Abstract
A fixed-horn antenna having a beam 10 degrees by 10 degrees , and a switched-load radiometer with traveling-wave-maser preamplifier were used to observe the 21-cm spectrum of the Virgo cluster of galaxies. An upper limit to the antenna temperature is 0.024 degrees K relative to regions outside the cluster with filters whose width is 2 Mc/sec. If the excitation temperature of the intergalactic hydrogen is enough greater than the background continuum radiation so that absorption can be ignored, and if the velocity spectrum is that defined by the galaxies, the density of optically thin neutral hydrogen in the cluster does not exceed that outside of the cluster by an amount that gives 5.6 x 10(12) solar masses in the cluster.
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