Deep Spitzer observations of infrared-faint radio sources: high-redshift 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.

Full text

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Deep Spitzer observations of infrared-faint radio sources:
high-redshift radio-loud AGN?
Ray P. Norris,1Jose Afonso,2,3Antonio Cava,4Duncan Farrah,5Minh T. Huynh,6
R.J. Ivison,7,8Matt Jarvis,9Mark Lacy,10Minnie Mao,1,6,11,12Claudia Maraston,13
Jean-Christophe Mauduit,6Enno Middelberg,14Seb Oliver,5Nick Seymour15and
Jason Surace6
1CSIRO Australia Telescope National Facility, PO Box 76, Epping, NSW, 1710, Australia,
email: Ray.Norris@csiro.au
2Observat´ orio Astron´ omico de Lisboa, Faculdade de Ciˆ encias, Universidade de Lisboa,
Tapada da Ajuda, 1349-018 Lisbon, Portugal
3Centro de Astronomia e Astrof´ ısica da Universidade de Lisboa, 1349-018 Lisbon, Portugal
4Departamento de Astrof´ ısica, Facultad de CC. F´ ısicas, Universidad Complutense de
Madrid, E-28040 Madrid, Spain
5Astronomy Centre, Dept. of Physics & Astronomy, University of Sussex, Brighton BN1
9QH, UK
6Infrared Processing and Analysis Center, MS220-6, California Institute of Technology,
Pasadena CA 91125, USA
7UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9
3HJ, UK
8Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
9Centre for Astrophysics, Science & Technology Research Institute, University of
Hertfordshire, Hatfield, Herts, AL10 9AB, UK
10NRAO, 520 Edgemont Road, Charlottesville, VA 22903, USA
11School of Mathematics and Physics, University of Tasmania, Private Bag 37, Hobart,
7001, Australia
12Anglo-Australian Observatory, PO Box 296, Epping, NSW, 1710, Australia
13Institute of Cosmology and Gravitation, Dennis Sciama Building, Burnaby Road,
Portsmouth, PO1 3FX, UK
14Astronomisches Institut, Ruhr-Universit¨ at Bochum, Universit¨ atsstr. 150, 44801 Bochum,
Germany
15Mullard Space Science Laboratory, UCL, Holmbury St Mary, Dorking, Surrey, RH5 6NT,
UK
ABSTRACT
Infrared-faint radio sources (IFRSs) are a rare class of object which are rel-
atively bright at radio wavelengths but very faint at infrared and optical wave-
arXiv:1105.0960v1 [astro-ph.CO] 4 May 2011

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lengths. 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 ∼ 1µJy 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
S3.6µm∼ 0.2 µJy 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.
Subject headings: galaxies: formation — galaxies: evolution — galaxies: star-
burst
1. Introduction
Infrared-faint radio sources (IFRSs) are a rare class of object which are relatively strong
at radio wavelengths but very faint at infrared (IR) and optical wavelengths. They were
first categorised in the Australia Telescope Large Area Survey (ATLAS: Norris et al. 2006)
as radio sources with no observable IR counterpart in the co-spatial Spitzer Wide-area IR
Extragalactic Survey (Lonsdale et al. 2004). Most have flux densities of a few hundredµJy
at 20 cm, but some are as bright as 20mJy. They may be related to the optically invisible
radio sources found by Higdon et al. (2005, 2008), which are compact radio sources with
no optical counterpart to R ∼ 25.7, although the IFRSs seem even more extreme than the
Higdon objects. Norris et al. (2006) and Middelberg et al. (2008a) have identified 51 such
sources out of 2002 radio sources in the ATLAS survey.
So far, four samples of IFRS have been identified: (a) the original ATLAS/SWIRE
samples identified by Norris et al. (2006) and Middelberg et al. (2008a), from which the
sources in this paper are drawn, (b) the sample in the Spitzer First-Look Survey, identified
by Garn & Alexander (2008), (c) the sample in ELAIS-N1 identified by Grant et al. (2011),
(d) a sample in the COSMOS field (Scoville et al. 2007) identified by Zinn et al. (2011).
These samples imply a sky density of of ∼7 per deg2for S20cm> 0.1mJy.
At the time of their discovery, the IFRS sources were unexpected, as SWIRE was thought
to be deep enough to detect all extragalactic radio sources at z?2, regardless of whether star
formation or active galactic nuclei (AGN) powered the radio emission. Possible explanations
were that these sources are i) high-redshift radio-loud AGN, ii) very obscured radio galaxies
at more moderate redshifts (1 < z < 2), iii) lobes of nearby but unidentified radio galaxies,

