Spectropolarimetry of high-redshift obscured and red quasars

Rachael M. Alexandroff*, Nadia L. Zakamska, Aaron J. Barth, Fred Hamann, Michael A. Strauss, Julian Krolik, Jenny E. Greene, Isabelle Pâris, Nicholas P. Ross

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


Spectropolarimetry is a powerful technique that has provided critical support for the geometric unification model of local active galactic nuclei. In this paper, we present optical [rest-frame ultraviolet (UV)] Keck spectropolarimetry of five luminous obscured (type 2) and extremely red quasars (ERQs) at z ≃ 2.5. Three objects reach polarization fractions of ≳ 10 per cent in the continuum. We prefer dust scattering as the dominant scattering and polarization mechanism in our targets, though electron scattering cannot be completely excluded. Emission lines are polarized at a lower level than the continuum. This suggests that the emission-line region exists on similar spatial scales as the scattering region. In three objects, we detect an intriguing 90°swing in the polarization position angle as a function of line-of-sight velocity in the emission lines of Ly α, CIV, and NV. We interpret this phenomenon in the framework of a geometric model with an equatorial scattering region in which the scattering material is outflowing at several thousand km s-1. Our model explains several salient features of observations by scattering on scales of a few tens of pc. Our observations provide a tantalizing view of the inner region geometry and kinematics of high-redshift obscured and ERQs. Our data and modelling lend strong support for toroidal obscuration and powerful outflows on the scales of the UV emission line region, in addition to the larger scale outflows inferred previously from the optical emission line kinematics.

Original languageEnglish
Pages (from-to)4936-4957
Number of pages22
JournalMonthly Notices of the Royal Astronomical Society
Issue number4
Early online date28 Jun 2018
Publication statusPublished - 1 Oct 2018


  • Polarization
  • Quasars: emission lines
  • Quasars: general
  • Scattering


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