Correlations between O VI Absorbers and Galaxies at Low Redshift

We investigate the relationship between galaxies and metal-line absorption systems in a large-scale cosmological simulation with galaxy formation. Our detailed treatment of metal enrichment and non-equilibrium calculation of oxygen species allow us, for the first time, to carry out quantitative calculations of the cross-correlations between galaxies and O VI absorbers. We find the following: (1) The cross-correlation strength depends weakly on the absorption strength but strongly on the luminosity of the galaxy. (2) The correlation distance increases monotonically with luminosity from ~0.5-1h^-1 Mpc for 0.1L* galaxies to ~3-5h^-1 Mpc for L* galaxies. (3) The correlation distance has a complicated dependence on absorber strength, with a luminosity-dependent peak. (4) Only 15% of O VI absorbers lie near >=Lz* galaxies. The remaining 85%, then, must arise ``near'' lower-luminosity galaxies, though, the positions of those galaxies is not well-correlated with the absorbers. This may point to pollution of intergalactic gas predominantly by smaller galaxies. (5) There is a subtle trend that for >~0.5Lz* galaxies, there is a positive correlation between absorber strength and galaxy luminosity in the sense that stronger absorbers have a slightly higher probability of finding such a large galaxy at a given projection distance. For less luminous galaxies, there seems to be a negative correlation between luminosity and absorber strength.Comment: uses emulateapj, 5 pages including 2 color figures and 2 tables, accepted for publication in ApJ Letter

The Intergalactic Medium and Soft X-ray Background

I present an overview of some of the recent advances in our understanding of the distribution and evolution of the ordinary, baryonic matter in the universe. Two observations that strongly suggest that most of the baryons seen at high redshift ($z\ge 2$) have turned into some forms yet undetected at $z=0$ are highlighted. With the aid of large-scale cosmological hydrodynamic simulations, it is shown that most of the baryons today are in a gaseous form with a temperature of $10^5-10^7~$Kelvin -- the "warm/hot gas", shock heated during the gravitational collapse and formation of the large-scale structure at low redshift. Primarily line emissions from this warm/hot gas may account for a large fraction of the residual (after removal of identifiable discrete sources) soft X-ray background at $h\nu < 1.0$keV. How this warm/hot gas may be detected by the next generation of EUV and soft X-ray instruments is indicated. Detection or non-detection of this warm/hot gas will have profound implications for cosmology.Comment: An invited overview presented at "Small Missions for Energetic Astrophysics", Los Alamos, March 1999, full paper with color figures can be obtained at http://astro.princeton.edu/~cen/PROJECTS/p1/p1.htm

Effective field theory approach to top-quark decay at next-to-leading order in QCD

We present analytical results for top-quark decay processes, in an effective field theory beyond the Standard Model, at next-to-leading order in QCD. We parametrize new physics effects using dimension-six operators, and consider all operators that give rise to non-standard interactions of the top quark. We investigate both the flavor-conserving and flavor-changing decay modes, including their two-body and three-body semi-leptonic final states. The QCD mixing among relevant operators are also taken into account. These results provide all information needed for a complete model-independent study of top-quark decay at next-to-leading order accuracy, paving the way to global analyses for new physics effects in an effective field theory approach.Comment: 34 pages, 7 figure