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 morpholo gically peculiar feature is approximately 30 '' X65 '' in size with a peak central amplitude of about -0.25 mK below the brightness temperat ure of the background sky at 3.6 cm. Within about one arcminute of thi s microwave decrement, we also found two radio quiet quasars, both at z=2.561, with a projected physical separation of about 1 Mpc (q(0)=0.5 , H-0=50 km/s/Mpc), suggestive of a possible galaxy cluster at high re dshift in the region. We are unable to account for this negative sourc e by systematic phase fluctuations within our observations, sidelobe a rtifacts, 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 fo rmation. (C) 1997 American Astronomical Society.