Detection of a Cosmic Microwave Background Decrement toward the [CLC][ITAL]z[/ITAL][/CLC] = 3.8 Quasar Pair PC 1643+4631A, B

In a 15 GHz Ryle Telescope observation of PC 1643+4631A, B, a pair of quasars at redshifts z = 3.79 and 3.83 separated by 198'' on the sky, we find a decrement in the cosmic microwave background (CMB) of -380 ± 64 μJy in a 110'' × 175'' beam. Assuming this to be a Sunyaev-Zeldovich effect due to an intervening cluster, the minimum magnitude of the central temperature decrement is 560 μK. A serendipitous ROSAT observation shows that there is no X-ray-luminous cluster in the direction of the decrement at z < 1. The implied gas mass is ≳2 × 10 14 M ☉ (assuming a temperature of ~5 keV), indicating a total mass of more than 10 15 M ☉ . This result demonstrates the existence of a massive system too distant to be detected by its emission, but which can be found via its imprint on the CMB.