Infrared-faint radio sources remain undetected at far-infrared wavelengths. Deep photometric observations using the Herschel Space Observatory

Abstract

Showing 1.4 GHz flux densities in the range of a few to a few tens of mJy, infrared-faint radio sources (IFRS) are a type of galaxy characterised by faint or absent near-infrared counterparts and consequently extreme radio-to-infrared flux density ratios up to several thousand. Recent studies showed that IFRS are radio-loud active galactic nuclei (AGNs) at redshifts >=2. This work explores the far-infrared emission of IFRS, providing crucial information on the star forming and AGN activity of IFRS and on the potential link between IFRS and high-redshift radio galaxies (HzRGs). A sample of six IFRS was observed with the Herschel Space Observatory between 100 um and 500 um. Using these results, we constrained the nature of IFRS by modelling their broad-band spectral energy distribution (SED). Furthermore, we set an upper limit on their infrared SED and decomposed their emission into contributions from an AGN and from star forming activity. All six observed IFRS were undetected in all five Herschel far-infrared channels (stacking limits: sigma = 0.74 mJy at 100 um, sigma = 3.45 mJy at 500 um). Based on our SED modelling, we find that the IFRS analysed in this work can only be explained by objects that fulfil the selection criteria of HzRGs. More precisely, IFRS could be (a) known HzRGs at very high redshifts (z >= 10.5), (b) low-luminosity siblings of HzRGs with additional dust obscuration at lower redshifts, (c) scaled or unscaled versions of Cygnus A at any redshift, and (d) scaled and dust-obscured radio-loud quasars or compact steep spectrum sources. We estimated upper limits on the infrared luminosity, the black hole accretion rate, and the star formation rate of IFRS, which all agreed with corresponding numbers of HzRGs.

15 pages, 10 figures, accepted for publication in A&A

References in corpus (11)

• The Massive Hosts of Radio Galaxies Across Cosmic Time
• Deep ATLAS radio observations of the CDFS-SWIRE field
• A Unified Catalog of Radio Objects Detected by NVSS, FIRST, WENSS, GB6, and SDSS
• Rapidly growing black holes and host galaxies in the distant Universe from the Herschel Radio Galaxy Evolution Project
• The XMM-Newton survey of the ELAIS-S1 field. I: Number counts, angular correlation function and X-ray spectral properties
• Infrared-Faint Radio Sources: A New Population of High-redshift Radio Galaxies
• VLBI detection of an Infrared-Faint Radio Source
• Deep 610-MHz Giant Metrewave Radio Telescope observations of the Spitzer extragalactic First Look Survey field - III. The radio properties of Infrared-Faint Radio Sources
• The first VLBI image of an Infrared-Faint Radio Source
• Infrared-Faint Radio Sources are at high redshifts
• Active galactic nuclei cores in infrared-faint radio sources: Very long baseline interferometry observations using the Very Long Baseline Array

Cited by in corpus (5)

• The radio spectral energy distribution of infrared-faint radio sources
• The Redshift Distribution of Infrared-Faint Radio Sources
• Infrared-Faint Radio Sources in the SERVS deep fields: Pinpointing AGNs at high redshift
• On the nature of infrared-faint radio sources in the SXDF and VLA-VVDS fields
• Multiwavelength Properties of Infrared-Faint Radio Sources Based on Spectral Energy Distribution Analysis