Evidence for a maximum jet efficiency for the most powerful radio galaxies

TL;DR

This study uses mid-infrared, radio, and optical data from z~1 radio galaxies to reveal a maximum jet efficiency, indicating a limit to how effectively accreted energy is converted into jet power.

Contribution

It provides evidence for a maximum jet efficiency in powerful radio galaxies, linking accretion rate and jet power with a fundamental efficiency limit.

Findings

Positive correlation between mid-IR, optical, and radio luminosities.

Existence of a maximum jet efficiency close to 100%.

Minimum accretion rate required for high jet power.

Abstract

We use new mid-infrared (mid-IR) photometry from the Spitzer Space Telescope to study the relations between low-frequency radio luminosity density L_151MHz, mid-IR (12um rest-frame) luminosity L_12um, and optical-emission-line ([OII]) luminosity L_[OII], for a complete sample of z~1 radio galaxies from the 3CRR, 6CE, 6C*, 7CRS and TOOT00 surveys. The narrow redshift span of our sample (0.9<z<1.1) means that it is unbiased to evolutionary effects. We find evidence that these three quantities are positively correlated. The scaling between L_12um and L_[OII] is similar to that seen in other AGN samples, consistent with both L_12um and L_[OII] tracing accretion rate. We show that the positive correlation between L_12um and L_151MHz implies that there is a genuine lack of objects with low values of L_12um at high values of L_151MHz. Given that L_12um traces accretion rate, while L_151MHz traces jet power, this can be understood in terms of a minimum accretion rate being necessary to produce a given jet power. This implies that there is a maximum efficiency with which accreted energy can be chanelled into jet power and that this efficiency is of order unity.