Quasar Radio-Loudness and the Elliptical Core Problem

TL;DR

This paper investigates the factors determining whether elliptical galaxies host radio-loud or radio-quiet QSOs, linking accretion rates, host size, and core properties to their radio emission characteristics.

Contribution

It introduces an evolutionary framework connecting galaxy core properties and merger history to QSO radio-loudness, based on host size and accretion rate analysis.

Findings

High x-ray luminosity QSOs are mostly radio-loud.

Large ellipticals (>10 kpc) tend to host radio-loud QSOs.

Core ellipticals are associated with radio-loud QSOs, while coreless with radio-quiet.

Abstract

The dichotomy between radio-loud and radio-quiet QSOs is not simply one of host morphology. While spiral galaxies almost exclusively host radio-quiet QSOs, ellipticals can host either radio-louds or radio-quiets. We find that a combination of accretion rate and host scale determines which type of QSO a given elliptical galaxy will host. QSOs with high x-ray luminosities (above 10^44.5 erg/s at 0.5 keV) are mostly radio-loud. But those with low luminosities divide fairly neatly in size (measured by the half-light radius, r_e). Those larger than about 10 kpc are radio-loud, while smaller ones are radio-quiet. It has recently been found that core and coreless ellipticals are also divided near this limit. This implies that for low-luminosity QSOs, radio-louds are found in core ellipticals, while radio-quiets are in coreless ellipticals and spirals. This segregation also shows up strongly for low-redshift objects, and in general, there is a loss over time of coreless, radio-loud QSOs. Since the presence or absence of a core may be tied to the galactic merger history, we have an evolutionary explanation for the differences between radio-loud and radio-quiet QSOs.