Superclusters are warm, not hot: the predicted flux distribution of soft X-rays

We present arguments showing that gas present in superclusters should have a characteristic temperature of approximately 106 K. In the most popular models of large-scale structure formation, including hot and cold dark matter, peculiar velocities of matter relative to the background are ≅ 1000 km s-1. This would imply temperatures exceeding 107 K. However, most of the peculiar motions can be attributed to large-scale, coherent velocities. Moreover, gas inside superclusters which is not concentrated in groups or clusters will cool adiabatically with the Hubble expansion. A temperature of 106 K implies that radiation from gas in superclusters should not be seen in X-rays of energy >500 eV. The most favourable range for observing this radiation is about 100-300 eV. We present the results of numerical simulations of the evolution of large-scale structure in a cold dark matter (CDM) scenario including the thermal history of the baryonic component. (author)