Deep Spitzer observations of infrared-faint radio sources: high-redshift radio-loud AGN?

TL;DR

This study uses deep near-infrared observations from Spitzer to investigate infrared-faint radio sources, suggesting most are high-redshift, dust-obscured radio-loud galaxies with extremely high radio-infrared flux ratios.

Contribution

The paper provides the first sensitive near-infrared observations of IFRSs, revealing their likely high-redshift, dust-obscured nature and extreme flux ratios, advancing understanding of these rare objects.

Findings

Most IFRSs are undetected at ~1 μJy in deep IR observations.

Stacked images suggest median flux density ≤ 0.2 μJy at 3.6 μm.

Majority are likely high-redshift, dust-obscured radio-loud AGN.

Abstract

Infrared-faint radio sources (IFRSs) are a rare class of object which are relatively bright at radio wavelengths but very faint at infrared and optical wavelengths. Here we present sensitive near-infrared observations of a sample of these sources taken as part of the Spitzer Extragalactic Representative Volume Survey (SERVS). Nearly all the IFRSs are undetected at a level of ~ 1 \mu$Jy in these new deep observations, and even the detections are consistent with confusion with unrelated galaxies. A stacked image implies that the median flux density is $S_{3.6\mu m} ~ 0.2$ \mu$Jy or less, giving extreme values of the radio-infrared flux density ratio. Comparison of these objects with known classes of object suggests that the majority are probably high-redshift radio-loud galaxies, possibly suffering from significant dust extinction.