675,295 research outputs found
Diffuse Background Radiation
A new determination of the upper limit to the cosmic diffuse background
radiation, at ~110 nm, of 300 photons s-1 cm-2 sr-1 nm-1, is placed in the
context of diffuse background measurements across the entire electromagnetic
spectrum, including new optical, infrared, visible, and gamma-ray background
measurements. The possibility that observed excess diffuse visible radiation is
due to redshifted cosmological Lyman alpha recomination radiation is explored.
Also, a new standard of units for the display of spectra is advocated.Comment: Nine pages and one figur
Diffuse Extragalactic Background Radiation
Attenuation of high--energy gamma rays by pair--production with UV, optical
and IR background photons provides a link between the history of galaxy
formation and high--energy astrophysics. We present results from our latest
semi-analytic models (SAMs), based upon a CDM hierarchical structural
formation scenario and employing all ingredients thought to be important to
galaxy formation and evolution, as well as reprocessing of starlight by dust to
mid- and far-IR wavelengths. Our models also use results from recent
hydrodynamic galaxy merger simulations. These latest SAMs are successful in
reproducing a large variety of observational constraints such as number counts,
luminosity and mass functions, and color bimodality. We have created 2 models
that bracket the likely ranges of galaxy emissivities, and for each of these we
show how the optical depth from pair--production is affected by redshift and
gamma-ray energy. We conclude with a discussion of the implications of our
work, and how the burgeoning science of gamma-ray astronomy will continue to
help constrain cosmology.Comment: 12 pages, 8 figures, to be published in the Proceedings of the 4th
Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, held
July 2008 in Heidelberg, German
Intergalactic UV Background Radiation Field
We have performed proximity effect analysis of low and high resolution data,
considering detailed frequency and redshift dependence of the AGN spectra
processed through galactic and intergalactic material. We show that such a
background flux, calculated using the observed distribution of AGNs, falls
short of the value required by the proximity effect analysis by a factor of
2.7. We have studied the uncertainty in the value of the required flux
due to its dependence on the resolution, description of column density
distribution, systemic redshifts of QSOs etc. We conclude that in view of these
uncertainties the proximity effect is consistent with the background
contributed by the observed AGNs and that the hypothesized presence of an
additional, dust extinct, population of AGNs may not be necessary.Comment: To be published in the Journal of Astronomy and Astrophysics aasms, 2
figures, 2 tables. Paper replaced to include the figure
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