The radio spectral energy distribution of infrared-faint radio sources^⋆

¹ Astronomisches Institut, Ruhr-Universität Bochum, Universitätsstr. 150, 44801 Bochum, Germany
e-mail: herzog@astro.rub.de
² Macquarie University, Sydney, NSW 2109, Australia
³ CSIRO Astronomy and Space Science, Marsfield, PO Box 76, Epping, NSW 1710, Australia
⁴ Western Sydney University, Locked Bag 1797, Penrith South, NSW 1797, Australia
⁵ International Centre for Radio Astronomy Research, Curtin University, Bentley, WA 6102, Australia
⁶ Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
⁷ Centro de Astrobiología (INTA-CSIC), Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
⁸ Sydney Institute for Astronomy, School of Physics, The University of Sydney, NSW 2006, Australia
⁹ National Radio Astronomy Observatory, PO Box O, 1003 Lopezville Road, Socorro, NM 87801, USA
¹⁰ Department of Physics, Chong Yeut Ming Physics Building, The University of Hong Kong, Pokfulam, Hong Kong, Japan
¹¹ National Centre for Radio Astrophysics, TIFR, Post Bag 3, Pune University Campus, 411007 Pune, India
¹² SKA SA, 3rd Floor, The Park, Park Road, 7405 Pinelands, South Africa
¹³ Department of Physics and Electronics, Rhodes University, PO Box 94, 6140 Grahamstown, South Africa
¹⁴ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
¹⁵ School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA
¹⁶ Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia
¹⁷ MIT Haystack Observatory, Westford, MA 01886, USA
¹⁸ ARC Centre of Excellence for All-sky Astrophysics(CAASTRO), The University of Sydney, Australia
¹⁹ Raman Research Institute, 560080 Bangalore, India
²⁰ International Centre for Radio Astronomy Research, University of Western Australia, 35 Stirling Hwy, WA 6009, Crawley, Australia
²¹ Department of Physics, University of Washington, Seattle, WA 98195, USA
²² School of Chemical & Physical Sciences, Victoria University of Wellington, PO Box 600, 6140 Wellington, New Zealand
²³ Department of Physics, University of Wisconsin–Milwaukee, Milwaukee, WI 53201, USA
²⁴ School of Physics, The University of Melbourne, VIC 3010, Parkville, Australia
²⁵ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
²⁶ National Centre for Radio Astrophysics, Tata Institute for Fundamental Research, 411007 Pune, India
²⁷ The Netherlands Institute for Radio Astronomy (ASTRON), Postbus 2, 7990 AA Dwingeloo, The Netherlands

Received: 20 July 2015
Accepted: 8 July 2016

Abstract

Context. Infrared-faint radio sources (IFRS) are a class of radio-loud (RL) active galactic nuclei (AGN) at high redshifts (z ≥ 1.7) that are characterised by their relative infrared faintness, resulting in enormous radio-to-infrared flux density ratios of up to several thousand.

Aims. Because of their optical and infrared faintness, it is very challenging to study IFRS at these wavelengths. However, IFRS are relatively bright in the radio regime with 1.4 GHz flux densities of a few to a few tens of mJy. Therefore, the radio regime is the most promising wavelength regime in which to constrain their nature. We aim to test the hypothesis that IFRS are young AGN, particularly GHz peaked-spectrum (GPS) and compact steep-spectrum (CSS) sources that have a low frequency turnover.

Methods. We use the rich radio data set available for the Australia Telescope Large Area Survey fields, covering the frequency range between 150 MHz and 34 GHz with up to 19 wavebands from different telescopes, and build radio spectral energy distributions (SEDs) for 34 IFRS. We then study the radio properties of this class of object with respect to turnover, spectral index, and behaviour towards higher frequencies. We also present the highest-frequency radio observations of an IFRS, observed with the Plateau de Bure Interferometer at 105 GHz, and model the multi-wavelength and radio-far-infrared SED of this source.

Results. We find IFRS usually follow single power laws down to observed frequencies of around 150 MHz. Mostly, the radio SEDs are steep (α < −0.8; 74⁺⁶_-9%), but we also find ultra-steep SEDs (α < −1.3; 6⁺⁷_-2%). In particular, IFRS show statistically significantly steeper radio SEDs than the broader RL AGN population. Our analysis reveals that the fractions of GPS and CSS sources in the population of IFRS are consistent with the fractions in the broader RL AGN population. We find that at least 18⁺⁸_-5% of IFRS contain young AGN, although the fraction might be significantly higher as suggested by the steep SEDs and the compact morphology of IFRS. The detailed multi-wavelength SED modelling of one IFRS shows that it is different from ordinary AGN, although it is consistent with a composite starburst-AGN model with a star formation rate of 170 M_⊙ yr^-1.