Research Note

GTC optical imaging of extremely red 5C radio galaxies at high redshift

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: andrew.humphrey@astro.up.pt
² Instituto Nacional de Astrofísica, Óptica y Electrónica (INAOE), Aptdo. Postal 51 y 216, 72000 Puebla, Pue., Mexico
³ Centro de Astrobiología (INTA-CSIC), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain

Received: 25 April 2015
Accepted: 26 August 2015

Abstract

Aims. We investigate the nature of seven unusual radio galaxies from the 5C catalogue that were previously known to have extremely red R−K colours, and for which emission lines were previously found to be weak or absent in their optical spectra.

Methods. We present and discuss u, g, or r images of these radio galaxies, obtained using the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS) at the Gran Telescopio Canarias (GTC).

Results. We have detected all seven targets in our g-band imaging. Their optical emission is extended, and we tentatively detect a radio-optical alignment effect in this sample. A subset of our sample (three sources) shows broad-band spectral energy distributions that flatten out near the wavelength range of the g-band, implying a dominant contribution there due to young stars and/or scattered or reprocessed radiation from the active nucleus.