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

Abstract

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 Hα. We have not detected polarized broad Brγ in the K band, however we can place a 3σ upper limit of 7 × 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 Hα 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° inclined cone of 45° 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.