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iv) very obscured, luminous starburst galaxies, such as high-redshift submillimetre-selected
galaxies (SMGs – Smail et al. 1997), v) high-latitude pulsars, or vi) an unknown type of
object. Of course it is also possible that they do not constitute a homogeneous class and
harbour examples of some or all of the above.
The nature of IFRSs has been hard to determine because nearly all the information on
them has been obtained at radio wavelengths. Spectroscopy is difficult because the hosts
are optically faint and the radio positions can also have uncertainties of the order of a few
arcsec. Norris et al. (2006) stacked the positions of 22 IFRSs in the Spitzer 3.6µm IRAC
images and found no detection in the averaged image, showing that they are well below the
SWIRE detection threshold. This was a surprising result at the time, as it was expected that
the IFRSs represented the tail of a distribution reaching just below the SWIRE detection
threshold, but it was confirmed by Garn & Alexander (2008) and by the new data presented
here.
Middelberg et al. (2008b) and Norris et al. (2007) targeted six IFRSs with the Australian
Long Baseline Array (LBA) and detected two of the sources. The Norris et al. (2007) LBA
detection constrained the source size to less than 0.03arcsec, suggesting a compact radio
core, powered by an AGN. Middelberg et al. (2008b) found the size and radio luminosity
of their LBA-detected source to be consistent with a high-redshift (z > 1) compact, steep-
spectrum (CSS) source. The VLBI detections rule out the possibility that these particular
IFRSs are simply the radio lobes of unidentified radio galaxies, or star-forming galaxies,
though the initial VLBI targets were inevitably amongst the most radio-bright examples in
the sample.
Garn & Alexander (2008) stacked IFRS sources in the Spitzer First Look Survey at
infrared wavelengths, as well as at 610MHz. The sources they find in the FLS are very
similar to the sources described here. They find that the IFRS sources can be modelled as
compact Fanaroff Riley type ii (FRii) radio galaxies at high redshift (z ? 4), and argue that
IFRSs are predominately high-redshift radio-loud AGN.
Huynh et al. (2010) used deep data from the Spitzer IRAC MUSYC Public Legacy in E-
CDFS (SIMPLE) project and the Far-Infrared Deep Extragalactic Legacy (FIDEL) Spitzer
survey to probe more deeply in the E-CDFS region, and detected two of the four IFRSs in
that region. However, the two non-detections, and the faintness of the two detected sources,
enabled Huynh et al. to place constraints on the sources. Their detailed modelling of their
spectral energy distributions shows that they are consistent with high-redshift (z > 1) AGN.
They also noted that the ratio of 20cm flux density, S20cm, to 3.6µm flux density, S3.6µm,
is higher than that of the general radio source population, and has significant overlap with
the population of high-redshift radio galaxies (HzRG) investigated previously with Spitzer

