Detection of a small-scale cosmic microwave background decrement at 3.6 cm

We report the detection of a feature with negative radio flux density in a sensitive, low resolution observation with the VLA. This morphologically peculiar feature is approximately 30″ x 65″ in size with a peak central amplitude of about -0.25 mK below the brightness temperature of the background sky at 3.6 cm. Within about one arcminute of this microwave decrement, we also found two radio quiet quasars, both at z = 2.561, with a projected physical separation of about 1 Mpc (q₀ = 0.5, H₀= 50 km/s/Mpc), suggestive of a possible galaxy cluster at high redshift in the region. We are unable to account for this negative source by systematic phase fluctuations within our observations, sidelobe artifacts, or instrumental noise. We discuss possible physical origins of this microwave source, in particular the inverse-Compton scattering of the cosmic microwave background by hot gas in a distant cluster of galaxies, and the implications for models of large-scale structure formation.