A Search for Intergalactic Hydrogen in the Virgo Cluster

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Science  07 Jan 1966:
Vol. 151, Issue 3706, pp. 71-73
DOI: 10.1126/science.151.3706.71


A fixed-horn antenna having a beam 10° by 10°, 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°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 1012 solar masses in the cluster.