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observations (Seymour et al. 2007) and is also consistent with the radio galaxy K−z relation
extended to high redshifts (Jarvis et al. 2001; De Breuck et al. 2002; Willott et al. 2003).
Middelberg et al. (2011) have measured the spectral indices of the radio emission at
several wavelengths of a sample of 17 strong IFRSs and find that they are significantly
different from the general radio source population, and also different from the general AGN
population. The spectra are steep, typically with α ∼ −1, and there is a particularly
prominent lack of sources with α > −0.7 (where Sν∝ να). Grant et al. (2011) also reported
steep spectral indices, and both Middelberg et al. (2011) and Grant et al. (2011) found
that several of the IFRSs were significantly polarised, suggesting an AGN rather than a
star-forming galaxy.
Each of these papers adds to a consensus view that the IFRSs represent a class of high-
redshift radio-loud galaxies. However, there is not yet unequivocal evidence for this, and it
is possible that the IFRS class constitutes several different types of objects.
Here we present near-IR data taken as part of the Spitzer Extragalactic Representative
Volume Survey (SERVS) project (Mauduit et al. 2011), which uses the data taken during
the Spitzer warm mission to probe extended regions of the sky to a sensitivity some five
times deeper than SWIRE. The SERVS project includes several square degrees around the
ELAIS-S1 and CDFS extended fields in which IFRSs were detected as part of the ATLAS
radio survey. These deeper data provide much tighter constraints on the nature of IFRSs.
Throughout this paper we assume a Hubble constant of 71kms−1Mpc−1, and matter
and cosmological constant density parameters of ΩM= 0.27 and ΩΛ= 0.73.
2.Observations and Analysis
The Australia Telescope Large Area Survey (ATLAS) radio survey is still in progress,
covering 7 deg2at 20 cm in two regions, surrounding the CDF-S and ELAIS-S1 fields to an
r.m.s. depth of typically 10µJy/beam. However, all the radio data used in this paper are
taken from the preliminary ATLAS catalogues (Norris et al. 2006; Middelberg et al. 2008a)
which surveyed the entire region to an r.m.s. depth of typically 20–30µJy/beam.
SERVS (Mauduit et al. 2011) is a medium-deep survey at 3.6µm and 4.5µm which
exploits the warm phase mission of Spitzer, to cover an 18 deg2field, which includes most of
the ATLAS fields.
For the work described in this paper, we used SERVS data in the CDFS and ELAIS-S1
fields. Only band 1 (at 3.6µm) of the SERVS data was used because it is intrinsically more

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sensitive for most galaxy spectral energy distributions than the band 2 (4.5µm) data. Fig. 1
shows the IR and radio coverage of these fields. The SERVS data on the CDFS area covers
nearly all of the area observed by SWIRE and ATLAS, but the ELAIS-S1 coverage is offset
from the centre of the SWIRE/ATLAS field by about a degree. Nevertheless, a total of 39
IFRSs are common to both datasets, and are listed in Table 1.
For each IFRS source detected at radio wavelengths, we took the initial positional un-
certainty (δi, in R.A. and Dec.) from the values listed by Norris et al. (2006) and Middelberg
et al. (2008a), which were calculated from the quadrature sum of the formal fitting uncer-
tainty and a 0.1 arcsec potential uncertainty in the position of the calibration source. We
also consider the formal positional uncertainty due to noise δnoise= 0.3 ∗ θb∗ (Srms/Speak),
where θbis the synthesised full-width-half-maximum beamwidth, Srmsis the local r.m.s. flux
density, and Speakis the peak flux density of the source, as discussed by Ivison et al. (2007).
We further considered any positional accuracy of less than 1arcsec to be unrealistic because
of systematic offsets and intrinsic source sizes. We derive a final 3σ positional uncertainty
δfinalin R.A. and Dec. as being the maximum of 3∗δi, 3∗δnoise, and 1 arcsec. These positional
uncertainties are listed in Table 1.
For each IFRS source, we examined the SERVS 3.6µm data for sources visible by eye
as a distinct peak above the noise which fell within the error ellipse. Using this technique,
we found that 3 of the 39 IFRS sources contained a 3.6µm candidate source within the 3σ
radio position error ellipse. Images of two representative IFRSs are shown in Fig. 2, with
their associated error ellipse.
We then performed aperture photometry on each of these candidates. The flux density
was measured using an aperture radius of 1.9 arcsec, which was found to be optimal in
SWIRE. An aperture correction of 1.4 was applied for this radius (Surace et al. 2005).
The uncertainty in the aperture flux density was estimated by measuring the r.m.s. of 100
randomly placed apertures placed near the source. The resulting flux densities are listed in
Table 1.
The r.m.s. noise of the SERVS data, measured in a region of sky free from visible sources,
can approach ∼ 0.2µJy. However, since SERVS data are typically confusion limited, such an
r.m.s. can be misleading if used to calculate uncertainties. Instead, we placed 1000 random
apertures across the image, and followed an iterative approach, rejecting apertures with a
flux density ? 2.5 σ and recalculating the r.m.s. until the process converged. The r.m.s. thus
obtained was 0.51µJy in the ELAIS-S1 field and 0.64µJy in the CDF-S field.
As the SERVS data approach the confusion limit of Spitzer, it is necessary to estimate
how many of the candidate cross-identifications are due to confusion. We estimated this in

