Evidence for a maximum jet efficiency for the most powerful radio galaxies
Abstract
We use new mid-infrared (mid-IR) photometry from the Spitzer Space Telescope to study the relations between low-frequency radio luminosity density L151MHz, mid-IR (12um rest-frame) luminosity L12um, 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 L12um and L[OII] is similar to that seen in other AGN samples, consistent with both L12um and L[OII] tracing accretion rate. We show that the positive correlation between L12um and L151MHz implies that there is a genuine lack of objects with low values of L12um at high values of L151MHz. Given that L12um traces accretion rate, while L151MHz 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.