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

Abstract

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 ∼30-40% 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 4σ detections of a HBLR in 11 of these galaxies (73% 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 Hα and Hβ components in polarized light for 10 targets, and just broad Hα 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.

Hidden Broad-line Regions in Seyfert 2 Galaxies: From the Spectropolarimetric Perspective

“…The degree of polarization in type 2 AGNs is generally very low (less than several percent), in most cases, only broad Hα lines in the polarized spectra are visible (e.g., Antonucci 1984;Tran et al 1992;Young et al 1996;Moran et al 2000;Ramos Almeida et al 2016). We assume that broad Hβ and Hα lines have the same widths in the polarized spectrum for each individual object, as has been demonstrated in several objects (Ramos Almeida et al 2016). Because the poor quality of polarized spectra would influence the measurements of the line dispersions (second momentum of the profile, σ), we only adopt the FWHM of polarized emission lines in the following analysis.…”

“…The hidden broad-line regions (BLRs) are observed in the polarized spectra of many type 2 AGNs (e.g., Antonucci 1984;Tran et al 1992;Young et al 1996;Moran et al 2000;Ramos Almeida et al 2016), and are thought to have scattering of BELs by the regions above the poles of accretion disks (called "polar scatter regions") (Antonucci & Miller 1985;Antonucci 1993). However, the detailed geometry and kinematics of BLRs hidden in the centers of type 2 AGNs remain elusive and are not yet fully understood.…”

Modeling optical and UV polarization of AGNs

“…We thus naturally expect highly polarized spectra for type-2 AGN as they are seen along the equatorial direction. Nevertheless the usual linear continuum polarization we observe is less than a few percent (e.g., Kay 1994; Ramos Almeida et al 2016). This is intriguing as the broad lines are detected in polarized flux.…”

The Panchromatic Polarisation Signatures of Active Galactic Nuclei

“…However, it is impossible to prove that all type-2 AGNs have a broad emission line region. High-resolution spectra and broadband observations (covering several Balmer lines) has proven to be necessary to observe faint broad lines in polarized flux [148], decreasing the estimated number of true type-2s, i.e. AGN that are genuinely lacking a BLR.…”