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two independent ways.
Our most reliable estimate was obtained by shifting the IFRS radio positions by an
arbitrary amount (typically ∼20arcsec) which is much greater than the beamsize of either
the radio or IR data, but much smaller than the scale size of variations in the image sampling.
We then examined the image to estimate how many IR sources fall by chance within the error
ellipse. This is a very robust way of estimating the confusion, as it builds in the varying error
ellipse size, and the non-uniform sensitivity, using the real data rather than a parameterised
representation. Because it uses real data, with exactly the same process in both cases, it is
immune to calibration or other systematic errors.
We then repeated this process ten times, shifting the positions by a different amount
(<1arcmin in all cases). A mean of 2.1 ± 1.2 of the shifted error ellipses contained a peak
in the Spitzer data. Thus, of our 3 candidate identifications with unshifted data, we expect
that 2.1 of these are due to confusion, leaving only zero or one genuine detections.
As a check on this result, we use the source counts calculated by Barmby et al. (2008).
Our radio position error ellipses have a total area of 266arcsec2, and we have found three
candidate identifications within this area, all of which are brighter than a flux density limit
of ∼ 1.1µJy. . Barmby et al. (2008) calculate the density of sources with S3.6µm> 1.1µJy as
205,000 deg−2, from which we estimate that we should detect ∼ 4 sources by chance. This is
greater than the number detected in the shifted data, but the difference is clearly dominated
by small number statistics. Nevertheless, the Barmby result does provide a useful rough
cross-check on our shifting technique.
In summary, of our 39 IFRSs which lie within the SERVS fields, we find that only 3 of
these have a source within the radio position error ellipse, none has a measured flux density
greater than five times the formal fitting uncertainty, and our basic Monte Carlo simulations
suggest that most or all of these detections are due to chance. We conclude that few or none
of our sources have reliable detections and the vast majority of IFRSs are undetected at
this level. Such faint emission from a source which is relatively strong at radio wavelengths
represents an extreme condition which is not common in the local Universe.
The S3.6µmdistribution of IFRS can be explored to even deeper levels by stacking 3.6µm
images at the IFRS positions. In Fig. 3, we show a median stacked image obtained by
summing 39 3.6µm images extracted from the SERVS data, centered on the IFRS radio
position. Because the stacking has reached the confusion limit for these data, the r.m.s.
noise no longer scales as the square root of integration time, and so the r.m.s. of this stacked
image is higher than would be obtained in an unconfused field, although it still offers a
significant improvement over the individual images. We note that Garn & Alexander (2008)

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faced a similar challenge in stacking 3.6µm data from 8 IFRS in the First Look Survey,
reaching a noise of approximately 1µJy in the image stack.
The measured r.m.s. of the median stacked image is 0.14µJy, and there is marginal
evidence for a source at the field centre whose flux density was measured to be 0.21± 0.14µJy.
Using a 3σ upper limit, we conclude that the IFRSs have a median flux density of ? 0.63µJy
at 3.6µm.
3. Radio/IR properties of IFRSs
Since we have no redshift information for any of the IFRSs, we focus here on two derived
quantities: the ratio of S20cmand S3.6µm, or its lower limit, and the S3.6µmflux density, or
its upper limit.
Of the 39 IFRS which lie within the SERVS fields, we find that, after allowing for
confusion, only 1 or 2 are detected and none has a measured flux density greater than five
times the formal fitting uncertainty. The remaining sources are undetected at this level.
From this we deduce that the vast majority of IFRSs have a ratio of S20cmto S3.6µmin the
range 200–2,000.
In Fig. 4 we show the ratios of S20cmto S3.6µmas a function of redshift for a representative
selection of models, and mark the limits obtained in this paper. It is clear from this figure
that the only objects known to have such a high ratio are radio-loud AGN, such as the high-
redshift radio galaxies (HzRGs). In particular, we can rule out any known type of galaxy
powered predominantly by star formation, such as ULIRGs, SMGs, etc., all of which fall well
below the region occupied by IFRSs.
If they are radio galaxies similar to those in the low-redshift Universe, then we can use
their brightness to obtain constraints on redshift. In Fig. 5. we show the S3.6µmof HzRGs
as a function of redshift, together with the limits obtained from our SERVS observations.
The HzRGs follow a relation between redshift and S3.6µm similar to the well-known K-z
relation for other radio galaxies (Willott et al. 2003). Although we caution that both these
relationships are unreliable above z>3, they imply that if the IFRSs are radio galaxies, then
their low 3.6µm flux densities constrain them to lie at high redshift.

