A Search for Hot, Diffuse Gas in Superclusters

The HEA01 A2 full sky, 2-10 keV X-ray map was searched for diffuse emission correlated with the plane of the local supercluster of galaxies and a positive correlation was found at the 99% confidence level. The most obvious interpretation is that the local supercluster contains a substantial amount of hot (10(exp 8) OK), diffuse gas, i.e. ionized hydrogen, with a density on the order of 2 - 3 x 10(exp -6) ions per cubic centimeter. This density is about an order of magnitude larger than the average baryon density of the universe and is consistent with a supercluster collapse factor of 10. The implied total mass is of the order of 10(exp 16) times the mass of the sun and would constitute a large fraction of the baryonic matter in the local universe. This result supports current thinking that most of the ordinary matter in the universe is in the form of ionized hydrogen; however, the high temperature implied by the X-ray emission is at the top of the range predicted by most theories. The presence of a large amount of hot gas would leave its imprint on the Cosmic Microwave Background (CMB) via the Sunyaev-Zel'dovich (SZ) effect. A marginal decrement (-17 muK) was found in the COBE 4-year 53 GHz CMB map coincident with the plane of the local supercluster. Although the detection is only 1beta, the level is consistent with the SZ effect predicted from the hot gas. If these results are confirmed by future observations they will have important implications for the formation of large-scale structure in the universe. Three other projects related directly to the HEAO 1 map or the X-ray background in general benefited from this NASA grant. They are: (1) "Correlations between the Cosmic X-ray and Microwave Backgrounds: Constraints on a Cosmological Constant"; (2) "Cross-correlation of the X-ray Background with Radio Sources: Constraining the Large-Scale Structure of the X-ray Background"; and (3) "Radio and X-ray Emission Mechanisms in Advection Dominated Accretion Flow"

A distorted view of the early universe

This was a solicited commentary on two papers in Physics Review Letters that were deemed by the American Physical Society to be of particular importance in 2011. My commentary was featured in the web based newsletter of the (British) Institute of Physics and also in a piece in the New Scientist. --author-supplied descriptio

Hard X-rays from Emission Line Galaxies and the X-ray Background: A Test for Advection Dominated Accretion with Radio Sources

Recent studies of the cosmic X-ray background (XRB) have suggested the possible existence of a population of relatively faint sources with hard X-ray spectra; however, the emission mechanism remains unclear. If the hard X-ray emission is from the radiatively inefficient, advection dominated accretion flows (ADAFs) around massive black holes in galactic nuclei, X-ray luminosity and radio luminosity satisfy the approximate relation $L_R\sim 7\times 10^{35}(\nu/15GHz)^{7/5}(M/10^7M_{\odot}) (L_x/10^{40} erg s^{-1})^{1/10} erg s^{-1}$ where $L_R=\nu L_\nu$ is the radio luminosity at frequency $\nu$, $M$ is the mass of the accreting black hole, and 10^{40} \simle L_x\simle 10^{42} erg s^{-1} is the 2-10 keV X-ray luminosity. These sources are characterized by inverted radio spectra $I_\nu \propto \nu^{2/5}$. For example, an ADAF X-ray source with luminosity $L_x\sim 10^{41} erg s^{-1}$ has a nuclear radio luminosity of $\sim 4\times 10^{36}(M/3\times 10^7M_{\odot}) erg s^{-1}$ at $\sim 20$ GHz and if at a distance of $\sim 10 (M/3\times 10^7M_{\odot})^{1/2} Mpc$ would be detected as a $\sim 1mJy$ point radio source. High frequency ($\sim 20 GHz$), high angular resolution radio observations provide an important test of the ADAF emission mechanism. Since $L_R$ depends strongly on black hole mass and only weakly on X-ray luminosity, the successful measurement of nuclear radio emission could provide an estimate of black hole mass. Because the X-ray spectra produced by ADAFs are relatively hard, sources of this emission are natural candidates for contributing to the hard, $>2$ keV, background.Comment: 16 pages, 2 figures, Ap