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

D. M. Alexander*, C. A. Heisler, S. Young, S. L. Lumsden, J. H. Hough, J. A. Bailey

*Corresponding author for this work

    Research output: Contribution to journalArticlepeer-review

    12 Citations (Scopus)


    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 Ha 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. A comparison of the [O III]λ5007 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.

    Original languageEnglish
    Pages (from-to)815-822
    Number of pages8
    JournalMonthly Notices of the Royal Astronomical Society
    Issue number4
    Publication statusPublished - 21 Apr 2000


    • Dust, extinction
    • Galaxies: Active
    • Galaxies: Individual: Circinus
    • Galaxies: Nuclei
    • Polarization
    • Scattering


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