Excess in the High Frequency Radio Background: Insights from Planck
  We conduct a stacking analysis using 1.4 GHz NRAO VLA Sky Survey (NVSS)
detections and Planck all-sky maps to estimate the differential source counts
down to the few 100 $\mu$Jy level at 30, 44, 70 and 100 GHz. Consequently, we
are able to measure the integrated extragalactic background light from discrete
sources at these frequencies. By integrating down to a 1.4 GHz flux density of
$\approx$2$ \mu$Jy, we measure integrated, extragalactic brightness
temperatures from discrete sources of $105.63\pm10.56 $mK, $21.76\pm3.09 \mu$K,
$8.80\pm0.95 \mu$K, $2.59\pm0.27 \mu$K, and $1.15\pm0.10 \mu$k at 1.4, 30, 44,
70, and 100 GHz, respectively. Our measurement at 1.4 GHz is slightly larger
than previous measurements, most likely due to using NVSS data compared to
older interferometric data in the literature, but still remains a factor of
$\approx$4.5 below that required to account for the excess extragalactic sky
brightness measured at 1.4 GHz by ARCADE 2. The fit to ARCADE 2 total
extragalactic sky brightness measurements is also a factor of $\approx$8.6,
6.6, 6.2, and 4.9 times brighter than what we estimate from discrete sources at
30, 44, 70 and 100 GHz, respectively. The extragalactic sky spectrum (i.e.,
$T_{\rm b} \propto \nu^{\beta}$) from discrete sources appears to flatten with
increasing frequency, having a spectral index of $\beta=-2.82\pm0.06$ between
1.4 and 30 GHz and $\beta=-2.39\pm0.12$ between 30 and 100 GHz. We believe that
the spectral flattening most likely arises from a combination of
Gigahertz-peaked sources and the spectral hardening of radio-detected sources
at higher frequencies, particularly at faint flux densities. However, the
precise origin of a hard component of energetic electrons responsible for the
emission remains unclear.