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4. What are IFRSs?
Whatever the nature of IFRSs, their properties are extreme, and not consistent with
any of the well-recognised classes of object. Here we review the options for what this class
of object is likely to be.
• Star-forming galaxies. Fig. 4 shows that no known class of galaxy powered predom-
inantly by star formation, such as ULIRGs, SMGs, etc., have ratios of 20cm to 3.6µm
flux densities comparable with those of IFRS. A star-forming galaxy might appear in
the region if it suffered from an unusually high extinction, but no star-forming galaxy
is known with such high extinction. Even Arp220 has a S20cm/S3.6µmratio a factor of
20 below the most moderate IFRS, and a factor of 4000 below the most extreme IFRS.
Such high extinction could in principle be found in a star-forming galaxy if it were
at high redshift (z > 3), where the observed 3.6µm emission is generated in visible
wavelengths in the galaxy rest-frame. However, no known star-forming galaxy gener-
ates sufficient radio power to reproduce the observed IFRS flux density. For example,
Arp220 at z = 3 would have an observed flux of ∼ 5µJy.
The observed 20cm radio luminosities of the IFRSs range from 7 × 1023WHz−1(for
the weakest IFRS at z = 1) to 1.4 × 1026WHz−1at z=1 to 7 × 1027WHz−1(for the
strongest IFRS at z = 5). The most luminous star-forming galaxies, typified by SMGs
at z ∼ 2 − 3, can have luminosities ∼ 1024WHz−1(e.g. Ivison et al. 1998; Seymour
et al. 2009), so that while the faintest of our galaxies could be caused by star-forming
galaxies, most are too radio-luminous.
In addition, two of the six IFRSs observed with VLBI were detected by Norris et al.
(2007) and Middelberg et al. (2008b). The VLBI detections rule out star formation
in these particular galaxies as the radio emission mechanism, since the synchrotron
emission from star-forming galaxies rarely has sufficient brightness temperature to be
detectable with VLBI (e.g. Kewley et al. 2000; Biggs et al. 2010).
Furthermore, Middelberg et al. (2011) and Grant et al. (2011) show that the radio
emission from several of the IFRSs studied by them is significantly polarised. This also
argues against star formation, in which polarisation is generally much lower than in
AGN.
We conclude that the majority of IFRSs are unlikely to represent star-forming galaxies.
• Radio lobes An early hypothesis was that IFRSs might be the extended radio lobes
of an AGN whose host may be located some distance away. The VLBI detections by
Norris et al. (2007) and Middelberg et al. (2008b) imply that at least about a third of

