The origin of the diffuse background gamma-radiation

Recent observations have now provided evidence for diffuse background gamma radiation extending to energies beyond 100 MeV. There is some evidence of isotropy and implied cosmological origin. Significant features in the spectrum of this background radiation have been observed which provide evidence for its origin in nuclear processes in the early stages of the big-band cosmology and tie in these processes with galaxy fromation theory. A crucial test of the theory may lie in future observations of the background radiation in the 100 MeV to 100 GeV energy range which may be made with large orbiting spark-chamber satellite detectors. A discussion of the theoretical interpretations of present data, their connection with baryon symmetric cosmology and galaxy formation theory, and the need for future observations are given

Photonuclear interactions of ultrahigh energy cosmic rays and their astrophysical consequences

Results of detailed Monte Carlo calculations of the interaction histories of ultrahigh energy cosmic-ray nuclei with intergalactic radiation fields are presented. Estimates of these fields and empirical determinations of photonuclear cross sections are used, including multinuclear disintegrations for nuclei up to 56Fe. Intergalactic and galactic energy loss rates and nucleon loss rates for nuclei up to 56Fe are also given. Astrophysical implications are discussed in terms of expected features in the cosmic-ray spectrum between quintillion and sextillion eV for the universal and supercluster origin hypotheses. The results of these calculations indicate that ultrahigh energy cosmic rays cannot be universal in origin regardless of whether they are protons or nuclei. Both the supercluster and galactic origin hypotheses, however, are possible regardless of nuclear composition

The FIR/submm window on galaxy formation

Our view on the deep universe has been so far biased towards optically bright galaxies. Now, the measurement of the Cosmic Infrared Background in FIRAS and DIRBE residuals, and the observations of FIR/submm sources by the ISOPHOT and SCUBA instruments begin unveiling the ``optically dark side'' of galaxy formation. Though the origin of dust heating is still unsolved, it appears very likely that a large fraction of the FIR/submm emission is due to heavily-extinguished star formation. Consequently, the level of the CIRB implies that about 2/3 of galaxy/star formation in the universe is hidden by dust shrouds. In this review, we introduce a new modeling of galaxy formation and evolution that provides us with specific predictions in FIR/submm wavebands. These predictions are compared with the current status of the observations. Finally, the capabilities of current and forthcoming instruments for all-sky and deep surveys of FIR/submm sources are briefly described.Comment: 10 pages, Latex, 5 postscript figures, to appear in ``The Birth of Galaxies'', 1999, B. Guiderdoni, F.R. Bouchet, T.X. Thuan & J. Tran Thanh Van (eds), Editions Frontiere

Small scale variations of abundances of transiently heated grains in molecular clouds

IRAS images of a variety of fragments in nearby molecular clouds show that the energy distribution of their IR emission varies widely from cloud to cloud and from place to place within a given cloud. These variations at small scale are all the more unexpected since the colors of the IR emission of cold material differ very little at large scale: the colors of the cirrus emission above the 3kpc molecular ring are the same as those of the cirrus emission in the solar neighborhood. To quantitatively study these variations, 12, 60, and 100 microns brightnesses were obtained of small areas centered at different positions within the set of clouds and complexes. The range of observed 12/100 micron colors is given for each cloud. Variations by an order of magnitude are found in most clouds. Variations by a factor of 2 to 3 are observed within a cloud on scales as small as 0.5pc, the resolution of this study. It is concluded that large variations of the abundances of small particles with respect to those of the large grains responsible for the 100 micron emission are required to explain the observed color variations and that these abundances have to vary by large factors; an order of magnitude from cloud to cloud

Diffuse infrared emission of the galaxy: Large scale properties

The Infrared Astronomy Satellite (IRAS) survey is used to study large scale properties and the origin of the diffuse emission of the Galaxy. A careful subtraction of the zodiacal light enables longitude profiles of the galactic emission at 12, 25, 60, and 100 microns to be presented

The cosmic far-infrared background at high galactic latitudes

Far-infrared background fluxes from various cosmic sources are predicted. These fluxes lie near the high frequency side of the blackbody radiation spectrum. These sources could account for a significant fraction of the background radiation at frequencies above 400 GHz which might be misinterpreted as a comptonization distortion of the blackbody radiation. Particular attention is paid to the possible contributions from external galaxies, rich clusters of galaxies and from galactic dust emission