Optically faint radio sources: reborn AGN?

¹ Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150–762 Porto, Portugal
e-mail: mfilho@astro.up.pt
² Leiden Observatory, University of Leiden, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Departamento de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre 687, 4169–007 Porto, Portugal
⁴ Centro de Investigação em Ciências Geo-Espaciais, Faculdade de Ciências da Universidade do Porto, Porto, Portugal
⁵ SIM, Faculdade de Ciências da Universidade de Lisboa, Lisboa, Portugal

Received: 5 August 2011
Accepted: 26 September 2011

Abstract

We present our discovery of several relatively strong radio sources in the field-of-view of SDSS galaxy clusters that have no optical counterparts down to the magnitude limits of the SDSS. The optically faint radio sources appear as double-lobed or core-jet objects in the FIRST radio images and have projected angular sizes ranging from 0.5 to 1.0 arcmin. We followed-up these sources with near-infrared imaging using the wide-field imager HAWK-I on the VLT. We detected K_s-band emitting regions, about 1.5 arcsec in size and coincident with the centers of the radio structures, in all sources, with magnitudes in the range 17–20 mag. We used spectral modelling to characterize the sample sources. In general, the radio properties are similar to those observed in 3CRR sources but the optical-radio slopes are consistent with those of moderate to high redshift (z < 4) gigahertz-peaked spectrum sources. Our results suggest that these unusual objects are galaxies whose black hole has been recently re-ignited but that retain large-scale radio structures, which are signatures of previous AGN activity.