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




We use new mid-infrared (mid-IR) photometry from the Spitzer Space Telescope to study the relations between low-frequency radio luminosity density inline image, mid-IR (12 μm rest frame) luminosity inline image and optical emission-line ([O ii]) luminosity L]O ii], 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 inline image and L[O ii] is similar to that seen in other active galactic nuclei samples, consistent with both inline image and L[O ii] tracing accretion rate. We show that the positive correlation between inline image and inline image implies that there is a genuine lack of objects with low values of inline image at high values of inline image. Given that inline image traces accretion rate, while inline image 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 this efficiency is of the order of unity.