Fanaroff-Riley dichotomy of radio galaxies and the Malmquist bias

TL;DR

This paper investigates whether the observed dependence of the Fanaroff-Riley classification break on optical host galaxy luminosity is actually due to Malmquist bias, rather than an intrinsic property, by analyzing correlations in a flux-limited radio galaxy sample.

Contribution

It demonstrates that the apparent correlation between radio luminosity and optical luminosity may be caused by Malmquist bias, challenging previous interpretations of the FR I/II dichotomy.

Findings

Correlations between redshift and optical luminosity are stronger than those between radio and optical luminosity.

Malmquist bias can produce spurious correlations in flux-limited samples.

Similar biases may affect other claimed correlations in radio galaxy studies.

Abstract

We examine the possibility that a claimed dependence of the FR 1/2 break value in radio luminosity on the absolute magnitude of the optical host galaxy could be due to the Malmquist bias, where a redshift-luminosity correlation appears in a flux-limited sample because of an observational selection effect. In such a sample, the redshift dependence of a phenomenon could appear as a luminosity-dependent effect and may not be really representing an intrinsic property of the radio sample. We test this on the radio complete MRC (Molonglo Reference Catalog) sample, where Spearman rank correlation and Kendall rank correlation tests show that the correlations are indeed stronger between the redshift and the optical luminosity than that between the radio luminosity and the optical luminosity, suggesting that the latter correlation perhaps arises because of the Malmquist Bias. We further show that similar effects of the Malmquist bias could also be present elsewhere in other correlations claimed in the literature between the radio luminosity and other observed properties of FR1 and FR2 sources.