A search for neutral hydrogen in primordial protoclusters at z = 3.33 and 4.92.

A search has been made for the redshifted neutral hydrogen emission from primordial protoclusters at early epochs in the history of the universe. A system with a 2.5-MHz bandwidth was used to investigate 20 intermediate-latitude fields at frequencies of 328 and 240 MHz which correspond to neutral hydrogen redshifts of z = 3.33 and 4.92. Upper limits of peak flux density as low as 50 and 11 mJy have been set in fields at the two frequencies. These results are used to derive a limit to the properties of protoclusters in the early universe on the model proposed by Sunyaev and Zeldovich; either their masses are less than or approximately equal to 3 x 10 to the 15th solar masses or the number of such objects in the early universe is less than or approximately equal to 10 to the 6th.