Sources of the Radio Background Considered Page: 3 of 12
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Sources of Radio Background 3
Figure 1. TOP: Scheamatic description of the observed
S2.5dN/dS distribution from different radio surveys. Source
counts from 8.4GHz surveys are in dark blue, 2.7GHz in light
blue, 5 GHz in yellow, 1.4 GHz in green, 610 MHz in pink, 408 MHz
in red, and 151 MHz in black. Details of the radio surveys used
with references are given in Table 1. Data from all frequencies is
normalized to flux bins at 1.4GHz using a 0.75 spectral index.
BOTTOM: The S2dN/dS distribution representing the surface
brightness per logarithmic flux bin of the same surveys, divided
by the surface brightness of CRB at 1.4 GHz reported by the AR-
CADE collaboration. The source counts tell roughly a consistent
story over a wide range of frequencies and resolutions. The peak
icontribution at ~., 100 mJy comes from bright radio galaxies. The
contribution decreases with decreasing flux density, but begins to
rise again below 1 mJy as a new population of sources becomes
important. Integrating over the entire flux range probed by these
surveys gives ., 26% of the total surface brightness of the CRB
at 1.4 GHz.
(see Ballantyne 2009, and references therein), with an addi-
tional possible contribution from radio-quiet and low-radio
power AGN (e.g. Ibar et al. 2009; Padovani et al. 2009). Sur-
veys probing fluxes as low as a few pJy are consistent with
the two population model (e.g., Fomalont et al. 2002; Biggs
& Ivison 2006).
2.1 High-Flux Population
The integrated (fractional) contribution of the high flux
population (S > So 1 mJy) to the measured cosmic ra-
dio background, fr dSS (dN/dS)/BCRB, is about ~ 16%
at 1.4 GHz, and the corresponding surface number density
dN / S
dS = k o
for Smin < S < So,
one can calculate the required value of Smin as a function of
y such that the sources in this range provide the rest of the
CBR. It is easy to show that for y > 2
Smn-So (1 H) (y - 2) BCRB ] 1/(2-f)
where the factor H is the estimated fractional contribution
from sources above So ~ 1 mJy, which we determine to be
0.16. The top panel of Figure 2 shows the variation of Smin
with y at v - 1.4 GHz. We can also estimate the corre-
sponding minimum number of required sources to be
koS1- [ S - 1-7
N (> Smzn) -N(> So) = O "" 1 S
1 , (6)
whose dependence on y is shown on the bottom panel of
Figure 2. Thus, if the background is to be made primarily
from low flux sources, their faint end index below 1 mJy
should be close to y - 2.5 in order to not exceed reason-
able estimates for the total number of non-dwarf galaxies
in the obervable Universe. In other words, in order for low
flux density sources to account for the observed CRB, they
must reach low flux values of less than 1 pJy and have a
surface density of (> 1010 sr-1), but not much higher than
A modeling of the dN/dS distribution of the low-flux
(< mJy) population in terms of a single power law, although
N(> So) - f dS (dN/dS) ~ 4.7 x 105 sr -1. One could ask
if a significant portion of the flux from the high flux pop-
ulation has been missed by radio surveys. If this were the
case, then the contribution of the high flux population to the
measured background would have been underestimated. One
possible source of the missing flux could be extended low sur-
face brightness sources. However, as mentioned above, the
surveys included in Figure 1, in spite of the fact that they
span a wide range of resolutions and are obtained from dif-
ferent interferometer arrays, show very good agreement on
the integrated contribution to the background. We will re-
turn to the contribution of low surface brightness sources
and quantify the possible missed flux further in 4.
2.2 Low-Flux Population
Unlike the high-flux population, the total contribution of
the low-flux population is not fixed by existing surveys,
since these do not constrain the low flux peak on the
S2(dN/dS) S plot. Even though at low fluxes (S < So)
source counts indicate a power-law distribution dN/dS a
S-", the measured faint end index y varies substantially
between different surveys, ranging from y - 2.11 claimed
by Fomalont et al. (2002) and y - 2.61 claimed by Owen &
Morrison (2008). The value of y = 2.5 is what would be seen
in a static, isotropic and homogeneous Euclidean universe.
In extrapolating to lower fluxes, as long as the faint end in-
dex is above 2, the integrated contribution from lower flux
2 sources to the background will continue to increase. Obvi-
ously, at some eventual lower flux, the faint end index must
drop below 2, to avoid Olbers' paradox.
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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/m1/3/: accessed November 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.