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_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.
