(Left) All-sky map of the test statistic Z (color bar; see text) quantifying the statistical excess of echo candidates in each sky pixel of ≈2 deg². The location of the Cas A remnant is marked. (Right) Histogram of the estimated surface brightness of echo candidates within ≈3^∘ of Cas A (Figure 5) in the W1 and W2 bands. The distributions drop off sharply at ≈21 Vega mag arcsec⁻² and ≈20 Vega mag arcsec⁻² in the W1 and W2 bands, respectively.