X‐Ray Emission from the Local Supercluster: Possible Evidence for Hot, Diffuse Gas

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Abstract
The HEAO 1 A2 full sky, 2-10 keV X-ray map was searched for emission correlated with the plane of the Local Supercluster of galaxies. After removing strong point and moderately extended sources (e.g., the core of the Virgo cluster), there remained a statistically significant component of "diffuse" X-rays in the plane of the supercluster. Fitting this diffuse component with a simple "pillbox" model of the local supercluster implies a volume X-ray emissivity of εX = 3.0 ± 0.3 × 1039(RSC/20 Mpc)-1 ergs s-1 Mpc-3, where RSC is the radius of the supercluster and the error is photon counting noise only. If one considers fluctuations in the X-ray background as an additional component of noise, then the detection is reduced considerably to 2.3 σ. However, the significance level implied by this value (i.e., 99%) is consistent with the distribution of fits to the model obtained by rotating the original data. If the source of the X-ray emission is bremsstrahlung from a uniformly distributed plasma with temperature Te, then the implied electron number density is Ne = 2.5 × 10-6(RSC/20 Mpc)-1/2(kTe/10 keV)-1/4 cm-3. This value is about an order of magnitude larger than the average baryon number density implied by nucleosynthesis and is consistent with a collapse factor of 10. A search for similar structure in the COBE 53 GHz microwave background map yielded a marginal detection with an amplitude of ~-17 ± 5 μK (statistical error only), which is consistent with the Sunyaev-Zeldovich (SZ) effect expected from 10 keV gas. This latter value is comparable to the amplitude of intrinsic large-scale fluctuations in the microwave background and should be considered to be a 1 σ result at best.