find marginalevidence for extended circumnuclear emission in NGC 3281. The highspatial resolution (1.7") of our data allows us to separate the flux ofthe nuclear point sources from the extended circumnuclear starburst (ifpresent). We complement our sample with literature data for a number ofnonactive starburst galaxies and relate the nuclear N-band flux topublished hard (2-10 keV) X-ray fluxes. We find tight and well-separatedcorrelations between the nuclear N-band flux and X-ray flux for bothSeyfert and starburst nuclei that span over 3 orders of magnitude inluminosity. We demonstrate that these correlations can be used as apowerful classification tool for galactic nuclei. For example, we findstrong evidence against NGC 1808 currently harboring an active Seyfertnucleus based on its position in the mid-infrared-X-ray diagram. On theother hand, we confirm that NGC 4945 is in fact a Seyfert 2 galaxy.