Sources of the Radio Background Considered Page: 2 of 12
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2 J. Singal ct al.
emitting halo of the intensity needed to explain the signal1.
Additionally, assuming a diffuse Galactic origin for the mea-
sured radio signal would require a source of radio emission
that does not follow the correlation with far-infrared emis-
sion observed in local galaxies, again making our Galaxy
anomalous. Isotropic radio emission from a local region on
the scale of the local bubble accounting for the level observed
would manifest an all-sky quadrupole polarization pattern
at a level visible in WMAP 23 GHz data, but such a pattern
is not seen. These lines of evidence are summarized in Kogut
et al. (2010). Furthermore, the considerations of 3 in this
paper can be used to place a strong constraint on the amount
of the total observed high latitude radio emission that can
Galactic in origin, as in the halo inverse compton scatter-
ing on the ambient light would produce a much stronger
X-ray background than observed, again strongly disfavoring
a Galactic origin as an explaination for the signal. In this pa-
per, then, we investigate possible origins for the radio back-
ground, assuming the extragalactic level is that reported by
the ARCADE 2 collaboration, although we conclude that
producing the background at the level observed is difficult
without a mechanism to cause significant redshift evolution
in the radio to far infrared correlation beyond what has been
observed thus far.
Interferometric surveys have achieved a relatively con-
sistent picture of radio source counts above the l0pJy level.
At high fluxes, say between 1 mJy and 1 Jy at 1.4 GHz, ra-
dio loud active galactic nuclei (AGN) dominate radio source
counts (e.g., Condon 2007; Windhorst et al. 1993), with their
differential counts following a power law dN/dS a S-7 with
y < 2.5 (the so-called Euclidean value). Below 1 mJy, this
trend tends to reverse and the source counts show a more
rapid increase with decreasing flux, perhaps indicating emer-
gence of a new population which does not contribute at
higher fluxes but dominates in this regime (e.g., Condon
2007; Simpson et al. 2006; Hopkins et al. 2003). Gervasi
et al. (2008) have calculated the radio brightness resulting
from fitting a two population model and extrapolating avail-
able radio source counts to fainter fluxes at frequencies from
150 MHz to 8.5 GHz. They derive a surface brightness that
is 3 to 6 times smaller than that reported by the ARCADE
2 collaboration. Clearly there must be significant contribu-
tions to the radio background from sources not considered
by Gervasi et al. (2008).
ARCADE 2 and other instruments used to determine
the diffuse background intensity have resolutions greater
than 1 degree, and cannot distinguish between a background
due to discrete sources of angular sizes smaller than one de-
gree, and ones that are truly diffuse. Therefore, we consider
several candidates for comprising the background.
The emission mechanism of sources producing the CRB
must be synchrotron radiation by relativistic electrons. This
puts powerful constraints on potential sources. They must
not overproduce the measured diffuse far-infrared back-
ground if related to star forming activity, nor overproduce
the measured diffuse X-ray/y-ray background through in-
1 One may add here that this is true even taking into ac-
count presence of 'anomalous large-scale radio continuum fea-
tures' present in some edge-on spirals (Hummel, van Gorkom,
& Kotanyi 1983; Elmouttie et al. 1995).
verse compton (IC) scattering of the cosmic microwave back-
ground (CMB) and other background photon fields by the
same electrons. Additionally, the source spectra must be
consistent with the CRB power-law spectrum of index 0.6,
In this paper, we investigate several kinds of possible
candidates. In 2 we analyze the existing data from radio
source counts, and using some approximate fitting proce-
dures estimate the contribution from sources observed in nu-
merous surveys. In 3 we use the X-ray/y-ray background to
limit the contribution from more diffuse emission, clusters,
and the intergalactic medium, and in 4 we examine whether
source count surveys have missed a significant amount of
flux. In 5.1 and 5.2 we explore the potential contribution
from radio supernovae and radio quiet quasars, and finally,
in 5.3 we conclude that emission from star forming galax-
ies may account for bulk of the measured radio background,
but only if the radio/far-infrared luminosity ratio increases
with redshift. A brief summary and discussion is presented
2 RADIO SOURCE COUNTS
Figure 1 shows a schematic but fairly accurate depiction of
radio source counts available in the literature (see Table 1).
In the top panel of the figure, we plot the S25 dN/dS distri-
butions given in different radio surveys (note that the differ-
ential count is per steradian per unit flux). The observations
were conducted at a range of frequencies (0.151 - 8.5 GHz).
We have converted all fluxes to 1.4 GHz assuming a general
power-law spectrum with an average (redshift independent)
spectral index equal to 0.75, the canonical value of the ex-
tragalactic radio sources. Note that even though the surveys
considered have a wide range of angular resolution (spanning
from 1 to 300 arcseconds) there is not only a clear agreement
among the counts at S > 1 mJy, but also in regards to the
presence of the relatively poorly constrained low-flux
population emerging at S < 1 mJy.
In the bottom panel of Figure 1 we plot a proxy for
1.4 GHz total surface brightness due to radio sources,
S2 (dN/dS), divided by the observed CRB brightness as
reported by the ARCADE collaboration in Fixsen et al.
BCRB(l) 1.4 GHz
TCRB(Q) = 1.17 x
The true surface brightness
Bcnts (S) = S dS
1 2 dN
(y- 2) dS
for a power law dN/dS = S--, is equal to the area under
the S2 (dN/dS) curve, and thus the bottom pannel of Figure
1 provides directly a minimum fractional contribution to the
CRB from the resolved objects. We estimate this fraction to
be ~ 26%. We note that the high-flux population is domi-
nated by bright radio-loud AGN, while the low-flux one is
generally thought to be dominated by starforming galaxies
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Singal, J.; Stawarz, L.; Lawrence, A. & Petrosian, V. Sources of the Radio Background Considered, article, August 22, 2011; United States. (digital.library.unt.edu/ark:/67531/metadc928747/m1/2/: accessed November 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.