Infrared-Faint Radio Sources in the SERVS deep fields: Pinpointing AGNs at high redshift

TL;DR

This study investigates the nature of infrared-faint radio sources using deep infrared surveys, revealing most are high-redshift radio-loud AGNs and proposing refined criteria for identifying such AGNs at very high redshifts.

Contribution

The paper provides new insights into the IR properties of faint IFRSs and refines selection criteria for high-redshift radio-loud AGNs using SERVS data.

Findings

Most IFRSs are consistent with high-redshift (z >= 3) radio-loud AGNs.

Faint IFRSs may also be nearer dust-enshrouded starburst galaxies.

Radio-to-IR ratio thresholds can distinguish redshift ranges of AGNs.

Abstract

Infrared-Faint Radio Sources (IFRS) represent an unexpected class of objects 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_{1.4GHz} >= 10 mJy) has provided evidence that most of them (if not all) contain an AGN. Still uncertain is the nature of the radio-faintest ones (S_{1.4GHz} <= 1 mJy). 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 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). 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 mum IR counterparts of the 64 sources located in the SERVS fields were searched for, and, when detected, their IR properties were studied. 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 they are mostly consistent with a mixture of high-redshift (z >= 3) radio-loud AGNs. The faintest ones (S_{1.4GHz} ~ 100 muJy), 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.