Optically Faint Radio Sources: Reborn AGN?,
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 Ks-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.
Many new radio sources have been discovered over the last 15 years and a significant fraction of these have been associated with faint star-forming galaxies. Some of these sources, however, have no detectable optical or near-infrared counterparts, and are believed to be associated with high-redshift Active Galactic Nuclei (AGN).
The team began by looking for radio sources located in the fields-of-view of known galaxy clusters. This was done by comparing the data from the maxBCG cluster catalog (obtained from the Sloan Digital Sky Survey) and the Faint Images of the Radio Sky at Twenty centimeter (FIRST) catalog, obtained with the Very Large Array. With this, the researchers were able to identify 291 cluster fields with a FIRST radio source located (in projection) near the center of the cluster.
During the process of radio source identification, the team discovered 8 FIRST radio sources with no optical counterpart in the SDSS images. These 8 targets were followed-up in the near-infrared with the Very Large Telescope, identifying a near-infrared source (host galaxy) coincident with the center of the radio structure. Using photometric redshifts, the team concluded that the sources presented redshifts higher than the ones of the galaxy clusters in which they were embedded and, therefore, were not physically associated with them.
The comparison of the photometric data of the radio sources with other classes of objects showed some similarities with the radio sources found in the Third Cambridge Revised Catalogue of Radio Sources (3CRR); however, the radio-to-near-infrared ratio (Fig. 2) is more consistent with gigahertz-peaked spectrum sources, which are thought to be young, compact radio sources.
In order to reconcile the observed large-scale (FIRST) radio structure, signature of a longer-lived jet (AGN) activity, with the young radio source nature of the radio-to-near-infrared ratio, the team has evoked the hypothesis known as the “double-double” radio galaxy scenario. During an active phase, an AGN will produce large-scale radio lobes fed by a jet. When the AGN is turned off, the jet ceases to feed the lobes and these fade away on timescales of about 107 years. Thus, if an AGN is reactivated within this period, it should be possible to observe the aged radio lobes, while still retaining the characteristics of a “young” radio source.
The work developed on this paper had a very strong contribution from CAUP members. Namely, the CAUP researchers obtained the observational data, performed the data reduction, analysis and modeling and also drew the conclusions presented in the paper.