We use absolutely calibrated data between 3 and 90 GHz from the 2006 balloon flight of the ARCADE 2 instrument , along with previous measurements at other frequencies , to constrain models of extragalactic emission .
Such emission is a combination of the Cosmic Microwave Background ( CMB ) monopole , Galactic foreground emission , the integrated contribution of radio emission from external galaxies , any spectral distortions present in the CMB , and any other extragalactic source .
After removal of estimates of foreground emission from our own Galaxy , and the estimated contribution of external galaxies , we present fits to a combination of the flat-spectrum CMB and potential spectral distortions in the CMB .
We find 2 \sigma upper limits to CMB spectral distortions of \mu < 5.8 \times 10 ^ { -5 } and |Y _ { \mbox { \scriptsize ff } } | < 6.2 \times 10 ^ { -5 } .
We also find a significant detection of a residual signal beyond that which can be explained by the CMB plus the integrated radio emission from galaxies estimated from existing surveys .
After subtraction of an estimate of the contribution of discrete radio sources , this unexplained signal is consistent with extragalactic emission in the form of a power law with amplitude 1.06 \pm 0.11 K at 1 GHz and a spectral index of -2.56 \pm 0.04 .