Infrared-faint radio sources in the SERVS deep fields

Pinpointing AGNs at high redshift

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, viale B. Pichat 6/2, 40127 Bologna, Italy
e-mail: alessandro.maini@studio.unibo.it
² Department of Physics and Astronomy, Macquarie University, Balaclava Road, North Ryde, NSW, 2109, Australia
³ INAF-IRA, via P. Gobetti 101, 40129 Bologna, Italy
⁴ CSIRO Astronomy & Space Science, PO Box 76, Epping, NSW 1710, Australia
⁵ Western Sydney University, Locked Bag 1797, Penrith South, NSW 1797, Australia
⁶ Australian Astronomical Observatories, PO Box 915, North Ryde, NSW 1670, Australia
⁷ NRAO, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁸ Netherlands Foundation for Research in Astronomy, PO Box 2, 7990 AA Dwingeloo, The Netherlands
⁹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands

Received: 25 July 2016
Accepted: 23 August 2016

Abstract

Context. Infrared-faint radio sources (IFRS) represent an unexpected class of objects which are relatively bright at radio wavelength, but unusually faint at infrared (IR) and optical wavelengths. A recent and extensive campaign on the radio-brightest IFRSs (S_{1.4 GHz}≳ 10 mJy) has provided evidence that most of them (if not all) contain an active galactic nuclei (AGN). Still uncertain is the nature of the radio-faintest IFRSs (S_{1.4 GHz}≲ 1 mJy).

Aims. The scope of this paper is to assess the nature of the radio-faintest IFRSs, testing their classification and improving the knowledge of their IR properties by making use of the most sensitive IR survey available so far: the Spitzer Extragalactic Representative Volume Survey (SERVS). We also explore how the criteria of IFRSs can be fine-tuned to pinpoint radio-loud AGNs at very high redshift (z > 4).

Methods. We analysed a number of IFRS samples identified in SERVS fields, including a new sample (21 sources) extracted from the Lockman Hole. 3.6 and 4.5 μm IR counterparts of the 64 sources located in the SERVS fields were searched for and, when detected, their IR properties were studied.

Results. We compared the radio/IR properties of the IR-detected IFRSs with those expected for a number of known classes of objects. We found that IR-detected IFRSs are mostly consistent with a mixture of high-redshift (z ≳ 3) radio-loud AGNs. The faintest ones (S_{1.4 GHz} ~ 100 μJy), however, could be also associated with nearer (z ~ 2) dust-enshrouded star-burst galaxies. We also argue that, while IFRSs with radio-to-IR ratios >500 can very efficiently pinpoint radio-loud AGNs at redshift 2 < z < 4, lower radio-to-IR ratios (~100–200) are expected for higher redshift radio-loud AGNs.