The radio properties of infrared-faint radio sources

¹ Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany
e-mail: middelberg@astro.rub.de
² CSIRO Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia
³ Sydney Institute for Astronomy, The University of Sydney, NSW 2006, Australia
⁴ Mullard Space Science Laboratory, UCL, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK
⁵ School of Chemical and Physical Sciences, Victoria University of Wellington, PO Box 600, Wellington, New Zealand
⁶ Infrared Processing and Analysis Center, MS220-6, California Institute of Technology, Pasadena CA 91125, USA
⁷ School of Mathematics and Physics, University of Tasmania, Private Bag 37, Hobart 7001, Australia

Received: 4 May 2010
Accepted: 20 October 2010

Abstract

Context. Infrared-faint radio sources (IFRS) are objects that have flux densities of several mJy at 1.4 GHz, but that are invisible at 3.6 μm when using sensitive Spitzer observations with μJy sensitivities. Their nature is unclear and difficult to investigate since they are only visible in the radio.

Aims. High-resolution radio images and comprehensive spectral coverage can yield constraints on the emission mechanisms of IFRS and can give hints to similarities with known objects.

Methods. We imaged a sample of 17 IFRS at 4.8 GHz and 8.6 GHz with the Australia Telescope Compact Array to determine the structures on arcsecond scales. We added radio data from other observing projects and from the literature to obtain broad-band radio spectra.

Results. We find that the sources in our sample are either resolved out at the higher frequencies or are compact at resolutions of a few arcsec, which implies that they are smaller than a typical galaxy. The spectra of IFRS are remarkably steep, with a median spectral index of −1.4 and a prominent lack of spectral indices larger than −0.7. We also find that, given the IR non-detections, the ratio of 1.4 GHz flux density to 3.6 μm flux density is very high, and this puts them into the same regime as high-redshift radio galaxies.

Conclusions. The evidence that IFRS are predominantly high-redshift sources driven by active galactic nuclei (AGN) is strong, even though not all IFRS may be caused by the same phenomenon. Compared to the rare and painstakingly collected high-redshift radio galaxies, IFRS appear to be much more abundant, but less luminous, AGN-driven galaxies at similar cosmological distances.