Upholding the unified model for active galactic nuclei: VLT/FORS2 spectropolarimetry of Seyfert 2 galaxies

Cristina Ramos Almeida, M. J. Martínez Gonzalez, A. Asensio Ramos, J. A. Acosta-Pulido, S. F. Hönig, A. Alonso-Herrero, C. N. Tadhunter, O. González-Martín. 2016. Upholding the Unified Model for Active Galactic Nuclei: VLT/FORS2 Spectropolarimetry of Seyfert 2 galaxies. Monthly Notices of the Royal Astronomical Society 461, 2, 1387-1403 DOI: 10.1093/mnras/stw1388

The origin of the unification model for active galactic nuclei (AGN) was the detection of broad hydrogen recombination lines in the optical polarized spectrum of the Seyfert 2 galaxy (Sy2) NGC 1068. Since then, a search for the hidden broad-line region (HBLR) of nearby Sy2s started, but polarized broad lines have only been detected in similar to 30-40 per cent of the nearby Sy2s observed to date. Here we present new VLT/FORS2 optical spectropolarimetry of a sample of 15 Sy2s, including Compton-thin and Compton-thick sources. The sample includes six galaxies without previously published spectropolarimetry, some of them normally treated as non-hidden BLR (NHBLR) objects in the literature, four classified as NHBLR, and five as HBLR based on previous data. We report >= 4 sigma detections of a HBLR in 11 of these galaxies (73 per cent of the sample) and a tentative detection in NGC 5793, which is Compton-thick according to the analysis of X-ray data performed here. Our results confirm that at least some NHBLRs are misclassified, bringing previous publications reporting differences between HBLR and NHBLR objects into question. We detect broad H alpha and H beta components in polarized light for 10 targets, and just broad Ha for NGC 5793 and NGC 6300, with line widths ranging between 2100 and 9600 km s(-1). High bolometric luminosities and low column densities are associated with higher polarization degrees, but not necessarily with the detection of the scattered broad components.

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