The Black-Hole Masses of High-Redshift QSOs

TL;DR

This paper proposes that high-redshift quasars' large black hole mass estimates may be overestimated due to beaming effects, similar to ultraluminous X-ray sources, implying they could have smaller, stellar-mass black holes.

Contribution

It introduces a model where beaming and outflows affect mass estimates of high-redshift quasars, challenging the assumption of supermassive black holes at early cosmic times.

Findings

Beaming causes overestimation of black hole masses in quasars.

Emission-line properties remain similar in beamed and unbeamed sources.

High-redshift quasars may host stellar-mass black holes, not supermassive ones.

Abstract

Observations of high-redshift quasars frequently promote suggestions of large black hole masses, whose presence so early in cosmic time is not easily explicable. I consider the parallel with ultraluminous X-ray sources (ULXs) -- now known to be stellar-mass black hole (and neutron star) binaries apparently radiating far above their Eddington luminosities $L_{\rm Edd}$. The true luminosity in ULXs is actually only of order $L_{\rm Edd}$, for {\it stellar-mass} accretors, but has a very anisotropic (`beamed') component, plus a near-isotropic component of similar luminosity but much lower specific intensity. Observers viewing ULXs from within the beam but assuming spherical symmetry deduce a luminosity $\gg L_{\rm Edd}$. These features appear because the accretors are fed mass at highly super-Eddington rates, most of it expelled in high-speed ($v >0.2c$) outflows from the accretion disc. I show that in similarly-beamed AGN, emission-line properties would be essentially the same as in unbeamed sources, but standard virial mass indicators unusable because velocity widths are dominated by the outflows, not bound motions about the black holes. In an ensemble of this kind the apparently most luminous systems are always the most distant, but have the lowest black hole masses. Interpreting observations of this ensemble without knowing that they are beamed leads instead to very high black hole mass estimates. The analogy with ULXs therefore suggests that high-redshift quasars might actually have central black hole masses which could have grown from stellar values within the lookback time. I consider how one might test these ideas observationally.