1,721,308 research outputs found
ANALYSIS OF A COMPLETE SAMPLE OF GALAXIES AT MZ LESS-THAN-OR-EQUAL-TO 14.5 - THE OPTICAL, RADIO AND FAR-INFRARED LUMINOSITY FUNCTIONS
No full textGALAXY COUNTS AND CONTRIBUTIONS TO THE BACKGROUND-RADIATION FROM 1 MU-M TO 1000 MU-M
No full textWe present detailed predictions for the possible outcome of surveys of extragalactic sources soon to be carried out with the forthcoming generation of both ground based and satellite-borne telescopes, over the full range from near-IR to mm wavelengths. Estimates of the confusion limits, due both to stars and galaxies, for several particularly interesting wavebands have been worked out. Predictions for the near- and IR bands are based on up-to-date models for chemical and photometric evolution of early- and late-type galaxies, and allow for luminosity evolution of Seyfert nuclei. These models are shown to provide a good fit to the recent deep counts in the K band; they imply that E/S0 and spiral galaxies are present in roughly equal numbers for K less-than-or-equal-to 19. At fainter magnitudes spirals dominate; ground based surveys down to K = 21 would allow to test photometric evolution models up to z congruent-to 1 - 3. Deep surveys at longer wavelengths, requiring for example a few hour exposures with the most sensitive filters of the ISO camera, such as that centered at lambda = 6.7-mu-m, are particularly well suited for studying the photometric evolution of E/S0 galaxies at high redshift. We argue that relatively shallow surveys at the same lambda-over an area of a few square degrees would be particularly effective in providing an unbiased sample of low luminosity AGNs. Predictions for 25-mu-m less-than-or-equal-to lambda less-than-or-equal-to 1.3 mm are obtained by extrapolating a model fitting the IRAS deep survey data. The adopted dust emission spectrum from a few microns to 1.3 mm takes into account simple but realistic size and temperature distributions of grains and of PAH molecules. Surveys at lambda > 100-mu-m (i.e. beyond the dust emission peak) would allow to investigate dust reradiation from distant and primeval galaxies. For example, a survey at 800-mu-m with a flux limit of 30 mJy would have a strong likelihood of discovering actively star forming galaxies at z congruent-to 5, if they exist. The measurements of the diffuse background spectrum and intensity being carried out by COBE, will provide important constraints but will require a delicate subtraction of foreground emissions
Constraints on the cosmic star formation history from the far-infrared background
No full textStudies of the far-infrared extragalactic background (FIRE) provide information on the star formation history of the Universe which is complementary to that given by optical observations, The upper limits on the FIRE intensity resulting from recent analyses of COBE/FIRAS data entail interesting constraints on the possibility that most of the star formation activity occurring during early phases of galaxy evolution takes place within dusty gas clouds, The tentative detection of the FIRE reported by Puget et al. suggests that the star formation and metal production histories might have been substantially different, at high redshifts, from those inferred from optical data
Intervening sequences in paralogous genes: a comparative genomic approach to study the evolution of x chromosome introns
No full textThe enlargement of the genome size and the decrease in genome compactness with increase in the number and size of introns is a general pattern during the evolution of eukaryotes. Among the possible mechanisms for modifying intron size, it has been suggested that the insertion of transposable elements might have an important role in driving intron evolution. The analysis of large portions of the human genome demonstrated that a relatively recent (50 to 100 MYA) accumulation of transposable elements appears to be biased, favoring a preferential insertion of LlNE1 transposons into sex chromosomes rather than into autosomes. In the present work, the effect of chromosomal location on the increase in size of introns was evaluated with a comparative analysis performed on pairs of human paralogous genes, one located on the X chromosome and the second on an autosome. A phylogenetic analysis was also performed on the X-encoded proteins and their paralogs to confirm orthology-paralogy and to approximately estimate the time of gene duplication. Statistical analysis of total intron length for each pair of paralogous genes provided no evidence for a larger size of introns in the gene copies located on the X chromosome. On the opposite, introns of autosomal genes were found to be significantly longer than introns of their X-finked paralogs. Likewise, LINE1 elements were not significantly more frequent in X-chromosome introns, whereas the frequency of SINE elements showed a marginally significant bias toward autosomal introns
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