Recently, Dr. YUAN Zunli and his colleagues from Yunnan Observatories (YNAO) of Chinese Academy of Sciences revealed the evolution mechanism of radio-loud active galactic nucleus (AGNs) that challenges the traditional viewpoint. This study was published in The Astrophysical Journal.

Based on comprehensive observed data, researchers calculated the radio luminosity function of radio-loud AGNs using a Bayesian method, which takes full consideration of the effect of spectral index distribution of samples. They found that the cosmic evolution of radio-loud AGNs can be reproduced by a simple combination of density evolution (DE) and luminosity evolution (LE). This mixture evolution scenario can naturally explain the luminosity-dependent evolution of radio-loud AGNs. Their models fitted a large amount of data on radio luminosity functions of steep-spectrum sources and multi-frequency source counts.

Researchers also obtained the dimensionless functions of DE and LE. The DE describes the trend that the number of AGNs vary with redshift, while the LE represents the changing of the typical luminosity of AGNs with redshift. Obviously, the DE and LE represent two physically distinct evolution mechanisms.

The traditional radio luminosity function models cannot specifically distinguish the contribution of DE and LE to the cosmic evolution of radio AGNs, known as the evolution degeneracy, however, in the mixture evolution scenario proposed by YUAN's group, the degeneracy of DE and LE could be broken.

The luminosity-dependent evolution of radio-loud AGNs has been found in the previous studies, i.e., powerful radio-loud AGNs undergo very rapid evolution to a redshift of z ~ 3 , while their lower luminosity counterparts only experience much milder positive evolution to z ~ 1. Astronomers usually assume that the low- and high-power radio-loud AGNs evolve differently.

However, YUAN's group argued that both populations experience the same combined evolution of DE and LE. A luminosity-dependent evolution is mimicked because the low-power sources are more sensitive to DE, while the high-power sources are more sensitive to LE.

"The nature of radio-loud AGN's cosmic evolution is still an open question,” YUAN says, “the mixture evolution scenario is a way to give an answer.”