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Centro de Astrofísica da Universidade do Porto

CAUP Researchers: Mercedes E. Filho, Jarle Brinchmann[also member of the Leiden Observatory], Catarina Lobo
Team at CAUP: Galaxies and Observational Cosmology
Other Researchers: Sonia Antón (PT)

Optically Faint Radio Sources: Reborn AGN?,
Astronomy and Astrophysics, Volume 536, pp. A35 (2011)

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.

Figure 1: The first radio source 2596. The near-infrared (white) superimposed on the radio (red) image of a part of the maxBCG 2596 cluster region, showing the FIRST radio source.

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.

Figure 2: The radio-NIR ratio as a function of redshift for different galaxy templates (solid and dashed lines). The dotted lines are the radio-NIR ratios of our sample sources. The radio flux density of the optically faint radio sources pertains to the total FIRST radio flux density, from the sum of the integrated power of all the FIRST components and the NIR emission pertains to the central NIR component, which is coincident with the center of the radio structure. The NIR magnitude system is Vega. The large dots are the median redshift values for the 3CR (dashed; Spinrad 1985) and GPS (solid; Labiano et al. 2007) sample sources, with a one s errorbar.

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Institute of Astrophysics and Space Sciences (IA) is a new but long anticipated research infrastructure with a national dimension. It embodies a bold but feasible vision for the development of Astronomy, Astrophysics and Space Sciences in Portugal, taking full advantage and fully realizing the potential created by the national membership of the European Space Agency (ESA) and the European Southern Observatory (ESO). IA resulted from the merging the two most prominent research units in the field in Portugal: the Centre for Astrophysics of the University of Porto (CAUP) and the Center for Astronomy and Astrophysics of the University of Lisbon (CAAUL). It currently hosts more than two-thirds of all active researchers working in Space Sciences in Portugal, and is responsible for an even greater fraction of the national productivity in international ISI journals in the area of Space Sciences. This is the scientific area with the highest relative impact factor (1.65 times above the international average) and the field with the highest average number of citations per article for Portugal.

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