Constraints on the nuclear emission of the Circinus galaxy:: optical and near-IR spectropolarimetry☆

We present optical and, for the first time, K-band spectropolarimetry of the nuclear region of the Circinus galaxy. In the optical we confirm the previous discovery of polarized broad H alpha. We have not detected polarized broad Br gamma in the K band, however we can place a 3 sigma upper limit of 7 x 10(-15) erg s(-1) cm(-2.) This corresponds to an upper limit of 7.7 mag of visual extinction along the broad-line scattered route assuming electron scattering and the Case B approximation. Analysis of these observations shows that galactic processes dominate in the optical total and polarized flux whilst the nucleus dominates the K-band polarized flux. From a sample of Seyfert 1 galaxies we find a tight non-stellar R-band continuum to broad H alpha correlation that allows us to constrain the nuclear continuum contribution at optical wavelengths. Based on these constraints we have modelled our observations as electron scatterers distributed in a 50 degrees inclined cone of 45 degrees opening half-angle, visually extincted by 5 mag through the Galaxy and Circinus galactic disc. At near-IR wavelengths the nuclear emission region becomes visible and dichroically polarized through the dusty torus; we find a visual extinction to this region of 35 mag gives the best results, consistent with that determined using other techniques. A comparison of the [O III]lambda 5007 emission line and infrared luminosity to other Seyfert 2 galaxies shows that the Circinus galaxy is the lowest powered object for which broad lines have been detected in polarized flux. As it is the closest Seyfert 2 galaxy to us this suggests considerable difficulty in finding polarized broad lines in other low-powered obscured AGN.