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IFRSs have high brightness temperature cores, which rules out the possibility that they
are radio lobes of AGN, since radio lobes do not have sufficient surface brightness to
be detectable with these VLBI observations. Furthermore, most IFRS are unresolved
in the high-resolution observations by Middelberg et al. (2011), making it unlikely that
they are extended radio lobes.
• Pulsars Cameron et al. (2011) tested the hypothesis that IFRSs may be pulsars, by
performing a pulsar search on a sample of IFRSs. Their results show that any putative
pulsars in the field have a pulsed flux density well below the observed flux density of
the IFRS, and they conclude that the IFRS are not radio pulsars.
• Radio-loud AGN The extreme values of the ratio of S20cm to S3.6µm reported in
Fig. 4 are known to occur in radio-loud AGN, and have not been observed in any other
type of extragalactic object. All the available observations (radio/IR flux densities,
radio/IR ratio, VLBI, polarisation) are consistent with the cores of radio-loud AGN,
and we conclude that radio-loud AGN are therefore a natural explanation of IFRS.
If IFRSs are caused by radio-loud AGN, their remaining unusual aspect is the extreme
faintness at 3.6µm. We now consider three possible causes:
• Dwarf galaxies that host a radio-loud AGN If a radio-loud AGN were to be hosted
by a dwarf galaxy at moderate redshift, then the low luminosity of the host galaxy
could reproduce both the faint 3.6µm flux density and the high S20cmto S3.6µmratio
of the IFRS. However, such objects are observationally unknown, and theoretically
unlikely given the weak potential well of a dwarf galaxy.
• Moderate-redshift radio-loud AGN with heavy dust extinction Fig. 5 shows
that known moderate-redshift (z ? 3) AGN would be detected in our 3.6µm obser-
vations. We can postulate moderate-redshift objects which are heavily obscured at
3.6µm, but the amounts of extinction required are very high. For example, 3C273 at
z = 1 would require an extinction of AV ∼ 50mto occupy the position of IFRSs in
Fig. 5. On the other at hand, at high redshift (z > 2), where the observed 3.6µm emis-
sion is generated in visible wavelengths in the galaxy rest-frame, as little as Av= 10
magnitudes of extinction is sufficient to raise the track of a radio-loud QSO into the
regime of the IFRS.
However, even if the AGN is obscured by dust, it is hard to obscure the host galaxy
to the low 3.6µm flux limits presented here. Even in Arp220, where as much as 100
magnitudes of extinction obscure the nucleus (e.g. Haas et al. 2001), the outer shell of

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the host galaxy is still bright at IR wavelengths. To reproduce the extreme IR-faintness
of the IFRS, the extinction must cover the entire body of the galaxy .
We therefore cannot exclude the possibility that the IFRSs could be a new class of
galaxy at z ∼ 1–2, in which the entire galaxy must be shrouded in dust, reducing its
integrated 3.6µm flux density by a factor of 100.
• High-redshift radio-loud AGN Fig. 5 and the K-z relation show that, if IFRSs
belong to a parent population similar to that of known radio-loud AGN, they must lie
at z ≥ 3. We note that obscured high-redshift radio galaxies and quasars have been
detected by Mart´ ınez-Sansigre et al. (2006); Dey et al. (2008); Yan et al. (2007) and
Sajina et al. (2007). While IFRSs bear some similarities to these galaxies, IFRSs are
even more extreme in their S20cm to S3.6µm ratios. For example, Mart´ ınez-Sansigre
et al. (2006) choose their sample by requiring that S3.6µm< 45µJy, whilst the IFRSs
have S3.6µm? 1µJy. While none of the IFRSs are detected at 24µm, the available
SWIRE data at 24µm are relatively insensitive compared to the 3.6µm SERVS data,
and, given the very low S3.6µm, only a very extreme spectral energy distribution would
enable them to be detected at 24µm.
We conclude that the most natural explanation for IFRSs is that they are very similar
to known classes of radio-loud AGN, but at a redshift ≥ 3. In all these properties, the IFRS
most resemble the HzRGs of Seymour et al. (2007), Ivison et al. (2008) and Jarvis et al.
(2009), but with even more extreme 20cm to 3.6µm flux density ratios. However, we cannot
exclude the possibility that they are a new class of lower-redshift (1 < z < 3) radio-loud
AGN in which the luminosity of the entire host galaxy is reduced by a factor of ∼ 100 by
dust extinction.
The 20cm flux densities of our IFRS range from 0.14 to 26mJy. The weakest end of
the range corresponds to a luminosity 7×1023WHz−1at z = 1 to 4×1025WHz−1at z = 5,
giving it the luminosity of an FRi galaxy at any reasonable redshift. On the other hand,
the brightest end of the range corresponds to a luminosity 1.4 × 1026WHz−1at z = 1 to
7 × 1027WHz−1at z = 5, making it a FRii galaxy at any reasonable redshift. Thus the
class of IFRS spans both the FRi and FRii luminosity classes, with the majority of galaxies
straddling the FRi/FRii break, depending on their redshift.
It would be instructive to consider these results in terms of black hole mass, MBH, but
Snellen et al. (2003) have shown there is no good correlation between radio luminosity and
black hole mass for radio-loud galaxies and so we cannot estimate a black hole mass except
in the most general terms. But we do have good observational evidence for the 3.6µm flux
densities of host galaxies of high-redshift radio galaxies, which is shown in Fig. 5.

