[en] Extragalactic Background Light (EBL) is the integrated light from all stars that have ever formed, and spans in a range of Infrared (IR) to Ultraviolet (UV). The interaction of very-high-energy (VHE: E > 100GeV) γ-rays emitted by Active Galactic Nuclei (AGN)at cosmological distances with the EBL results in electron-positron pair production that leads to an energy-dependent attenuation of the observed VHE flux. Here it is introduced a method based on the attenuation to measure the EBL photon number density. Then it is applied this method on simultaneous blazar data-PKS 2155-304-to determine the optical density at z = 0.12 and compare it with the optical densities predicted by popular EBL models.

No abstract available

[en] Optical and radio wavelength data available on fast S and E galaxies in regular clusters were analyzed. These galaxies are deficient in gas, have slower star formation rates and less active nuclei when compared to similar field galaxies. The slow massive galaxies in central regions of rich clusters, on the contrary, contain more gas and have more active nuclei

[en] The growth of galaxy clusters is energetic and may trigger and/or quench star formation and black hole activity. The ENISALA⁴ project is a collection of multiwavelength observations aimed at understanding how large-scale structure drives galaxy and black hole evolution. Here, we introduce optical spectroscopy of over 800 Hα emission-line galaxies, selected in 14 z ∼ 0.15–0.31 galaxy clusters, spanning a range of masses and dynamical states. We investigate the nature of the emission lines in relation to the host galaxy properties, its location within the cluster, and the properties of the parent cluster. We uncover remarkable differences between mergers and relaxed clusters. The majority of Hα emission-line galaxies in merging cluster fields are located within 3 Mpc of their center. A large fraction of these line emitters in merging clusters are powered by star formation irrespective of cluster-centric radius, while the rest are powered by active galactic nuclei (AGNs). Star-forming galaxies are rare within 3 Mpc of relaxed clusters and AGNs are most abundant at their outskirts (∼1.5–3 Mpc). We discover a population of star-forming galaxies with large equivalent widths and blue UV–optical colors found exclusively in the merging clusters in our sample. The widespread emission-line activity in merging clusters is likely supported by triggered activity in recently accreted, gas-rich galaxies. By contrast, our observations for relaxed clusters match established models in which black hole activity is enhanced at the virial radius and star formation is quenched within the infall region. We conclude that emission-line galaxies experience distinct evolutionary paths in merging and relaxed clusters.

[en] In a short note main reports on the problem of active galaxies made at the 18th General Assembly of the International astronomical Union are presented. A Wilson (England) presented a vast report on resemblance of activities of different galaxies. A. Fabian (England) investigatied x radiation of active galaxy nuclei. P. Wilkinson (England) observed on 18 cm and 6 cm waves small outbursts near the nucleus of galaxies M 87 and 3C 273. G. Mailey (Holland) found out a dependence between the width of emission lines in the visible spectrum range and radio luminosity radiosources. V. Van Brugel (Holland) investigated the 3C 293 radiosource, the nucleus of which consists of two components. M. Berbidge (USA) reported on the study of a strong radiosource 3C 303 having a complex structure. E. Hachikian (USSR) presented the results obtained at the Burakan Astrophysical Observatory on the problem of galaxies with ultraviolet excess

[en] We quantify the importance of the mechanical energy released by radio galaxies inside galaxy groups. We use scaling relations to estimate the mechanical energy released by 16 radio-active galactic nuclei located inside X-ray-detected galaxy groups in the COSMOS field. By comparing this energy output to the host groups' gravitational binding energy, we find that radio galaxies produce sufficient energy to unbind a significant fraction of the intragroup medium. This unbinding effect is negligible in massive galaxy clusters with deeper potential wells. Our results correctly reproduce the breaking of self-similarity observed in the scaling relation between entropy and temperature for galaxy groups.

[en] Modern 21 cm hydrogen-line redshifts (published and unpublished), accurate to approx.8 km s^-1, are used as standards to derive zero point corrections, rms uncertainties (sigma_V), and the percentage of discordant (i.e., differing by more than 3 sigma_V from the zero point redshift) optical and 21 cm redshifts from earlier catalogs. Optical redshifts have a typical uncertainty of approx.100 km s^-1 (independent of the epoch of the source of redshift), but the variation in sigma_V from source to source is considerable. Optical redshifts by Kelton (sigma_V = 43 km s^-1), the Center for Astrophysics (Davis et al.) (sigma_V = 37 km s^-1), and Rubin, Ford, and Thonnard (sigma_V = 8 km s^-1) are especially noteworthy. The rms correction factor appropriate to optical redshifts listed in the First Reference Catalog (de Vaucouleurs and de Vaucouleurs) is 1.33 and that for the Second Reference Catalog (de Vaucouleurs, de Vaucouleurs, and Corwin) is 1.42. A new catalog of galaxy redshifts (CGR) containing known redshifts for approx.4000 galaxies and reliable estimates of rms uncertainties was compiled and is available for circulation

[en] The effect of relativistic electrons on a photoionized emission-line region is investigated, considering optically thick models. The main effect of these electrons is to heat the gas while their influence on the ionization structure is not very important. The calculated emission-line fluxes are compared to the available observational data. (author)

[en] The size and nature of any large-scale anisotropy in the three-dimensional distribution of galaxies is still little understood. Recent studies have indicated that large fluctuations in the matter distribution on a scale from tens up to several hundreds of megaparsecs may exist. Work at the South African Astronomical Observatory (SAAO) in recent years has made major contributions to studies of the large scale distribution of galaxies, as well as to solving the problems of the galactic and extragalactic distance scale. Other studies of galaxies undertaken at SAAO include: quasars in the fields of nearby galaxies; dwarf irregular galaxies; IRAS galaxies; Seyfert galaxies; 'hot spot' galaxies; supernovae in NGC 5128 and NGC 1559 and superclusters. 4 figs

[en] A list is given of 136 galaxies with ultraviolet excess found with the 40-in. Schmidt telescope of the Byurakan Observatory with a 1.5-deg objective prism. Of these, 58 were observed at the primary focus of the 2.6-m telescope of the Byurakan Observatory, and 12 at the primary focus of the 6-m telescope of the Special Astronomical Observatory of the USSR Academy of Sciences. These observations and Palomar Sky Survey prints were used for a morphological description of the galaxies. Descriptions are given of the spectra of 17 galaxies obtained with the 6-m telescope of the Special Astronomical Observatory, the 2.6-m telescope of the Byurakan Observatory, and 90-, 107-, and 200-in. telescopes in the United States