Spectropolarimetry of high-redshift obscured and red quasars

Abstract

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 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% in the continuum. We propose a model in which dust scattering is 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 is 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 degree swing in the polarization position angle as a function of line-of-sight velocity in the emission lines of Lyα, C IV and N V. We interpret this phenomenon in the framework of a geometric model with an equatorial dusty scattering region in which the material is outflowing at several thousand km sec −1 . Emission lines may also be scattered by dust or resonantly. This 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 extremely red quasars. Our data and modeling 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.

Host galaxies of high-redshift extremely red and obscured quasars

“…This regime has only recently been probed by numerical simulations (S adowski & Narayan 2016; Dai et al 2018), which demonstrate that at high accretion rates, wide-angle out-flows ubiquitously form and carry an appreciable fraction of the accretion power. The extreme wind activity seen in our targets on multiple scales Alexandroff et al 2018;Perrotta et al 2019) is qualitatively consistent with super-Eddington accretion, so our multi-wavelength observations of ERQs support this scenario, and other populations of candidate super-Eddington accretors also show powerful outflow activity (Done et al 2007;Ni et al 2018). Furthermore, simulations confirm earlier suggestions that due to photon advection, near-and super-Eddington accretion tends to be radiatively inefficient (Abramowicz et al 1988).…”

“…In the last target neither position angle is well determined, but our best attempt yields ∼ 40 degrees. The clearest measurement is in SDSS J1515+1757, with Figure 7 demonstrating that the position angle of the major axis of the fitted galaxy is 30 degrees E of N, whereas Figure A2 of Alexandroff et al (2018) shows that the reddest polarization position angle measurement is 80 degrees E of N, leading to the offset between the two of 50±10 degrees.…”

ERQs are the BOSS of quasar samples: the highest velocity [O iii] quasar outflows

“…Another possibility is that the continuum light from the quasar is scattered into our line of sight by the surrounding medium. Alexandroff et al (2018) found that the rest-frame UV continua of two ERQs have a polarization between 10 and 15 per cent. Costa et al (2018) include trapped IR radiation pressure in cosmological radiation-hydrodynamic simulations.…”

“…F 20c m is the 20 cm radio flux from FIRST where no entry means the source was not covered by FIRST, 0.0 indicates a non-detection with 5σ upper limit ∼1 mJy (Becker et al 1995;Helfand et al 2015), non-zero entries are measurements with SNR > 3 as recorded in DR12Q. Alexandroff et al (2018) the core ERQs (see Hamann et al 2017). For convenience, we will refer to this entire sample as "ERQs" throughout the remainder of this article.…”

“…Seven sources are part of previous X-shooter and GNIRS programs (Zakamska et al 2016 andAlexandroff et al 2018; see Table 1). In particular, the spectrum of J135608.32+073017.2 is unpublished from the GNIRS program by Alexandroff et al (2018).…”