The Extreme Red Excess in Blazar Ultraviolet Broad Emission Lines

TL;DR

This paper investigates quasars with redward asymmetric UV broad emission lines, revealing that their redshifted emission originates from a broad component near the black hole, linked to low accretion rates and weak ionizing continua, which enhances the redshifted emission.

Contribution

It demonstrates that red asymmetric BELs in blazars are due to a luminous, redshifted broad component from gas near the black hole, driven by low ionizing flux and relativistic effects, a novel explanation for this phenomenon.

Findings

Red BELs are from gas in a broad component near the black hole.

Low Eddington rates and jet power ratios correlate with red BELs.

Weak ionizing flux explains high efficiency of redshifted BEL emission.

Abstract

We present a study of quasars with very redward asymmetric (RA) ultraviolet (UV) broad emission lines (BELs). An excess of redshifted emission has been previously shown to occur in the BELs of radio loud quasars and is most extreme in certain blazars. Paradoxically, blazars are objects that are characterized by a highly relativistic blue-shifted outflow towards Earth. We show that the red emitting gas resides in a very broad component (VBC) that is typical of Population B quasars that are defined by a wide H$\beta$ BEL profile. Empirically, we find that RA BEL blazars have both low Eddington rates ($\lesssim1\%$) and an inordinately large (order unity) ratio of long term time averaged jet power to accretion luminosity. The latter circumstance has been previously shown to be associated with a depressed extreme UV ionizing continuum. Both properties conspire to produce a low flux of ionizing photons, two orders of magnitude less than typical Population B quasars. We use CLOUDY models to demonstrate that a weak ionizing flux is required for gas near the central black hole to be optimally ionized to radiate BELs with high efficiency (most quasars over-ionize nearby gas, resulting in low radiative efficiency). The large gravitational redshift and transverse Doppler shift results in a VBC that is redshifted by $\sim 2000 -5000$~km~s$^{-1}$ with a correspondingly large line width. The RA BELs result from an enhanced efficiency (relative to typical Population B quasars) to produce a luminous, redshifted VBC near the central black hole.