Infrared-faint radio sources are at high redshifts

Abstract

Context. Infrared-faint radio sources (IFRS) are characterised by relatively high radio flux densities and associated faint or even absent infrared and optical counterparts. The resulting extremely high radio-to-infrared flux density ratios up to several thousands were previously known only for high-redshift radio galaxies (HzRGs), suggesting a link between the two classes of object. However, the optical and infrared faintness of IFRS makes their study difficult. Prior to this work, no redshift was known for any IFRS in the Australia Telescope Large Area Survey (ATLAS) fields which would help to put IFRS in the context of other classes of object, especially of HzRGs. Aims. This work aims at measuring the first redshifts of IFRS in the ATLAS fields. Furthermore, we test the hypothesis that IFRS are similar to HzRGs, that they are higher-redshift or dust-obscured versions of these massive galaxies. Methods. A sample of IFRS was spectroscopically observed using the Focal Reducer and Low Dispersion Spectrograph 2 (FORS2) at the Very Large Telescope (VLT). The data were calibrated based on the Image Reduction and Analysis Facility (IRAF) and redshifts extracted from the final spectra, where possible. This information was then used to calculate rest-frame luminosities, and to perform the first spectral energy distribution modelling of IFRS based on redshifts. Results. We found redshifts of 1.84, 2.13, and 2.76, for three IFRS, confirming the suggested high-redshift character of this class of object. These redshifts and the resulting luminosities show IFRS to be similar to HzRGs, supporting our hypothesis. We found further evidence that fainter IFRS are at even higher redshifts. Conclusions. Considering the similarities between IFRS and HzRGs substantiated in this work, the detection of IFRS, which have a significantly higher sky density than HzRGs, increases the number of active galactic nuclei in the early universe and adds to the problems of explaining the formation of supermassive black holes shortly after the Big Bang.

Infrared-faint radio sources remain undetected at far-infrared wavelengths

“…Furthermore, Norris et al suggest that IFRS might follow the correlation between 3.6 µm flux density and redshift found for the sample of HzRGs by Seymour et al, similar to the K − z relation for other radio galaxies (Willott et al 2003). Collier et al (2014) and Herzog et al (2014) tested this hypothesis in the reachable redshift range between 2 and 3 and find IFRS follow this correlation. This adds evidence to the suggestion that IR-fainter IFRS are located at even higher redshift, potentially reaching z ≈ 5 or 6.…”

“…Additionally, the results by Collier et al (2014) and Herzog et al (2014) finally disposed of the alternative hypothesis that IFRS are hotspots of spatially separated radio galaxies. If the IR detections were real counterparts of the radio detections, the redshift distribution of these objects would be expected to follow that of Fanaroff Riley type 2 (FRII) galaxies which peaks at z ∼ 1.…”

“…The outlier is expected to be a misidentification or an SFG with an AGN in its centre. Herzog et al (2014) measured redshifts of three IFRS in the deep ATLAS fields based on optical spectroscopy from the Very Large Telescope (VLT) and found them to be in the range 1.8 < z < 2.8. The IFRS with spectroscopic redshifts, both from Collier et al and Herzog et al, lie at the IR-bright boundary of this class of objects with IR flux densities between 14 µJy and 30 µJy at 3.6 µm or 3.4 µm.…”

“…Based on the redshifts measured in their work, Herzog et al (2014) present the first redshift-based SED modelling of IFRS. They find the templates of an RL AGN (3C 273) and a compact-steep spectrum (CSS) source (3C 48) in agreement with all available photometric data, while an ultraluminous IR galaxy (ULIRG, Arp 220) or a Seyfert galaxy (Mrk 231) are clearly ruled out.…”

ATCA detections of massive molecular gas reservoirs in dusty, high-zradio galaxies

“…The two HzRGs studied via their CO lines by Emonts et al (2011) also display no spectral curvature between 1.4 and 30 GHz (observed frame). Herzog et al (2016) also report that a high fraction (∼75%) of the radio SEDs of infrared-faint radio sources (IFRS) can be modelled by a simple power law, extending as far as 105 GHz (observed frame). The radio properties of IFRS are reported to be consistent with the general population of radio-loud AGN at high redshift, with similar fractions of com- pact steep spectrum (CSS) and gigahertz-peaked spectrum (GPS) sources.…”