Mutual distance dependence drives the observed jet-power–radio-luminosity scaling relations in radio galaxies

• Published in: Monthly notices of the Royal Astronomical Society 2016-02, Vol.456 (2), p.1172-1184
• Main Authors: Godfrey, L. E. H., Shabala, S. S.
• Format: Article
• Language: English
• Subjects: Astrophysics; Correlation; Correlation analysis; Dynamics; Feedback; Luminosity; Mathematical models; Radio; Radio galaxies; Slopes; Star & galaxy formation; X-ray astronomy
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stv2712

The kinetic power of radio jets is a quantity of fundamental importance to studies of the AGN feedback process and radio galaxy physics. A widely used proxy for jet power is the extended radio luminosity. A number of empirical methods have been used to calibrate a scaling relationship between jet power (Q) and radio luminosity (L) of the form log (Q) = β L  log (L) + C. The regression slope has typically been found to be β L  ∼ 0.7–0.8. Here we show that the previously reported scaling relations are strongly affected by the confounding variable, distance. We find that in a sample of FRI X-ray cavity systems, after accounting for the mutual distance dependence, the jet power and radio luminosity are only weakly correlated, with slope β L ≈ 0.3: significantly flatter than previously reported. We also find that in previously used samples of high-power sources, no evidence for an intrinsic correlation is present when the effect of distance is accounted for. Using a simple model we show that β L is expected to be significantly lower in samples of FRI radio galaxies than it is for FRIIs, due to the differing dynamics for these two classes of radio source. For FRI X-ray cavity systems the model predicts β L (FRI) ≳ 0.5 in contrast to FRII radio galaxies, for which β L (FRII) ≳ 0.8. We discuss the implications of our finding for studies of radio mode feedback, and radio galaxy physics.