Could a Foreground Residual Fake the Cold Spot? The Required Residual Amplitude at 143 Gigahertz and Its Frequency Fingerprint

A foreground residual cold enough to imitate the Cold Spot would betray itself by its spectrum, because galactic emission is not blackbody. This paper computes the fingerprint. A dust-like residual producing -150 microK at 143 gigahertz would produce -303.2 microK at 217 gigahertz and -103.2 microK at 100 gigahertz; a synchrotron-like residual would reach -341.3 microK at 100 gigahertz; a free-free-like residual would give -113.0 microK at 217 gigahertz. The spot in component-separated Planck maps is achromatic to within about twenty microK across these bands. Each single-component foreground hypothesis therefore misses the observed achromaticity by 153.2 microK or more at at least one band, several times the tolerance. A foreground origin for the Cold Spot's core would require a conspiracy of components cancelling each other's spectra at the spot's position, which is not a foreground model but a coincidence. The study is computed, not measured.