The environment of radio sources in the VLA-COSMOS survey field^⋆

¹ University of Bologna, Department of Physics and Astronomy (DIFA), v.le Berti Pichat 6/2, 40127 Bologna, Italy
e-mail: nicola.malavasi@unibo.it
² INAF–Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille), UMR 7326, 13388 Marseille, France

Received: 13 October 2014
Accepted: 24 December 2014

Abstract

Context. Several problems with the process of galaxy formation are still open. One of them is the role played by active galactic nuclei (AGNs) phenomena in galaxy build-up and, in particular, in quenching star formation (SF). On the other hand, the theory of AGN formation predicts that these phenomena are correlated with the host-galaxy environment, therefore opening links among SF quenching, environment and AGN phenomena in galaxy formation, and the evolution paradigm.

Aims. This work focuses on the correlations among environmental density, radio AGN presence and the probability that a galaxy hosts a radio AGN.

Methods. Using data from the photometric COSMOS survey and its radio 1.4 GHz follow-up (VLA-COSMOS), a sample of radio AGNs has been defined. The environment was studied using the richness distributions inside a parallelepiped with base side of 1 Mpc and height proportional to the photometric redshift precision. Richness distributions were compared as a function of both the redshift and the relative evolution of the stellar masses of galaxies and AGN hosts up to z = 2.

Results. Radio AGNs are always located in environments that are significantly richer and denser than those around galaxies in which radio emission is absent, so the environment seems to enhance the probability of a galaxy hosting a radio AGN. Moreover, a distinction between high-power and low-power radio AGNs leads to the conclusion that the significance of the environmental effect is only maintained for low-power radio sources. By studying the evolution of stellar masses, it is possible to conclude that radio AGN presence is a phenomenon related to quiescent galaxies up to z = 2, with a significant increase in the fraction of quiescent galaxies hosting a radio AGN with decreasing redshift. Hints of an environmental effect are present as well.

Conclusions. The results found with this work lead to concluding that denser environments play a significant role in enhancing the probability that a galaxy hosts a radio AGN and, in particular, low-power ones.