Excess in the High Frequency Radio Background: Insights from Planck

TL;DR

This study measures the extragalactic radio background across multiple frequencies, revealing a significant excess compared to discrete sources and suggesting spectral flattening likely due to specific source populations.

Contribution

It provides new measurements of the extragalactic background light from discrete sources at GHz frequencies, highlighting a persistent excess over previous estimates and analyzing spectral behavior.

Findings

Measured brightness temperatures at multiple GHz frequencies.

Found a significant excess over the contribution from discrete sources.

Observed spectral flattening with increasing frequency.

Abstract

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.