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 L-nu 151 (MHz), mid-IR (12 mu m rest frame) luminosity nu L-nu 12 (mu m) and optical emission-line ([O II]) luminosity L-[O II], for a complete sample of z similar to 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 nu L-nu 12 (mu m) and L-[O II] is similar to that seen in other active galactic nuclei samples, consistent with both nu L-nu 12 (mu m) and L-[O II] tracing accretion rate. We show that the positive correlation between nu L-nu 12 (mu m) and L-nu 151 (MHz) implies that there is a genuine lack of objects with low values of nu L-nu 12 (mu m) at high values of L-nu 151 (MHz). Given that nu L-nu 12 (mu m) traces accretion rate, while L-nu 151 (MHz) 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.
Published inMonthly Notices of the Royal Astronomical Society
RelationsSchool of Physics, Astronomy and Mathematics
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