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We have detected 51 IFRS in the 7deg2area of ATLAS, implying a density of IFRSs on
the sky of ∼7 per deg2for S20cm> 0.1mJy. It is difficult to make a meaningful comparison
of the numbers of IFRSs with source counts at the present time without a better constraint
on redshift. The SKADS simulation (Wilman et al. 2008) gives a sky density of 0.5 FRii
galaxies deg−2at z ? 4, implying that only 3–4 of the IFRSs discussed here are FRii
galaxies at z ? 4. However, the space density of all but the highest-luminosity, FRII-type
radio sources are very poorly constrained at high redshifts, and AGN models are very poorly
constrained by current observations at the low flux densities probed here. Thus it is possible
(Zinn et al. 2011) that the IFRS could be radio-loud AGN of moderate radio luminosity and
much higher space density than their more powerful counterparts.
5. Conclusion
We have searched for IR counterparts to 39 IR-faint radio sources using deep 3.6µm
observations from SERVS. Even though the sensitivity is 3–5 times better than the previous
observations (with SWIRE), few or none of the IFRSs are detected after taking into account
the number of chance associations, and a stacked image indicates a median 3.6µm flux
density of 0.21 ±0.14µJy. This places extreme constraints on the properties of these sources,
making it likely that they are radio-loud AGN at redshifts z ? 3, or heavily dust-obscured
radio-loud AGN at redshifts z ? 1. While some may have radio-to-IR ratios similar to
3C273, but at a much higher redshift, the most extreme of them require several magnitudes
of obscuration in the optical/NIR to remain undetected by deep imaging.
While we cannot rule out the possibility that more than one type of object may be
represented by IFRSs, the evidence suggests that a significant proportion, if not all, of the
IFRS sources are either
• radio-loud AGN (similar to known high-redshift radio-loud AGN) at z ? 3, or
• a new class of lower-redshift (1 < z < 3) radio-loud AGN in which the luminosity of
the entire host galaxy is severely reduced by dust extinction.
Acknowledgements
This work is based in part on observations made with Spitzer, which is operated by the
Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA.
Support for this work was provided by NASA through an award issued by JPL/Caltech. This

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research has made use of the NASA/IPAC Extragalactic Database (NED) which is operated
by the Jet Propulsion Laboratory, California Institute of Technology, under contract with
the National Aeronautics and Space Administration. The Australia Telescope is funded by
the Commonwealth of Australia for operation as a National Facility managed by CSIRO.
JA gratefully acknowledges the support from the Science and Technology Foundation (FCT,
Portugal) through the research grant PTDC/FIS/100170/2008. MYM acknowledges the
support of an Australian Postgraduate Award as well as Postgraduate Scholarships from
AAO and ATNF.
We thank P. Barmby for providing unpublished details of her source density calculations,
Barnaby Norris for generating the stacked median image, and George Hobbs for helpful
comments on the pulsar density at high galactic latitude.

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Fig. 1.— Field coverage of the data presented here. The left hand image shows the region
surrounding the CDFS field, while the right hand image shows the ELAIS-S1 image. The
greyscale shows the radio images, taken from Norris et al. (2006) and Middelberg et al.
(2008a) and the circles show the positions of the IFRSs. The solid lines show the SERVS
coverage. The 39 sources discussed in this paper, and listed in Table 1, are those shown here
which also lie within the SERVS coverage.

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Fig. 2.— Two representative IFRS sources. The greyscale is the 3.6µm SERVS data, and
the contours are the 20 cm image, with contour levels of (1, 2, 3, 4, 5) mJy/beam. The left
hand image is a non-detection(CS0194) and the right-hand image is a candidate detection
(CS0114).

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Fig. 3.— The 3.6µm median stacked image of the IFRSs, obtained by calculating the median
of 39 images extracted from the SERVS 3.6µm data, centered on the IFRS radio positions.
The r.m.s. noise of the image is 0.14µJy, and the marginal detection at the centre has a flux
density (measured using aperture photometry) of 0.21 ±0.14µJy. Both axes are in units of
pixels, each of which is 0.6*0.6 arcsec.