Peculiar emission line spectra of core extremely red BOSS quasars at z∼2–3: orientation and/or evolution?

Abstract

Context. Core extremely red quasars (core ERQ) have been proposed to represent an intermediate evolutionary phase in which a heavily obscured quasar blows out the circumnuclear interstellar medium with very energetic outflows before it becomes an optical quasar. Aims. We investigate whether the properties of core ERQ fit the AGN orientation-based unification scenario. Methods. We revised the general UV and optical emission line properties of core ERQ in the context of the orientation-based scenario. We used diagnostic diagrams based on UV emission line ratios and UV-to-optical line kinematic information to compare the physical and kinematic gas properties of core ERQ with those of other luminous narrow- and broad-line AGN. In particular, we provide a revised comparison of the [OIII] kinematics in 21 core ERQ (20 from Perrotta et al. 2019, MNRAS, 488, 4126 and SDSS J171420.38+414815.7, based on GTC EMIR near-infrared spectroscopy) with other samples of quasars with matching luminosity with the aim of evaluating whether core ERQ host the most extreme [OIII] outflows. Results. The UV line ratios suggest that the physical properties (e.g., density and metallicity) of the ionised gas in core ERQ are similar to those observed in the broad-line region of blue nitrogen-loud quasars. The [OIII] outflow velocities of core ERQ are on average consistent with those of very luminous blue type 1 quasars, although extreme outflows are much more frequent in core ERQ. These similarities can be explained in the context of the AGN unification model under the assumption that core ERQ are viewed with an intermediate orientation between type 2 (edge-on) and type 1 (face-on) quasars. Conclusions. We propose that core ERQ are very luminous but otherwise normal quasars viewed at an intermediate orientation. This orientation allows a direct view of the outer part of the large broad-line region from which core ERQ UV line emission originates; the extreme [OIII] outflow velocities are instead a consequence of the very high luminosity of core ERQ.

Carbon-loud SDSS BOSS type II quasars at z > 2: high-density gas or secondary production of carbon?

“…be viewed, in addition to the low density NLR. As noted by Villar Martin et al (2020), the core-ERQs lie at the high N v/C iv edge of the QSO2 cloud (see Fig. 1), implying they have have some of the highest values of N abundance and/or ionization parameter of any obscured quasars.…”

“…• The UV emission line ratios of the core-ERQs place them within the cloud defined by the QSO2s that have extreme values for the C iv/He ii ratio (C iv/He ii > 4), suggesting that the observed emission line gas in both classes of object has broadly similar physical conditions, i.e., strong contamination from intermediate/high density gas in the outer BLR, likely due to orientation and/or geometrical effects (see also Villar Martin et al 2020).…”

“…• Looking at the HzRGs and QSO2s, we suggest that the differences between them is due to (i) an orientation effect where the QSO2s are viewed at an angle that reveals high/intermediate density gas in the outskirts of the BLR, and/or (ii) the extension of high/intermediate density gas above the plane of the obscuring torus Villar Martin et al (2020).…”

“…We also include four 'core extremely red quasars' (core-ERQs; see Ross et al 2015, Zakamska et al 2016, Hamann et al 2017, Villar Martin et al 2020 in the redshift range 2.3 < z < 2.6. It has been proposed that core-ERQs are in a strongly dust-obscured evolutionary stage, where the central engine is in the process of removing gas and dust from its vicinity via powerful outflows, after which it will evolve into a classical unobscured quasar (Zakamska et al 2016).…”

“…1 and 2), the observational data shows an order of magnitude range in U = 2×10 −3 U = 1.0 10 2 cm −3 10 4 cm −3 10 6 cm −3 10 7 cm −3 10 8 cm −3 10 −2 10 0 10 2 10 −2 10 −1 10 0 10 −2 10 0 10 2 10 −2 10 −1 10 0 10 −2 10 −1 10 0 Models using secondary Nitrogen and primary Carbon Z/Z = 0.5 Z/Z = 1.0 Z/Z = 2.0 Z/Z = 3.0 Z/Z = 4.0 Z/Z = 5.0 NV/CIV NV/HeII Figure 1. Comparison of the observed emission line ratios from the SDSS type II quasars divided in objects with C iv/He ii < 4 and C iv/He ii > 4 (purple filled diamond and yellow filled circles, respectively), Keck II HzRGs from Vernet et al (2001) (blue filled triangles), BLRG 2025-218 (red filled triangle) from Humphrey et al (2008b) and core-ERQ (green filled hexagon) from Villar Martin et al (2020) with photoionization models using ionizing continuum power law index α = -1.0. Each diagram presents a different gas metallicity, i.e., Z/Z = 0.5, 1.0, 2.0, 3.0, 4.0, 5.0.…”