Detection of a Small Scale Cosmic Background Anisotropy at 3.6 cm

We report on the detection of a feature with negative radio (cid:13)ux in a sensitive, low resolution observation with the VLA. This morphologically peculiar feature is approximately 25 00 (cid:2) 65 00 in size with a peak central amplitude of about {0.25 mK compared to the brightness temperature of the background sky at 3.6 cm. Within about 1 0 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 (q o = 0.5, h = 0.5), suggestive of a possible galaxy cluster in the region. We are unable to account for this negative source by systematic phase (cid:13)uctuations 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.